INTERMITENT EXOTROPIA

Theories – HistoricalTheories

– Innervational imbalance between convergence and divergence (Duane)

• Divergence: active or passive– Active- Divergence burst cells

» Mesencephlic reticular formation (near oculomotor nucleus) in monkeys

– Passive» Relaxation of accommodation» Absence of simultaneous contraction of LR

– Anatomic (Bielschowsky)– Abnormal position of rest

• Theory of Defective fusion (Worth)• Hemiretinal suppression (Knapp and Jampolsky).• Uncorrected refractive errors (Donders)

ExotropiaExodeviation less common than esodeviation in

WestF > MExodeviation found areas with more sunlightFrequently seen in neonatal period – resolvesRedevelops before 2nd year of life (35-40%)Often positive family historyCommon with facial asymmetry and

neurological defects

Classic presentation• Begins as exophoria – shortly after birth

– Binocular fixation and NRC– Progresses to intermitent exotropia X(T)

• Adults have diplopia• Children develop hemiretinal suppression• Periods of phoria and tropia

• Frequent pseudo-oblique dysfunction– Associated with tight laterals– Called “leash phenomenon”

Natural History• Progression occurs in 35% - 75% of pts.• Improvement occurs in 16% - 65%• Factors:

– Age • Decreasing tonic convergence

– Suppression sets in• “The key to unlocking fusion”

– Worsening seen at distance first

– Decreased accommodation– Orbital divergence from maturation of facial

features

Signs and Symptoms• Eyestrain

– Blurred vision, prolonged reading problems, headaches

– Diplopia• Photophobia

• One eye closed to avoid diplopia– Decreased fusional convergence– Decreased binocular photophobia threshold

Intermittent Exotropia• Onset

– Infancy to 5 years– May progress

• Noticed by the parents when the child is tired or ill or during daydreaming

• very little visual symptoms• Transient diplopia or asthenopia especially after

prolonged reading.• Diplophotophobia, or closure of one eye in bright light is a very common symptom

Phase - Deviation -SensoryI Exophoria at distance, orthophoria at near Asymptomatic

II Intermittent exotropia for distance, Symptomatic for orthophoria/ exophoria at near distance

III Exotropia for distance, exophoria or BV for near, intermittent exotropia at near. suppression scotoma for distance

IV Exotropia at distance as well as near Lack of binocularity.

classification

Burian has classified IDS into 4 types based on measurements of the distance and near deviation.

1. Basic type: distance deviation and near deviation are within 10 PD of each other.

2. Divergence excess type: distance measurement 10 PD or greater than the near deviation.

3. Convergence insufficiency type: near deviation 10 PD greater than the distance.

4. Simulated or pseudo divergence excess type: near deviation is less than the distance deviation but it increases to within10 PD of distance deviation after 30 – 60 minutes of monocular occlusion

Kushner has further modified this classification by taking into account AC/A ratio. He introduced two new groups

1)Pseudo divergence with tenacious proximal fusion (near deviation increase after 60 minutes of monocular occlusion)

2)Divergence with High AC/A ratio.

Basic Type XT• Burian and Kushner

– Distance = Near– Normal AC/A

Convergence Insufficiency

• Burian– Near 10 PD more than

at Distance

• Kushner– Low AC/A– Fusional convergence

insufficiency– Pseudo-convergence

insufficiency• Patching increases

distance deviation to match near

Divergence Excess

• Burian– Distance 15 PD more

than at Near

• Kushner– True Divergence

Excess– Simulated

– Tenacious proximal fusion

Examination• Comprehensive ophthalmologic examination• Check fusion and stereopsis before

occluding eyes for visual acuity examination – So ARC and suppression do not set in

• Check stereo acuity for distance and for near• Versions• Cover test

Examination• Important to obtain cycloplegic retinoscopy• “Pseudoamblyopia”

– Exodeviation often manifests with fixation preference

– Misinterpreted as evidence of amblyopia– Excellent fusion and stereopsis when eyes are

aligned– Patching may worsen situation and induce

iatrogenic loss of fusion and stereopsis

Measurement of the deviation

1. Prolonged alternate cover test: To maximally suspend the tonic fusional convergence during the ACT the occluder must be placed in front of either eye for a sufficient duration and alternated .

2. Patch Test: Mono ocular occlusion for 30 – 60 minutesIt differentiates true and pseudo divergence excess

3. High AC/A ratio (Lens gradient test / +3.0 D test): This test helps in diagnosing the patients with divergence excess due to a high AC/A ratio. In such cases the near deviation increases by 20 PD or more on addition of a +3.0 D lens.

4. Far distance measurement: Apart from the near and distance (20 feet) measurement, the deviation must also be measured for far distance (100 – 200 feet).

XT response patterns with tests• Occlusion

– Distance deviation increases– If Near = Distance with occlusion

• Simulated divergence excess– Near<distance

• +3.00 lens– Near= Distance

» High AC/A– Near< Distance

» True divergence excess

Fusional ControlThe level of fusional control is an baseline evaluation and indicator of progression.1.Home control: percentage of waking hours when the squint is noticed by the parents . Deviation manifesting more than 50% of waking hours indicates poor control..

2. Clinical control: fusional control can be assessed using cover test

a. Good control: The patient resumes fusion rapidly without blinking or re-fixation.b. Fair control: Patient blinks or re-fixates to control the deviation.c. Poor control: Patient breaks spontaneously without any disruption

3. Stereo-acuity: (mainly for distance) Indicator of both control of the deviation and deterioration of fusion. The distance stereoacuity can be assessed usingRandom dot E test or Mentor B- Vat tests.

The unique features of IDS that make its management controversial are1. Variable angle of deviation2. Unpredictable course of progression (i.e. deterioration of control)3. Good binocularity (for near till late)4. Rarity of amblyopia or ARC (abnormal retinal correspondence)

• Treatment1) Non-Surgical2) SurgicalGoal of Non-surgical treatment is -To improve the neuro-physiological vergence control mechanism to• decrease the frequency of the manifest

phases • prevent the progression from latent to

constant squint.

Ideal candidates for conservative therapy1. Young patients (4-5 years old)2. Phase I or II.3. Fair or better control of deviation.4. Angle of deviation d”20 – 25 PD.These patients must be monitored closely for any signs of progression.

Treatment-Non surgical • Correction of refraction

– Over-minus• To produce High AC/A• XT usually recurs

– Hyperopia• Mild to moderate – no treatment• High – treatment (partial to full plus)

• Prisms – Not generally used, base in prisms– Decreases convergence amplitude

• Occasionally slow weaning from prism– may improve convergence amplitude

Treatment• Patching

– Preoperatively• May reduce frequency and magnitude of deviation

– XT becomes X(T)– X(T) becomes X

– Unknown mechanism• Speculation

– Reduces depth of suppression

– XT typically recurs when patching stopped

Treatment• Orthoptics

– Antisuppression treatment.• Controversial

– Diplopia awareness• May lead to intractable diplopia• “Orthoptic cripples”

– Fusional amplitudes• Pencil pushups• Convergence amplitudes

• Observation– Frequency

Signs of Progression1. Gradual loss of fusional control

evidenced by the increasing frequency of the manifest phase of squint

2. Development of Secondary convergence insufficiency3. Increase in size of the deviation4. Development of suppression as indicated by absence of diplopia during manifest phase5. Decrease of Stereoacuity

Surgical Treatment

• The primary aim for any squint surgery is the

Restoration of appropriate binocular functionImprove cosmesis.

INDICATIONS

• Large angle deviations (> 20 PD)• Documented worsening of the deviation

(Signs of progression)• Deviation present for more than 50% of

waking hours.• Failure of conservative therapy.

The Newcastle Control Score (NCS)It differentiates and quantifies the various levels of severity in IXT

NCS total-Home+Clinic near+Clinic distance 3 groups1. Well controlled (NCS 2): high probability of stable course or spontaneous recovery. Candidates for vision therapy

2. Poorly controlled (NCS 7 or more): Definite surgical intervention needed, as spontaneous recovery is unlikely.

3. Moderate control (NCS 3- 6) show better results with surgical management as compared to conservative approach.

NEW CASTLE SCORE Component• Home control0. Squint eye do sure never noticed1. Squint eye do sure seen occasionally (<50% of time child observed) for distance2. Squint eye do sure seen frequently (>50% of time child observed) for distance3. Squint eye do sure seen for distance & near fixation• Clinic control near0.Manified only after cover test and resumes fusion without need for blink or refixation1. Blink or refixate to control after CT2. Manified spontaneously or with any form of fusion disruption without recovery

Timing of surgeryEarly surgery may offer to - prevent the development of sensory changes - risk of consecutive esotropia leading subsequently to monofixation.

Delayed surgery -advantage of accurate diagnosis and - quantification of the amount of deviation and - to avoid consecutive esotropia.

For small angle deviations (< 20 PD) and for young children (< 4 years) Defer the surgery and Use vision therapy with a close watch for signs of progression.

SURGERY OF CHOICE

• Pure divergence excess type- bilateral lateral rectus recession

• Simulated divergence excess type and basic types - unilateral recess- resect procedure.

Goal of surgeryVisually immature infant: avoid consecutive esotropias(can have the consequences of amblyopia and loss ofbinocularity)

Older children, who develop intermittent exotropia afterage 10 years, aim for orthotropia on the firstpostoperative day.

Adults with longstanding IXT will tolerate undercorrection, but will have symptomatic diplopia whenovercorrected.

Surgical SuccessAnatomical Cure: Alignment within 8 – 10 PD of orthotropia.Functional Cure: Near Stereopsis between 40 – 60 sec of Arc

UNDERCORRECTION

Patients with• large angle deviations • high myopic refractive error • Undiagnosed oblique muscle overaction • small vertical deviations

OVER CORRECTION• Small over correction of up to 10 – 15 PD in a

visually mature patient is acceptable• A high AC/A ratio and • Undiagnosed lateral incomitance are the risk

factors for larger angle esodeviation.• A 6 – 8 week trial of conservative measures like part time occlusion, bifocals (high AC/A ratio) or neutralizing prisms may be tried before resorting to surgery.

• Overcorrection– ET > 6months– Significant risk of amblyopia– Risk for monofixation syndrome– Treatment:

• Maximum plus in Rx• Prisms for deviation

–Fresnel prism early ,then built in–Fusion restoration–Wean slowly of prism

• Reoperation(BMR)

Convergence Insufficiency• XT greater at near• Asthenopia with near work• Decreased convergence amplitudes• Uncommon < 10 years of age• Females > males• Treat with pencil push ups• May need surgery

– BMR recess

Conclusion• Duration of constant strabismus is the

controlling factor– Aligned within 12 months of the onset of strabismus

• Do well – Those whose strabismus persisted beyond 12

months • Do very poorly

• Better the sensory outcome the greater likelihood of long-term stability of the alignment