ocular torsion reveals the mechanisms of cyclovertical strabismus the weisenfeld lecture @ arvo 2007...

Ocular Torsion Reveals the Mechanisms of Cyclovertical

StrabismusThe Weisenfeld Lecture @ ARVO

2007Guyton DL

IOVS, March 2008, Vol 49 (3):847 - 857

Introduction

• Torsion was not well understood in 1970s

• This lecture is the chronology of understanding torsion and its implications for cyclovertical strabismus

Topics to be covered:

• Measurement of ocular torsion

• Apparent Oblique muscle over / underaction and ‘A’ and ‘V’ patterns

• Changes in strabismus over time

• Dissociated vertical deviation

• Congenital superior oblique paresis

Introduction

• Measurement of ocular torsion• Oblique muscle over / underaction and ‘A’ and

‘V’ patterns• Changes in strabismus over time• Dissociated vertical deviation• Congenital superior oblique paresis

Measurement of torsion

• Torsion cannot reliably be seen by external landmarks unlike H and V deviations

• Subjective torsion: double Maddox rods, Lancaster Red-Green Test [Hess Variant - uses a line, not a dot]

• Objective torsion: blind spot mapping, fundus photos, Indirect ophthalmoscopy

• In 1970s, abnormal fovea-disc relationship was called foveal ectopia; considered pathological, not a strabismic variant


• von Noorden: why does a torted eye not right itself on covering the other eye?..but the torted image subjectively straightens?

• On comparing subjective vs objective torsion significant adaptation was discovered


• Measurement of the blind spots on visual fields and fundus photos were impractical

• Developed a scheme for measurement of torsion on indirect ophthalmoscopic examination

Grading of torsion (LE, inverted view)

LK:1/3: trace or slight3/3: huge 2/3: in between

Introduction


• Oblique muscle over / underaction and ‘A’ and ‘V’ patterns




Oblique muscle over / underactions‘A’ and ‘V’ patterns

• Fundus extorsion correlates with ‘V’ and IOOA; intorsion with SOOA and ‘A’ [e.g. to follow]

• ‘A’ and ‘V’ patters can be explained by rotation of the planes of action of the recti [e.g. to follow]

• The amount of torsion correlates with the pattern and the abnormality of eye movements

BE IOOA

Fundus Xtorsion correlates with ‘V’ & IOOA

‘V’ pattern and Upshoot in adduction

Oblique muscle over / underactions‘A’ and ‘V’ patterns

• How did the torsion develop?

• How is it sustained?

Introduction






Changes in strabismus over time

• Lack of fusion predisposes to H, V and T

deviations • H: sensory exotropia [ET if hyperopic]• T :sensory torsion

• Are these progressive changes due to change in neurological tone or actual muscle length change?

Changes in strabismus over time

• Skeletal muscles known to adapt lengths by addition or removal of sarcomeres

• Shorter muscle: sarcomere density• Stimuli for length adaptation:

• Tension or lack of tension• Mechanical stretch or lack of stretch• Neurological stimulation : if increased, muscle shortens by

losing sarcomeres and and decreased stimulation muscle lengthens by adding sarcomeres

Modeling of Ocular Alignment Control System

• Each EOM has basic length and a basic density of sarcomeres and

• Each EOM receives resting level of stimulation (vergence tonus) → functional muscle length → aligned eyes


• A change happens ( new glasses/ orbital growth/ scar elongation etc) leads to misalignment → retinal image disparity → fast fusional vergence changes stimulation → new functional lengths to align eyes


Sustained fast fusional vergence → vergence adaptation →

adjustments in basic level of vergence tonus →

eases the burden on fast fusional vergence


• Change in vergence tonus → muscle length adaptation → initial neurological feedback mechanisms subside

Modelling of Ocular Alignment Control System

Breakdown of Ocular Alignment Control System

• Most common abnormality is absence of fast fusional vergence

• Neurological feedback continues at basal level, albeit without guidance

• New strabismus arises and progresses

• Cyclofusion loss leads to torsional misalignment

Modelling of Ocular Alignment Control System

ACCURATE FUSION RESPONSE IS LOST SENSORY XT

IF NO CONVERGENCE STIMULATION REQU’D, LESS CONV STIM PROGRESSIVE XT

Breakdown of Ocular Alignment

• [XT] who were overcorrected and lost fusion: 43 % developed ‘A’ or ‘V’ patterns

• [XT] who had fusion after surgery: <5 % developed ‘A’ or ‘V’ patterns

Introduction






Dissociated Vertical Deviation

• Scleral search coil technique – DVD is always associated with latent nystagmus

• DVD damps torsional component of MLN

• DVD is a learned, anticipatory response when switching fixation to one eye


• Which muscles are responsible for DVD?


• DVD can be broken down into vergence and version eye movements – both obeyed Hering’s law

• Horizontal component of LN is dampened by the convergence

• Cyclovertical component is dampened by the oblique muscles

Dissociated Vertical DeviationVertical Vergence response

• Previous research by Guyton had shown that the vertical fusional vergence is mediated by the oblique muscles

• Superior oblique in the fixing eye recruited to intort the eye and apply “brakes” on T MLN (consequent depression)

• Inferior oblique in the other eye forms the other part of vergence response and causes extorsion, elevation of other eye

Dissociated Vertical DeviationVertical Version response

• Vertical version is mediated by the superior recti of both eyes to elevate the fixing eye to primary position

• Fixing eye = SO (↓ + in ∩) + SR (↑ + in ∩)

= net intorsion

• DVD eye = IO (↑ + ex ∩) + SR (↑ + in ∩)

= net elevation


• Head tilt can be used alternately to damp LN

• DVD gets better when head tilt is used

• DVD frequently manifest when fine vision is required or when fusion is suspended

• DVD solved!

Introduction






Congenital SO Paresis

• Most common cyclovertical disturbance in practice

• Are all unexplained extorsion + hypertropia due to SO paresis?

• Where is the non-paretic analog of the basic esotropia in the cyclovertical plane?

• Demer JL – 50 % of clinical SO palsies had normal muscle thickness and contractility

Congenital SO ParesisMechanisms of Hypothetic “Basic Cyclovertical”

deviation

• Changes in strabismus are always bilateral


deviation

• EOMs do not adapt to their lengths to the position in which they are held neurologically

• FIGURE 12. A particularly telling patient, a 33-year-old woman with esotropia (crossed eyes) since birth. Her right eye, only, was operated for the esotropia at age 21⁄2, but the esotropia recurred (top). The right eye turned in again—or did it? She had fixed with her left eye only, as long as she could remember, because of mild hyperopia and amblyopia in her right eye. But the extraocular muscles of the two eyes had not adapted to these positions, because when she was placed under deep anesthesia (bottom), both eyes deviated rightward—the right eye because of the original surgery turning it outward, and the left eye because of the original esotropia, with still further esotropia developing later. When awake, her muscles were stimulated neurologically to hold her eyes so that she could fix with her better eye—her left eye, but the muscle lengths had not adapted long-term to this type of stimulation

• An 80-year-old woman with dense amblyopia in her left eye since childhood, fixing with her right eye only, all her life. Note the left “sensory” exotropia (top)—the poorly seeing left eye drifted out over the years because the vision was not good enough for sensory fusion with the other eye. But under general anesthesia, when the eyes reached their stable, deep-anesthesia state (bottom), both eyes turned out, equally, and significantly farther than the usual divergence seen under general anesthesia (in 110 patients with exotropia, Apt and Isenberg26 documented an increased divergence under general anesthesia of only 11.9 prism diopters). Both lateral rectus muscles were equally and abnormally tight when operated. The strabismus had occurred bilaterally.


deviation

• EOMs have version stimulation and vergence stimulations

• Version stimulations do not lead to length adaptations

• Vergence stimulations help ocular alignment hence lead to length adaptations bilaterally


deviation

• Evidence shows that the muscle fibres involved in versions and vergence responses are different


deviation• Chronic low-grade vergence response in the

absence of fusion leads to initiation and progression of strabismus

• For example, hyperopic children develop accomodative esotropia due to convergence

• Similar mechanism may operate in cyclovertical plane involving obliques and recti

Congenital SO ParesisExperimental evidence of hypothesis

• A haploscopic device was created to induce vertical disparity and subjects were adapted to fusion

• Pattern of vertical vergence and torsion suggested a strong role of obliques in vertical fusional movements

• Investigations are on to simulate the signs of “Cong. SO palsy” in normal patients with adaptation

Congenital SO Paresis

• “Though the mechanism explained has not been confirmed, we are coming closer” - Guyton

In the Hogwart of strabismus….

• If torsion is Lord Voldemort………David Guyton is

P.S. Torsion did not kill Guyton’s parents

ocular torsion reveals the mechanisms of cyclovertical strabismus the weisenfeld lecture @ arvo 2007...

Documents

v patternschanges

v deviationssubjective

v patternshow

v patters

adductionoblique muscle

dotobjective torsion

v iooav pattern

decreased stimulation