Diagnosis of Lateralized Lumbosacral DiskHerniation with Magnetic Resonance Imaging

Jonathan N. Chambers, DVM

Barbara A. Selcer, DVM

Stacey A. Sullivan, DVM

Joan R. Coates, DVM, MS

From the Departments ofSmall Animal Medicine (Chambers,

Sullivan, Coates) andAnatomy and Radiology (Selcer),

College of Veterinary Medicine,The University of Georgia,

Athens, Georgia 30602.

A left-lateralized, lumbosacral intervertebral disk herniation, which was not apparenton epidurography, was diagnosed in a dog with magnetic resonance imaging.Precise, preoperative localization and characterization of the lesion allowed surgicalapproach and excision with minimum disruption of surrounding tissues.J Am Anim Hosp Assoc 1997;33:296–9.

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IntroductionThe use of continuously improved spinal imaging techniques hasallowed more precision in the preoperative localization and charac-terization of surgical lesions. The once ritualistic and often excessive“exploratory” surgery to confirm the presence, nature, and extent of alesion rarely is necessary, and thus treatment successes have in-creased and morbidity has decreased as patients have benefited fromless normal tissue disruption.1

Epidurography has proven to be a sensitive test for the typical,ventral midline, lumbosacral degenerative disk herniations seen inlarge-breed dogs.2 When combined with a consistent history, physicalsigns, and electromyogram, the positive epidurogram usually is suffi-cient to make a decision regarding the indication for surgery. Addi-tional corroborative imaging procedures are indicated only if thediagnosis is equivocal or if further information may alter the treat-ment plan.3,4 Magnetic resonance imaging (MRI) and computed to-mography (CT) have revolutionized the characterization andmanagement of lumbosacral disease in humans. The appearances ofthe normal and the degenerated lumbosacral spines of dogs usingthese techniques have been described recently.4–7 Reported here is acase where imaging beyond epidurography was required forpresurgical localization and distinction.

Case ReportA six-year-old, female Labrador retriever was admitted to the Univer-sity of Georgia Veterinary Teaching Hospital with a history of aspontaneously appearing dysfunction of the left pelvic limb that hadbeen progressive over five weeks. The problem first was noted as areluctance and then a refusal to jump. The dog would use the affectedlimb with a noticeable limp while walking and running, but wouldhold the paw slightly off the ground when standing. The owner be-lieved that the limb lacked normal strength and coordination and thatthe dog occasionally exhibited pain when touched in the pelvic region.

Upon physical examination, the off-weight bearing standing pos-ture of the left pelvic limb was as the owner had described. Thewalking gait was abnormal with deficient hock flexion during theswing phase of the limb. The withdrawal reflex was weak and waspredominantly stifle flexion without hock flexion. The cranial tibialreflex was markedly weaker compared to the opposite side, but thequadriceps reflex was normal. There was moderate, generalized atro-phy in the limb, including the muscles surrounding the hip. Hyperes-

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thesia was noted when pressure was applied over thelumbosacral junction. The remainder of the physicalexamination was normal. A tentative diagnosis of apredominately left-sided, seventh lumbar (L7) to firstsciatic (S1) nerve root lesion was made.

Routine laboratory tests including a complete bloodcount (CBC), serum chemistry profile, and urinalysiswere normal. The dog was placed under general anes-thesia for electrodiagnostics and an epidurogram. Ab-normal spontaneous activity was identified withelectromyography, predominantly in the muscles inner-vated by the left sciatic nerve (i.e., interosseous, gas-trocnemius, cranial tibial, and biceps femoris muscles)and the muscles of the proximal tail. Other electrodiag-nostic tests consisting of tibial nerve conduction veloc-ity and repetitive nerve stimulation were normal.

A lateral survey radiograph [Figure 1A] was ob-tained prior to the epidurography. Epidurographicviews included neutral, flexed and extended laterals,and dorsoventral [Figures 1B–1E]. The radiographicstudy was considered inconclusive.

Figure 1A— Lateral radiograph of the lumbosacral (L-S) junctiondemonstrating sclerosis and smooth-to-irregular remodeling ofthe cranial endplate of the sacrum. Partial bridging spondylosisdeformans is seen ventrally.

Figure 1B— Lateral view of the lumbosacral (L-S) epidurogrammade with the L-S spine in neutral position. No definitivecompressive lesion is noted.

Figure 1C— Lateral view of the lumbosacral (L-S) epidurogrammade with pelvic flexion. No abnormalities are noted.

Figure 1D— Lateral view of the lumbosacral (L-S) epidurogrammade with pelvic extension. No abnormalities are noted.

Figure 1E— Dorsoventral view of the lumbosacral (L-S)epidurogram. There is slight narrowing of the epidural spaceover the L-S junction (arrow).

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The dog was reanesthetized one week later forMRI of the lumbosacral spine using a 0.5 Telsa su-perconducting scanner.a Slices (3-mm thick) were ob-tained with the dog in sternal recumbency with thepelvic limbs extended. T1b-weighted images (repeti-tion time [TR], 550 to 750 msec; echo delay time[TE], 25 msec) were obtained in sagittal [Figure 2A],axial [Figure 2B], and dorsal [Figure 2C] planes.Changes identified on the MRI study included loss ofthe normal T1 hypointense (i.e., uniform black) sig-nal from the intervertebral disk at L7-S1 and the pres-ence of a left-sided, hypointense signal massobscuring the normally hyperintense (i.e., brightwhite) signal from the fat-filled intervertebral fora-men at L7-S1 [Figure 2A]. The exiting nerve roots onthe left side of the intervertebral foramen at L7-S1were visualized poorly [Figure 2B]. Possible newbone formation or mineralized disk material was iden-

tified at the level of the left L7-S1 intervertebral canal[Figure 2C].

The lesion was approached surgically from the dor-sal aspect, which included excision of the interarcuateligament, limited midline laminectomy of L7 and S1,and near total left facetectomy. A spherical mass (5mm in diameter) protruded from the disk space intothe left L7-S1 intervertebral foramen. The mass con-sisted of a combination of herniated intervertebraldisk material and a large osteophyte from the marginof the L7 vertebral body. The L7 nerve root was com-pressed between the herniated disk and the L7 pedicle,and it was swollen. The compressive mass was ex-cised with rongeurs, curettes, and a pneumatic burr.

The dog recovered uneventfully, showed progres-sive improvement in limb function, and was withoutpain for the subsequent six months, at which time shewas readmitted for a left hemiparesis that subse-quently proved to be caused by an intradural nerve-sheath tumor located at the first-to-second cervicalregion. Electromyography at that time revealed per-sisting, abnormal spontaneous activity in the left

Figure 2A— T1b-weighted sagittal image of the lumbosacral(L-S) junction. The image plane is through the left intervertebralforamen. A hypointense (i.e., dark) signal mass is present withinthe foramen, obscuring the normal, uniform, hyperintense (i.e.,bright white) signal from the epidural fat.

Figure 2B— T1b-weighted axial image of the lumbosacral (L-S)junction at the level of the exiting nerve roots. The normallypresent hyperintense (i.e., white) signal from the epidural fatsurrounding the nerve roots exiting the left intervertebral fora-men is not visualized (L=left; R=right).

Figure 2C— T1b-weighted dorsal plane image of the lumbosa-cral (L-S) junction. The normally present hyperintense (i.e.,white) epidural fat surrounding the exiting nerve roots is notseen on the left side. Instead, a hypointense (i.e., dark) “masseffect” is present along the cranial margin of the sacrum,possibly protruding into the left intervertebral foramen andvertebral canal (arrow) (R=right; L=left).

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interosseous and proximal tail muscles, but no ab-normal activity in the other previously affectedmuscles.

DiscussionMagnetic resonance imaging and CT have specificroles of delineating lateralized lesions in dogs withcompressive lumbosacral disease.4,6 An entirely pre-sumptive diagnosis of lateralized disk herniationcould have been made in this case based on the his-tory, neurological signs, and electromyography, andthe definitive diagnosis could have been made at ex-ploratory surgery; but an epidural versus extraspinalplexus lesion could not be ruled out preoperatively.Similar signs could have been caused by a foreignbody reaction, penetrating injury, abscess, or neo-plasm.8 An extraspinal lesion most likely would bepoorly detected, characterized, and managed via adorsal midline surgical approach. The MRI allowedcharacterization and precise localization of the lesionsuch that a good prognosis could be offered, and theoperation was individualized to address only the spe-cific problem, with resultant reduction in surgicaltrauma.1

a MR MAX; General Electric, Milwaukee, WIb Transverse plane to longitudinal plane proton relaxation time

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