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Developmen tal Abnormalit ies of the Central Nervous System Pediatric Neurosurgery Mahmod Saad

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Page 1: Developmental Cns Anomalies

Developmental Abnormalities of the Central Nervous SystemPediatric Neurosurgery

Mahmod Saad

Page 2: Developmental Cns Anomalies

Develop. Anomalies of CNS

Spinal Dysraphism and Related Conditions

Contents

Definitions Classification of Spina Bifida Epidemiology of Spina Bifida Etiology of Spina Bifida Pathology Principles of Treatment of Congenital Malformations Spina Bifida Occulta Neural Tube Defects (MC, MMC) Dermal Sinus and Associated Dermoid and Epidermoid Tumors Spinal Lipomas Combined Spina Bifida, Split Cord, Neurenteric Cysts Thickened Filum, Myelocystocoele Caudal Agenesis Encephaloceles Chiari II Malformation Tethered Spinal Cord

 

Definitions

• Spinal dysraphism. A generic term describing pathologic conditions related to improper closure of the caudal neuropore. Encompasses all conditions associated with spina bifida.

• Craniorachischisis. Total failure of neurulation. It is the most severe form of spinal dysraphism. There is no dorsal axial skeleton or dermal covering.

• Myeloschisis. Midline clefting of the spinal cord, partial or complete.

• Spina bifida cystica. Refers to a meningocele or meningomyelocele.

• Spina bifida aperta. A defect open to the environment.

• Neural plaque (or placode). The dorsal neural tissue contained within the meningomyelocele.

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Classification of Developmental Abnormalities of the CNS

• A number of schemes have been described. The most straightforward is one that divides anomalies into open and closed. This has practical utility as most closed defects can be observed in the early period. It does not, however, provide a logical etiological framework to link the wide range of dysraphic states.

Embryological Classification

Anomalies due to defects of neural folding (neural tube defects such as MC and MMC).

Anomalies due to incomplete dysjunction (dermal sinus and associated dermoid and epidermoid tumors).

Anomalies due to premature dysjunction. (spinal lipomas). Anomalies due to disorders of gastrulation (combined spina bifida, split cord,

neurenteric cysts). Anomalies due to disordered secondary neurulation (thickened filum,

myelocystocoele). Anomalies due to failure of caudal neuraxial development (caudal agenesis). Anomalies due to disordered postneurulation development (encephaloceles). Secondarily acquired CNS anomalies due to myelodysplasia (Chiari II malformation).

Anatomical Classification

Open Neural Tube Defects

Meningocele Meningomyelocele Spina bifida aperta Encephalocele

Closed Neural Tube Defects

Spina bifida occulta Lipoma Lipomyelomeningocele Dermal sinus tract Thickened filum Myelocystocele Caudal agenesis Diastematomyelia Diplomyelia Neurenteric cyst

Epidemiology

• Spina bifida occulta occurs in 20 to 30% of the population.

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• Incidence for open spina bifida has been declining. Before early 80's, about 1-2/1000 live births in US and 4-5/1000 in Ireland. Now down to 3-6/10000 in US. Declining incidence is probably due to numerous factors. Most are unknown except possibly vitamin supplementation and the impact of antenatal screening and termination.

• F>M; whites > blacks and Orientals. Increased incidence in those belonging to poorer socioeconomic classes.

Genetics

• There is familial tendency, but not Mendelian inheritance. Probably multiple genes involved with variable penetrance. Risk of one child with spina bifida is 0.05%. If one sibling affected, risk increases to 5%. If 2 affected, risk increases to 12-15%. Genetic counseling should be offered to all parents.

 

Pathology

Etiology

• Unknown. Probably multifactorial.

o Genetic (see above)o Nutrition (Folate, Zinc)o Drugs (valproate, carbamazepine)o Environmental (maternal hyperthermia)

• Numerous teratogens have been suggested. Folate supplementation of 0.4 mg/day before conception and through early pregnancy reduces occurrence of spina bifida by 72% in high-risk and first pregnancies. Folic acid antagonists (aminopterin) produce neural tube defects in lab animals.

Gross

• Frequently, much of the CNS is affected in addition to the obvious defects. In addition, there may be GI, GU and CV abnormalities. See path. Under individual conditions.

• A large variety of brain abnormalities are seen including polymicrogyria, lobar agenessis, holoproencephaly, cerebellar dysplasia, heterotopias, schizencephaly, gyral abnormalities, agenesis of corpus callosum, septum pellucidum cysts, lipomas, arachnoid cysts, and high frequency of hydrocephalus.

• Spinal cord 85% of lesions are in distal thoracic, lumbar, and sacral areas (10% thoracic, 5% cervical).

 

Principles of Treatment of Congenital Malformations

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1. Counselling of patient and parent.2. Care of neurogenic bladder.3. Care of associated anomalies (GI, CVS, etc.).4. Medical management of spasticity, neurogenic pain, and associated medical conditions.5. Appropriate surgical procedure.6. Post-operative care.7. Rehabilitation and the use of rehabilitation aids.8. Orthopaedic referral.9. Regular and close follow up.

I. Spina Bifida OccultaDefinition

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• Absence of the spinous process along with minor amounts of the neural arch. Usually, the CNS, cauda equina, and peripheral nervous system are not involved. It is a radiological diagnosis.

• Occurs in 20-30% of the general population, most common at L5 or S1.

• Etiology represents failure of fusion of the posterior arch.

• Clinically there are no obvious external markings. When associated with a skin dimple, hair patch, telangiectasia or other cutaneous marking, there should be increased suspicion of an underlying anomaly (occult spinal disraphysm). No obvious clinical signs or symptoms are associated with this diagnosis.

Imaging

• Plain films show absence of the spinous process along with minor amounts of the neural arch. Most common at L5 or S1.

This AP view of the lumbar spine demonstrates a bifid posterior arch at the S1 level. S1 itself is partially lumbralized. No neurological abnormality was present with this incidental finding.

Treatment and Results

• Requires no treatment. Questionable association with chronic back pain and enuresis.

 

II. Abnormalities Due to Defective Neural Tube Folding

Meningocele and Myelomenigocele6

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o Definition

• A meningocele is a defect consisting of a herniation of meningeal tissue through a defect in the skull and/or spine.

• A myelomeningocele (MMC) is a defect consisting of a herniation of meningeal tissue and nervous tissue through a defect in the skull and/or spine.

• Myeloschisis is an old term which represents a completely open caudal neural tube with no evidence of a central canal. Placode is not displaced by underlying CSF.

• The placode is the open part of the neural tube which remains attached to the surface ectoderm.

• Meningocele "manque" refers to an elongation of the conus nerve roots or neuroglial tissue, with attachment to the posterior dura with tethering, with or without a lipoma, but without a disruption of the posterior neural arch and overlying tissues. (rare).

o Epidemiology

• For MMC see above. Meningoceles are 10% of all patients with spina bifida.

o Etiology

• Two theories.

1. Primary failure of neural tube closure (originally proposed by von Recklinghausen in 1886).2. Rupture of a previously closed neural tube due to overdistension (Gardner; unpopular theory).

o Embryology

• Closure of the rostral and caudal neuropores occurs by 4 weeks. Therefore open neural tube defects are felt to result from a primary neural tube closure defect vs. rupture of already closed neural tube.

• Closed neural tube defects are felt to arise as a complication of secondary neurulation, that is involved in formation of the caudal part of the spinal cord from the neural cell mass caudally. Normally the intervening somatic and epithelial tissue between the main (rostral) neural tube and the caudal mass regresses as these areas unite to complete neurulation, leaving behind a long thin filum terminale, as a remnant. Maldevelopment at this stage (which occurs between the 4th and 6th week of gestation) results in incorporation of mesodermal (lipomyelomingocoele) or epithelial elements (dermal sinus) or incompleted fusion of some elements (e.g. meningocele), all of these defects are covered with skin as in this time of development the caudal mass is already skin covered.

o Pathology of Meningocele

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• Pathogenesis is a post-neurulation disorder. Neurulation and dysjunction of the cutaneous ectoderm have occured normally, but subsequent development of overlying mesenchymal and cutaneous ectoderm components is aberrent.

• Gross. Dorsal half of the vertebra(e) is gone, and the contents of the bulge are limited to CSF, meninges and skin.

• Microscopic. Meningocele consists of loose firovascular tissue. May contain microscopic islands of aberrent nervous tissue. The cyst is ependyma-lined, under which is a thin glial layer. Hallmark is that glial tissue is superimposed on collagenous tissue without intervening meninges.

o Pathology of Myleomeningocele

• Pathogenesis is a problem of primary neurulation. Caudal neuropore closes day 26-28. Rostral neuroprore at day 22-24. Failure to close will produce MMC and encephalocele, respectively (other authors argue that because the encephalocele contains brain tissue, the embrylogical defect must occur after rostral neural tube closure and differentiation).

• The open neural placode represents the embryologic form of caudal end of spinal cord. The narrow groove passing down placode in midline represents primitive neural groove which in normal development forms the spinal canal. The placode is displaced dorsally by underlying CSF. Normal skin surrounds the neural tissue, with an intermediate zone of thin skin.

• The dura is usually found underlying the skin within about 1-2 mm from edge. The true epidural space with fat often found. The underlying vertebral bodies are flattened and widened. The pedicles are everted and lie horizontal. The laminae are often hypoplastic and spinous processes are absent. Paraspinal muscles lie anteriorly and act as flexors rather than extensors of the spine.

o Pathophysiology of Neural Dysfunction

1. Early embryonic intrauterine injury.2. Irritation at birth via delivery through birth canal.3. Infection.4. Hydrocephalus.5. Tethered cord (causing vascular insufficiency, anoxia, mechanical traction, axial distortion of axons).

o Gross

• Most frequently located in the thoracolumbar area.

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Thoracic   3%T/L        45Lumbar 20L/S       22Sacral     10

A lumbar myleomeningocele with an irregular sac.

• Contains CSF, meninges, cauda equina and abnormal spinal cord. Vertebral anomalies include absence of spinous processes and lamina, reduced AP diameter of vertebral body, increased interpeduncular distance, laterally extending transverse process.

• All patients have some form of Chiari II malformation. Over 75% develop hydrocephalus. The neural placode looks like an open book, with the neural groove and central canal visible. The dorsal roots exit from the lateral anterior portion of the cord. About 40% have hydro/syringomyelia.

Clinical Features

• Prenatal diagnosis is by measurement of alpha-fetoprotein (AFP). Open defects allow AFP to enter amniotic fluid and maternal bloodstream. Elevations peak at 15-18 weeks. Maternal AFP serum assays have a 60-70% accuracy in identifying fetuses with open NTD. If AFP levels are positive on 2 separate occasions, amniocentesis is recommended (if ultrasound is negative). AFP and acetylcholinesterase analysis of amniotic fluid has accuracy of 97% with false positive of 0.4%. Ultrasound in experienced hands can have 100% detection rate. Genetic counselling may help detect those with 2nd or 3rd degree relatives. In the absence of craniomegaly from hydrocephalus, vaginal delivery is recommended by most, unless there is an obstetric indication for C-section (e.g., breech). One study (Luthy et al. NEJM, 1991) suggests that C-secion delivery of MMC babies produces better neurological outcome.

• Postnatal assessment includes keeping baby NPO, prone or lateral decubitus position, appropriate dressings (usually to keep sac moist), normothermia, and appropriate intermittent catheterization. On examination, it is crucial to check for associated CV, GI, GU, and orthopaedic abnormalities. Birth trauma should be excluded. Neurological examination should include size, appearence, and shape of lesion. Assess spinal level with motor and

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sensory exam. It is essential to check rectal tone prior to performing repair. Post-op deterioration can occur.

• Meningocele usually causes no disability. Low incidence of associated anomalies and hydrocephalus.

• MMC usually causes sensory and motor changes distal to the anatomic level producing varying degrees of weakness, anesthesia, urine and fecal incontinence.

Laboratory and Imaging

• EMG's and nerve stimulation show normal peripheral nerve and non-denervated muscle. The paralysis secondary to an MMC is thus due to an upper motor neuron lesion, either within the plaque or just above it. Sensory pathways to and from the plaque are intact (by evoked potentials). Thus, interference with afferent conduction is at level of plaque.

• Plain films, CT, MRI, ultrasound, cardiac echo, GU investigations can be done but usually direct imaging of the sac does not alter surgical management.

An antenatal ultrasound at demonstrating a saggital image of a fetal spine. The myelomeningocele sac can be seen at the lower end of the spine. Nerve roots travel from the placode down to the anterior spinal canal. 

 Treatment and Results

• For meningocele, surgical excision is recommended. Excellent results are the rule.

• Surgery for anterior sacral meningocele is through posterior laminectomy, closure/ligation of the dural opening, and aspiration of meningocele contents.

• For myelomeningocele, surgical indications are to preserve intellectual, sensory and motor function and prevent CNS infections.

• Principles.

Goal is to restore anatomy with early closure and preserve all neural tissue.Placode should be kept moist before repair.Neural tissue and coverings are reconstructed and a water-tight dural closure is done.

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• Surgical results.

Operative mortality close to 0%.95% 2 year survival.12% wound infection, dehiscence rare.18% develop ventriculitis.80% have IQ in normal range.If ventriculitis or ICH, 30% have normal IQ.80% will walk with some assistance.<10% are economically independent.

Outcome

• Major cause of death in first 2 years is due to brain stem (dysphagea, apnea, bradycardia, etc.) and upper cervical cord dysfunction (spasticity, hypotonia, and scoliosis). These symptoms may be aggravated by hydrocephalus. Thus, the Chiari II malformation (see below) remains problematic.

• Delayed deterioration requires a "top-down" approach.

1. Hydrocephalus or shunt malfunction.2. Symptomatic Chiari.3. Hydrosyringomyelia.4. Associated dysraphic lesion.5. Tethering at original site.

• Rule of 80's (80% have hydrocephalus, 80% require shunt, and 80% will have IQ >80 (but most are 80-95)).

• Delayed complications are common.

CNS

hydrocephalus (80-90% of MMC kids) seizures (30% of patients, possibly shunt malfunction) spasticity hydrosyringomyelia brainstem dysfunction (swallowing difficulties, apnea, lower cranial nerve dysfunction, bradycardia, and auditory difficulties) intraspinal tumor tethered cord psychosocial difficulties learning disabilities ascent of spinal level

MSK

kyphoscoliosis contractures

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pes cavus leg deformities pressure sores MMC repair site tenderness

Ophthalmologic

Papilledema visual field defects visual acuity defects amblyopia optic atrophy nystagmus strabismus

Genitourinary. Bowel incontinence

Growth disturbances.

Endocrine. Most MMC kids are short. Treatment with growth hormone has been attempted with some success. They are euthyroid.

III. Abnormalities Due to Incomplete DysjunctionDermal Sinus Tracts, Dermoid, and Epidermoid Tumors

Definition

• Dysjunction is the separation of the neural tube from the overlying ectoderm.

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• A dermal sinus is a tract of cutaneous ectoderm which extends to varying degrees through the underlying mesenchymal tissues, sometimes as far as the dura of the neural tube.

Pathogenesis

• Etiology is due to incomplete dysjunction, during week 3-5 of gestation.

• Other theories are invoked.

1. Persistent caudal neuropore.2. Ligamentum caudale.3. Terminal cysts of the filum terminale.4. Cutaneous ectoderm along course of filum.5. Iatrogenic.

By Location

Misplaced cutaneous ectodermal elements. Midline lesions (cutaneous anlage entrapped). Lateral lesions (incomplete separation ectoderm from mesoderm). Caudal lesions (remnant of caudal neuropore).

Pathology of Dermal Sinus Tract

• Pathophysiology is tethering of the spinal cord, or neural element compression.

• Gross. About 60% end in a epidermoid or dermoid tumor, since ectodermal, or ectodermal and mesodermal elements can be trapped along the tract. 30% of dermoid or epidermoid tumors are associated with a sinus tract.

• May occur at any level of the neuraxis. Have a predilection for the lumbosacral region (70%; caudal neuropore site) and frontobasal region (rostral neuropore site).

• In the spine, usually penetrate between two laminae, or between bifid laminae. Usually enter dura right beneath the cutaneous lesion, from which point they can extend cephalad as far as the conus. (since dura doesn't migrate, but the cord does). The dermoid or epidermoid tumors may occur anywhere along its length (i.e; intradural or extradural).

• Cranial dermal sinuses arise either from the frontobasal skull, or the occipital squama.

• A nasal dermal sinus tract arises from the anterior lip of the anterior neuropore at the commissural plate and connects the cranial neural tube with the midline nasal skin either at the glabella, the site of juncture between the frontal and nasal bones (funticulus nasofrontalis), or anywhere along the bridge of the nose. It is the result of a failure to regress/obliterate of a dural tract which connected these structures during embryogenesis. Remenants of the tract in the adult skull are the foramen cecum. The tract may extend intracranially to variable degrees.

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Dermoid tumors may occur anywhere along it. Sequestered neuroepithelium along the tract in the prenasal space may give rise to "nasal glimas".

• Posterior parietal dermal sinus arises from the caudal lip of the anterior neuropore. Goes from the region of the quadrigeminal cistern, passes posteriorly around the splenium of the corpus callosum, and through the two leaves of the falx to end in the parietal midline.

• Occipital dermal sinus arises from the roof of the fourth ventricle, the cerebellar vermis, or the subdural space posterior to the cerebellum and penetrates dura to end up in the occipital epithelium.

Epidermoids

• Gross. Very thin, transluscent capsule. Pearly cysts full of cottage cheese-like debris. In the skull, a lamina of dense bone (sclerotic rim seen on X-ray) surrounds the lesion.

• Microscopic. Cyst lining composed of keratinizing squamous epithelium. Contents are full of keratinaceous debris.

Dermoids

• Gross. Possess a firm fibrous capsule which contain hair, sebaceous secretion and squamous debris.

• Microscopic. Capsules, in addition to squamous epithelium, have coexistance of adnexal tissue (sebaceous eecrine, apocrine glands) and hair follicles.

Clinical Features

• Cutaneous anomalies are evident such as a skin dimple, hair tuft, cutaneous nevi or hemangiomas. A large or small opening may be present. Differential diagnosis is an innocent coccygeal dimple. Dermal sinuses always arise higher, cranial to the intergluteal cleft. Must differentiate from pilonidal sinus tracts, which arise closer to the anus.

• Complications include meningitis, spinal cord abscess (mass effect), spinal cord tethering, arachnoiditis.

Imaging

• Plain films may demonstrate a bony defect. Superseded by high resolution CT.

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• Myelography demonstrates a mass lesion in cauda equina, block, arachnoiditis, or intramedullary lesion.

• MRI. Both dermoids and epidermoids appear isodense with CSF and are invaluble for defining intracranial anatomy.

Treatment and Results

• Principle is to excise lesion before it causes infection. Surgical excision is a cure.

IV. Abnormalities Due to Premature Dysjunction

Spinal lipomas

Definition

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• The lipomatous content of a spina bifida lesion.

Epidemiology.

Probably the most frequent occult dysraphic malformation. F = M.

Classification

1. Intradural lipomas. 4% of total, surrounded by pia within the dural sac. Are ensconced within the cord. Spinal canal is modestly altered with bifida anomalies.2. Lipomyelomeningoceles. Represent 84% of total. Associated with dural defect, with the lipoma spanning from the spinal cord to (in varying degrees) the skin. The subjascent spinal cord is cleft, and resembles the placode of an MMC. The cord is usually tethered. Basically, it's an MMC with a lipoma.3. Lipomas of filum are incidental findings in 4-6% of normal adults. Some fat is found in the filum of 90% of tethered cord patients and in 20% of asymptomatic adults.

Pathology

• General pathological characteristics (they may conatin skeletal muscle, cartilage, bone etc.).

1. Broad indistinct interface with the conus medullaris.2. Substantial congenital posterior dural defect.3. Contiguous subcutaneous component.

• Etiology.

Thought to arise from premature separation of the neuroepithelium from the cutaneous ectoderm. The surrounding mesenchyme thus has access to the ependymal surface of the open neural tube.

• Pathophysiology. Symptoms are due to compression of surrounding elements or due to associated abnormalities.

• Gross. Most frequently involve the lumbosacral spinal cord, conus, and filum terminale. Some are exclusively intrathecal. Most have both intrathecal and extradural components, so that the thecal space is contiguous with a subcutaneous mass through bifid laminae, and a dural defect. The spinal cord is usually dysraphic, with the lipoma arising from the dorsal surface of the cord, medial to the dorsal root entry zones.

• Microscopic. Composed of mature adipose tissue, without any atypical features. Occasional islands of n. rootlets or meninges may be seen, in addition to fibrous tissue, muscle, cartilage, bone, vascular tissue. May be regarded as a mesenchymal hamartoma.

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A T1-weighted saggital MRI demonstrating a low-lying spinal cord that blends into an intraspinal lipoma through which nerve roots often traverse.

Variants

• Three types exist (dorsal, terminal, and transitional).

1. Dorsal

• Large dural defect.• Lipoma has a subcutaneous component which copnnect with the spinal canal through a spina bfida.• Lipoma/conus interface has no identifiable cleavage plane and the conus is tethered posteriorly.• The posterior roots emerge from the conus just ventral to the plane of fusion between lipoma, dura, and conus.• On MRI, the lipoma is typically tethered to the posterior dura with a capacious thecal sac more distally.

2. Terminal

• Lipoma is attached predominatly to the caudal rather than the dorsal apect of the conus. Less commonly may have a subcutaneous component. Extends for a variable distance into the thecal cul de sac before exiting through a dural defect.• Often a non-descript skin lesion such as a sacral pit.• Caudal nerve roots traverse the lipoma within its ventral portion.• Untethering these lesions relatively more simple as it is analogous to dividing a thick filum.• On MRI, present the picture of a more attenuated elongated conus expanding into a terminal lipoma which obliterates the cul de sac.

3. Transitional

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• A lesion that incorporates features of both. The more rostral portion generally has features of the dorsal type while the caudal aspects of the lesion tend take on the caudal type of lipoma.• One proceeds with surgical resection from rostral to caudal.

Clinical Features

• Asymptomatic, or subcutaneous lump. May have symptoms related to associated abnormalities such as tethering of cord or neural element compression. Include motor and sensory deficits, reflex changes, gait abnormalities, bowel/bladderproblems, musculoskeletal problems.

• Spinal lipomas are thought to produce progressive problems, starting at any age. Therefore, prophylactic surgery of these lesions is recommended by most American centers. French neurosurgeons, however, are much mroe conservative in their management of these lesions.

Treatment and Results

• Surgical. Aim is to stop symptom progression.

V. Anomalies due to Disorders of GastrulationA) Split Cord Malformations (SCM)

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Definitions

• Combined spina bifida implies anterior and posterior spina bifida.

• Anterior meningocele (or anterior spina bifida). Usually consists of a pelvic mass containing usually just CSF. Usually in lumbosacral area.

• Diastematomyelia. Spinal cord is split into two hemicords, each having one set of dorsal and ventral nerve roots. Each has its own dural sheath. They are sepatated by fibrous, bony, or cartilaginous septae. Associated with bony abnormalities such as split or fused vertebrae.

• Diplomyelia. Complete duplication of the spinal cord, each having two sets of ventral and dorsal nerve roots. The two cords have a common dural seath with no intervening structures.

Classification

Type I SCM (distematomyelia)Type II SCM (diplomyelia)

Epidemiology

• F:M :: 3:1.

Pathology

• Etiology suggested by four theories which all account for split or duplicated notochords.

1. Splitting of the notochord during elongation due to adhesions between the epiblast and hypoblast.2. Formation of an accessory neurenteric canal by a dorsal herniation of endoderm causing splitting of the notochord.3. Duplication of the notochord as a primary abnormality.4. Disordered gastrulation: Abnormally wide primitive streak during gastrulation causing formation of two notochords.

• Gross. 70% in lumbar or thoracolumbar area. Features of diastematomyelia and diplomyelia as in definitions. There is likely significant overlap between the two, as a common embryonic origin is postulated, i.e. diastematomyelia is an incomplete cord duplication. Split cord malformations (SMCs) have a variety of associated anomalies including combined spina bifida, myelomeningoceles, neurenteric cysts.

• The hemicords usually reunite above and below the split. The conus is usually low.

• The septum in diastematomyelia is usually the cause of the tethering. However, additional tethering may occur from filum thickening, tumor, or tethering nerve roots.

Clinical Features

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• Associated features include cutaneous changes (see below), segmentation abnormalities with hemivertebra, butterfly vertebra, block vertebra, in about 85%. Kyphoscoliois in 50-60%, typically as a result of the former.

1. Cutaneous Stigmata

Hypertrichosis (a hairy patch with triangular outline is specific forDiastematomyelia ("horse's mane")

Nevus Dimples Dermal sinus tract Capillary hemangioma Subcutaneous lipoma

2. Orthopaedic

Progressive back Leg pain and/or weakness Foot deformity, (neurogenic pes cavus) Scoliosis

3. Neurologic Changes

Weakness Atrophy Gait disturbance Sensory deficit Reflex changes (Absence of Ankle Jerk) Neurogenic Bowel/Bladder, Incontinence (Only One Leg Affected In 85%)

Imaging

• Plain films, CT/myelogram, and MRI.

• Radiologic findings:

Level of septum L1-L4.Broadened spinal canal.Spina bifida.Vertical laminar fusion.Fused vertebral bodies, split vertebral bodies.Hemivertebral hypoplasia.Fused or deformed spinous processes.Scoliosis.

Treatment and Results

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• Medical. Many are asymptomatic nd need conservative, expectant management.

• Surgical indications are for, i) progressive neurological deficit, and ii) as a preliminary to surgical correction for scoliosis.

• Goals are to remove septum flush with vertebral bodies, untether the cords laterally, and reconstitute dura if possible.

• Key points. Epidural plexus surrounding the bony spur deep to the two hemicords is substantial. Complete removal of the septum is necessary for successful untethring. filum should be divided if it is thickened and if this can be done without significantly enlarging the exposure.

• Results:

Pain relief is very good.Neurological improvement is usually modest.Need longterm follow up to assess for retethering.

B) Neurenteric Cysts

Definition

• A congenital lesion of foregut origin, most frequently seen as a right mediastinal mass in a child with respiratory or swallowing difficulties. It may be accompanied by vertebral and neural anomalies, and may demonstrate spinal cord compression secondarily.

Embryology

• Neurenteric cysts believed to be result of split notochord syndrome. Primitive streak forms a groove in the dorsal aspect of the embryo's ectodermal layer. At the anterior end of the primitive streak, Hensen's node appears. From here, cells forming the notochord begin their cephalad migration between the bilaminar disk composed of ectoderm dorsally and endoderm ventrally. As the notochord develops, it becomes adherent to the underlying endoderm. The notochord unfolds forming the notochordal plate and the roof of the archenteron by intercalation. Adjacent to Hensen's node, a channel forms called the neurenteric canal. This is usually a transient structure. As the embryo grows axially, Hensen's node and the canal are displaced caudally. The notochordal plate then separates from the endoderm by excalation and the tubular notochord is reformed and the neurenteric canal disappears. This process allows the paraxial mesoderm on either side of the notochord to form the sclerotomes which are the precursors of the vertebrae. The notochord induces neural tube closure.

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Etiology

1. Feller and Sternberg (1929). Group of Hensen's node cells become separated from rest of cells and interferes with normal development of the notochord and neural tube.2. Saunders (1943). A double or cleft notochord as the cause of these anomalies. Dorsal herniation of the endoderm causing all the abnormalities.3. Bremer (1952). Accessory neurenteric canal.4. Fallon, Gordon, Lendrum (1954). Traction diverticulum forms as the notochordal plate separates from the endoderm preventing fusion of apposing sclerotomes and normal vertebral body formation.

Pathology

• Gross. Clear to milky white fluid. May have previous hemorrhage and is frequently adherent to ventral spinal cord.

• Microscopic. Low cuboidal, ciliated, non-ciliated, gastric mucosa type epithelium which stains positively for PAS. A basement membrane is always present. May see glandular elements, ganglion cells, lymphoid tissue, and muscle. Occasionally see pancreatic tissue.

Pathological Variants

1. Fistula. A patent communication of the intestinal tract with the skin on the back traversing a DSM or spina bifida.2. Sinus. A persistence of the dorsal portion of a fistula with or without posterior or combined spina bifida.3. Posterior enteric diverticulum. Persistence of the ventral portion of the fistula.4. Posterior enteric cysts. Includes prevertebral (mediastinal), intraspinal, and post-vertebral cysts with or without spina bifida.

Clinical Features

1. Peristaltic or respiratory activity of tissue on the back in severe cases.2. Compression of the esophagus and tracheobronchial tree.3. Usually all found before age 5 years.4. 80% male preponderance.5. Pain localized to the cyst at whatever level.6. Spinal cord compression.

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Imaging

• Plain films reveal widened interpedicular distance, fused vertebral bodies, hemivertebra, etc.

• CT Myelogram. Smooth cyst causing partial or complete block.

Treatment

• Complete removal is the procedure of choice. Residual cyst can cause chemical meningitis.

 

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VI. Abnormalities due to Disordered Secondary Neurulation

A) Myelocystocele

• Definition. Rare occult dysraphic lesions in which the central canal of the caudal spinal cord expands to become a CSF containing glial or ependymal lined terminal cyst. Rare lesions.

• Etiology. Secondary neurulation is the stage of neurulation which starts after disjunction. i.e., when the embryo is covered with cutaneous ectoderm. The resultant abnormalities are always skin-covered. May evolve secondary to embryonic hydromyelia, which produces a posterior trumpeting of the spinal cord and spinal fluid through a bifid spine. The dilated terminal cord is tethered in the sacral area and is surrounded by an ectatic dural sleeve. An associated lipoma is almost universal.

• Clinical features. In the absence of release of tethering, progressive neurological deficit occurs. Other associated malformations are frequent (imperforate anus, ambiguous genitalia, vertebral segmetation anomalies). Hydrocephalus is extremely uncommon.

• MRI is the test of choice.

• Treatment is surgical excision.

B) Thickened Filum

• Definition. The filum is short and thickened, >2 mm, and tethers the conus at an abnormally caudal position.

• Etiology. May represent the least serious consequence of premature dysjunction.

• Gross pathology. Is usually shortened, thickened (>2 mm), and frequently inflitrated with adipose tissue. Always associated with at least spina bifida occulta. 50% have some cutaneous marker. Tip of conus is below L2. Fibrolipomas may be present in up to 90%.

• Clinical features. Weakness (76%), pain (42%), bladder dysfunction (35%), and sensory loss (21%).

• Indications for treatment are based on the presence of clinical symptoms.

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VII. Abnormalities due to Failure of Caudal Neuraxial Development

Caudal (Sacral) Agenesis

Definition

• Caudal agenesis is a group of caudal malformations characterized by partial or complete absence of a variable number of lumbar or sacral vertebrae, together with corresponding areas of the neural tube.

• The exact etiology is unknown. May result from an insult to the caudal mesoderm before the 4th gestational week. Vascular hypoperfusion favored in sirenomelia - single large artery diverts blood flow from caudal portion of the embryo to placenta.

• 15% have diabetic mothers - insulin, hyperglycemica, vs. ketone bodies.

• Genetic - several familial cases described.

Pathology

• Gross. Complex vertebral anomalies such as absence of a vertebra, hemivertebrae, wedge vertebrae, fused vertebrae, sacralization of lumbar vertebrae, spina bifida. The distal cord is absent. 50% have associated myelomeningoceles. Has associated limb, visceral (imperforate anus, malformed genitalia) and urogenital abnormalities. Unilateral agenesis produces severe scoliosis. Sirenomelia, or "mermaid syndrome", is a congenital fusion of both lower limbs as a severe consequence of caudal agenesis.

• Microscopic. Affected muscle show a virtual absence of myocytes with lots of fibrous tissue.

Clinical Features

• Flattened buttocks, short intergluteal cleft, prominent ileac crests. Motor deficits correspond to the level of agenesis. Sensory sparing occurs, probably due to preservation of neural crest cells.

• Most have neurogenic bladder; the most severely affected also have anal atresia and GU anomalies.

Imaging Features

• Prenatal ultrasound is usually diagnostic in the third trimester.

• Plain films demonstrate bony hypogenesis.

• MRI i) caudal vertebral agenesis ii) characteristic saber-shaped, blunted distal cord.

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Treatment and Results

• Operative treatment is frequently necessary if the cord is tethered. Manangement of associated abnormalities also needed.

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VIII. Abnormalities due to Disordered Postneurulation Development

Encephaloceles

Definition

• A herniation of meningeal and CNS tissue through a defect in the skull.

Classification

Basal

Sphenopharyngeal. Through spenoid into epipharynx. Intranasal. Through cribriform plate into nasal cavity. Spheno-orbital. Through superior orbital fissure into orbit. Sphenomaxillary. Throughinferior orbital fissure into pterygopalatine fossa.

Sincipital

Nasofrontal. Between nasal and frontal bones (at glabella). Fronto-ethmoidal. Between frontal, nasal and ethmoidal bone. Naso-orbital. Between frontal, lacrimal andethmoidal bone.

Convexity (most common group)

Frontal. Parietal (saggital). Least common type. Occipital. Most common type. Occipito-cervical.

Atretic

Alopecic (parietal). Nodular (occipital).

Epidemiology

• 1/5000 live births worldwide. Occipital encephaloceles more common in females, anterior in males. Occipital encephaloceles most common in North America.

Pathology

• Initially thought to be a neurulation defect. However, the CNS tissue is too well developed. Thus, the present theory is that a defect in mesenchyme formation, with the concomitant explosive growth of CNS tissue, causes fully neurulated, often well organized neural tissue to

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herniate through a defect in the anterior basal or occipital mesenchyme. Teratogens may be involved: Sodium arsenate, clofibrate, vitamin A, Trypan blue, irradiation.

• Gross. Saccular outpouchings of variable size, most commonly in the occipital region. The protuberance is encircled by a collar of hair-bearing skin which gives way to translucent epidermis or fibrous tissue.

Occipital encephalocele. Bony defect is posterior to the posterior fontanelle, may extend from occipital squame as far back as the foramen magnum and may involve the posterior arch of C1. Skull base is deformed, with small anterior and middle fossae, and large posterior fossa. The mass varies in size from tiney to huge. The brain tissue is usually an asymmetric herniation of a hemisphere. The sac may contain protions of cerebellum brainstem, and vascular structures. The dural sinuses are usually not in the encephalocele, which may split them.

• Microscopic. Meningocele consists of loose firovascular tissue. The encephalocele consists of cerebral tissue, usually disorganized glioneural nests surrounded by collagenous septae. However, the cerebral cortex may be indistinguishable from normal tissue, or it may contain areas of fibrosis, ischemia, or haemorrhage.

Clinical Features

• Basal encephaloceles usually diagnosed delayed, when a nasal or glabellar mass forms, or when hypertelorism is being corrected. Deficient olfaction should be documented.

• Occipital encephalocele may be diagnosed prenatally with AFP and/or ultrasound. It is immediately apparent at birth.

Imaging

• Plain X-rays show bony defect in occipital encephalocele. CT ± 3D reconstruction is useful for fronto-ethmoidal/ basal defects. CT and MRI will show the contents of the encephalocele. MRI useful to delineate associated CNS anomalies (i.e. porencephaly).

Treatment

• Basal and sincipital lesions carry excellent prognosis and need to be treated for reasons of 1. cosmesis 2. function (compressive effect on eye/nose contents) 3. prevention of infection.

• Repair defect via an intra- and extradural anterior fossa approach. Transcect the stalk intradurally and use fascial patch to prevent CSF leak. ENT/plastic surgery then required to perform the facial component, dissecting the stalk to the nose.

• Prognosis with occipital encephaloceles is very difficult to determine. Thus, all children need operative repair for reasons of.

1. Cosmesis, ability to nurse child.2. Prevention of eventual sac rupture and CSF leak.3. Decrease irritability due to exposed dura.4. Prevent progressive hydrocephalus.

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• Repair done by incising lesion circumferentially, leaving enough skin to close defect. Release CSF, explore contents, excise herniated tissue flush with plane of skull, reconstruct dura, trim skin edges and close, usually transversely. If hydrocephalus occurs, a CSF shunt is necessary. Cranioplasty may be needed later in life.

Results (Occipital)

• 60-70% require shunts. Presence of hydrocephalus is not a factor in quality of outcome if it is treated promptly. 20% have microcephaly, a poor prognostic sign for later intellectual function. Most improtant factor is presence or absence of brain tissue in the encephalocele. If brain is not present (meningocele), 60-80% are normal. If brain is present, 10% are normal.

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IX. Secondarily Acquired CNS Abnormalities due to Myelodysplasia

Chiari II Malformation

Definition

• A complex disorder encompassing the whole neuraxis. most prominent is the caudal displacement of the cerebellar tonsils and vermis. Also, elongation, kinking and caudal displacement of the brainstem below foramen magnum, and superior displacement of cerebellum through a hypoplastic incisura.

• Epidemiology. All patients have a myelomeningocele.

History

• Cleland (1883) describes general pathological changes of one type of these anomalies.

• Chiari (1891-1896) reported a number of anomalies in cases, and identified 4 types of cerebellar abnormalities. Type I described was limited to cerebellar displacement. Type II described as displacement of the part of the inferior vermis, displacement of the pons and displacement of the medulla into the cervical canal and elongation of the IVth ventricle into the cervical canal.

• Arnold (1900-1907) describes single case of type II disorder. Schwalbe and Gredig coin term Arnold-Chiari malformation.

Embryology

• The anterior neuropore closes at about day 24; the posterior neuropore closes 2-5 days later. As the neural tube closes, three flexures form shaping the cephalic end of the tube. First is the cephalic flexure, placing the forebrain at a right angle to the hindbrain. Then the cervical flexure forms. The pontine flexure forms last (about day 29) and is probably caused by increased growth within the rhombencephalon. Because of the pontine flexure formation, the thin roof of the rhombencephalon becomes creased transversely, and this fold is known as the plica chorioidea. It contains the choroid plexus that will come in direct contact with the ependymal lining, and ultimately correlate with the foramina of Luschka. All of the cerebellum will develop rostral to this line and come to overlie the posterior portion of the ventricular roof as a result of later explosive growth.

• During the following week, there is rapid neuroblastic proliferation of the alar plate rostral to the pontine flexure, forming the rhombic lip. The paired cerebellar primordia are pushed upward and medially, meeting in the midline and starting their fusion at about the third month of gestation. The nodulus and the choroid remain caudal to the developing cerebellar hemispheres.

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Structural Features

Cerebrum. Polymicrogyria, cortical heterotopias, enlarged massa intermedia, hydrocephalus (85%), and agenesis of the corpus callosum (partial in 80%, complete in 33%).Meninges. Hypoplasia of falx, low set tentorium with with enlarged incisura, and low torcula and lateral sinuses.Cerebellum. Caudal displacement of vermis and fourth ventricle, cerebellum smaller than normal, and fourth ventricular choroid plexus may reside ouside of the ventricle.Midbrain. Tectal "beak" (beaked colliculi) in 75%, and aqueduct stenosis and forking.Brainstem. Caudal displacement of pons and medulla, kinking of the cervicomedullary junction (70%), displaced basilar and vertebral arteries, stretching of lower cranial nerves, and hypoplasia of cranial nerve nuclei and medullary olives.Spinal cord. Compression of upper segements and associated with hydrosyringomyelia (80-90%). Course of the upper cervical nerve roots is up or horizontal.Skull and Spine. Craniolacunia (Luckenschadel deformity) which are multiple punched out areas measuring up to 1 cm. Disappears at 6 months. Enlarged foramen magnum, small posterior fossa, scalloped petrous bone, and enlarged upper cervical canal.Systemic. May be associated with cardiac defects (transposition of great vessels), GU, and GI malformations.

Pathology

• Etiology accounted by five types of theories.

1. Dysgenesis/developmental arrest theories. Primary dysgenesis of neuraxis, failure of pontine flexure to form. This lack of flexion causes elongation of the hind brain.2. Hydrocephalus/hydrodynamic theories. Fetal hydrocephalus displaces posterior fossa contents caudally.3. Traction theories. Taction of the caudal spinal cord by a myelomeningocele may pull hindbrain caudally (of historical interest).4. Small posterior fossa/overgrowth theory. Postulates a primary disorder of the paraxial mesoderm resulting in a small posterior fossa which cannot accomodate its contents.5. CSF escape theory. CSF escape through an open caudal placode interferes with proper ventricular enlargement which eventually produces multiple CNS and posterior fossa bony abnormalities.(Mnemonic is "THODE"; Traction, Hydrocephalus/hydrodynamic, Overgrowth, Dysgenesis, and Escape).

• Three frequently quoted theories are (although best for syringomyelia).

1. Hydrodynamic theory (Gardner). Physiologic outlets of IV ventricle (foramina of Luschka and Magendie) are delayed in their development due to indadequate or late rupture of rhombic roof. Pulsatile CSF flow in fourth ventricle is obstructed from its natural egress, and the pressure is dissipated through the obex into the central canal.

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The contents of the post-fossa are forced downward by this, and by the hydrocephalus which forms.

2. Differential pressure theory (Williams). Differential pressures between the intracranial and intraspinal compartments, especially as a result of Valsalva manouvers are normally dissipated. However, arachnoid adhesions and/or tissue in the foramen magnum area impede CSF flow and pressure gradient dissipation. An alternate path of decompression produces syrinx cavities and progressive caudal displacement of tissue.

3. Systolic pressure theory (Oldfield 1994). As brain fills with blood during systole, a CSF pressure wave is formed, and is normally accomodated by normal subjects by dissipation into the high cervical area. If CSF flow is plugged at the foramen magnum, the cerebellar tonsils move downward like pistons, producing a pressure wave in the spinal compartment which acts on the surface of the cord to enlarge a syrinx by pressing on it (causing longitudinal dissection) and/or forcing CSF through the perivascular and interstitial spaces.

Clinical Features

• Almost always associated with a myelomeningocele. Usually symptomatic in infancy. Between 5 and 33% of patients with MMC will have symptoms from their Chiari II in infancy. The majority will have their symptoms stabilize. However, some could have life threatening apneic episodes, aspirations, etc. Symptomatic Chiari II is the leading cause of death in treated MMC in the first two years of life.

• Signs and symptoms.

Infancy

Inspiratory stridor (cranial nerve X paresis)Episodic apneaDepressed gagNystagmusFixed retrocollisWeak crySpastic upper extremity weaknessPeripheral cranial nerve VII palsy

Childhood

NystagmusSpastic quadriparesisAtaxiaRecurrent aspiration pneumoniaDepressed coug

Adolescence

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Upper and lower extremity spasticity and weaknessSuspended dissociated sensory loss (syrinx)AtaxiaScoliosisHand atrophy (syrinx)Cranial nerve XII palsy

Comparison of Chiari Malformations

Feature Type I Type IIAge Adult Infant

Hydrocephalus Occasional Over 90%Brainstem Compression Signs Seldom May be acute

Caudal Displacement of Cerebellar Tonsils Yes YesCaudal displacement of Inferior Vermis and

IVth VentricleNo Yes

Caudal displacement of Medulla Oblongata No YesDorsal kink of Cervico-Medullary Junction No Yes

Course of Upper Cervical Roots Normal CephaladSpina Bifida Absent Present

Relief by Shunting Occasional UsualPosterior Fossa Decompression Needed Occasional

Decompression Elective Can be urgent

Imaging

• Plain films show a variety of features.

Wide upper cervical canal. Small posterior fossa. Calvarial lacunar pattern. Segmentation disorders, bony fusion abnormalities. Basilar impression, invagination. Scalloping of the petrous pyramids and clivus. Luckenshadel pattern (thinning of calvarial bone) in 80% of cases.

• CT Myelogram. Tonsillar descent.

• MRI shows cerebellar abnormality, medullary kink, and syringomyelia.

• Angiography. Descent of the inferior loop of PICA.

MRI of entire neuraxis, ENT and Opth evaluation of lower cranial nerve function, & usual evaluation related to the MMC.

Treatment and Results

• Treatment of hydrocephalus takes precedence.

• "Top-down" approach.

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Treat hydrocephalus or revise shunt.Posterior fossa decompression with or without laminectomy.

• Surgical treatment of symptomatic Chiari II is controversial because it is uncertain whether the symptoms are due to compression, or due to the primary myelodysplasia of the CNS.

• Following laminectomy, dural opening is to below the tip of the cerebellar tongue and the medullary kink. NOT above level of foramen magnum.

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X. Tethered Spinal CordDefinition

• A condition in which the spinal cord is fastened to an immovable structure, such as a lipoma, vertebra, dura, or skin. Always associated with at least spina bifida occulta and other causes below.

Etiology

Primary Causes

Neural tube defects: MMC (posterior, lateral, anterior) Dermal sinus and associated dermoid and epidermoid tumors Intraspinal tumors Spinal lipomas Combined spina bifida, Split cord (diastematomyelia, diplomyelia) Neurenteric cysts Thickened filum, myelocystocoele Caudal Agenesis

Secondary Causes

Arachnoiditis Dermoid Trauma Suture granuloma Spinal tumors "Re-tethering"

Pathology

• Pathogenesis. The filum is formed by secondary neurulation by canalization caudal to posterior neuropore. The caudal cell mass involutes, leaving the filum. Failure of the caudal cell mass to involute produces a number of secondary neurulation anomalies, including thickened filum and myelocystocele which tether the cord.

• Pathophysiology. Vertebral structure that move as a result of growth, daily activity, or pathologic skeletal change stretch the spinal cord, producing abnormal tension. Neuronal, axonal and vascular changes ensue. Spinal cord blood flow is decreased in the levels immediately rostral to the tether. Measurements of mitochondrial redox state show alterations in the tethered cord. Secondarily, structures supplied by the tethered cord are also altered. The result is muscular weakness, bowel and bladder difficulties, sensory deficits and skeletal deformity.

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• Gross. Cord is stretched, decreased in diameter, pale, with absent pulsations. Nerve roots leave the cord at a 90 degree angle. The dorsal vasculature is stretched and linear with decreased vessel diameter. Radicular arteries also appear stretched.

Clinical Features

• Majority in pediatric age groups. Minority in adults.

• Changes are in areas subserved by lumbosacral cord and nerve roots.

• Cutaneous lesions in 50%.

Dimple Hypertrichosis Subcutaneous fat lipoma Dermal sinus tract Naevus Capillary hemangioma Tuft of hair

• Presentation is usually orthopaedic, neurological, or urological.

1. Orthopaedic. Scoliosis (convex curvatures to left in males), back stiffness, cavo-varus foot, cock-up toe, postural changes, valgus or varus ankle deformities, leg length discrepancy, congenital hip dislocation.2. Neurological. A mixture of upper and lower motor neuron signs, often asymmetrical. Lower extremity fatigue, pain, muscular weakness, muscle wasting, absent ankle reflexes, and sensory changes. Gait disturbances are common.3. Urological. Enuresis, stress incontinence, and frequent UTI's.

• Symptoms in order of prevalence are weakness (76%), pain (42%), bladder dysfunction (35%), sensory loss (21%).

• Differences exist between children and adults. Pain common in adults (80%), rare in children. Cutaneous stigmata common in children (80%), rare in adults. Scoliosis common in children, rare in adults.

Laboratory and Imaging

• SSEPs may show decreased amplitude and increased response latencies. EMG's demonstrate denervation of affected muscles.

• Urodynamics demonstrate a neurogenic bladder, detrusor-sphincter dysinergia.

• Spine X-rays. Spina bifida always present. Scoliosis easily demonstrated.

• MRI shows conus to be invariably at L2 or below. Cause of tethering almost always seen.

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Treatment and Results

• Counseling of patient and parents.

• Appropriate care of neurogenic bladder.

• Medical treatment of associated conditions such as spasticity and neurogenic pain.

• Surgery to relieve tethering.

• Rehabilitation.

• Orthopaedic referral.

• Surgical indications are to stop progression of CNS, MSK and GU disability. Early surgery is best. Thus should do as soon as patient is diagnosed.

• Pain stops in 80-90% of patients. Sensory and motor symptoms only improve if of short duration. Urologic dysfunction rarely improves.

 

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