Strabismus
Definition
Ocular misalignment that may be horizontal or vertical; comitant (same angle of deviation in all positions of gaze) or incomitant (angle of deviation varies in different positions of gaze); latent, manifest, or intermittent.
Phoria
Latent deviation.
Tropia
Manifest deviation.
Esotropia
Inward deviation.
Exotropia
Outward deviation.
Hypertropia
Upward deviation.
Hypotropia
Downward deviation (vertical deviations usually are designated by the hypertropic eye, but if the process clearly is causing one eye to turn downward, this eye is designated hypotropic).
Etiology
Congenital or acquired local or systemic muscle, nerve, or neuromuscular abnormality; childhood forms are usually idiopathic. See below for full description of the various types of strabismus (horizontal, vertical, miscellaneous forms). See also cranial nerve palsy, chronic progressive external ophthalmoplegia (CPEO), and myasthenia gravis.
Symptoms
Asymptomatic; may have eye turn, head turn, head tilt, decreased vision, binocular diplopia (in older children and adults), headaches, asthenopia, and eye fatigue.
Signs
Normal or decreased visual acuity (amblyopia), strabismus, limitation of ocular movements, reduced stereopsis; may have other ocular pathology (i.e., corneal opacity, cataract, aphakia, retinal detachment, optic atrophy, macular scar, phthisis) causing a secondary sensory deprivation strabismus (usually esotropia in children < 6 years old, exotropia in older children and adults).
Differential Diagnosis
See above; pseudostrabismus (epicanthal fold), negative angle kappa (pseudoesotropia), positive angle kappa (pseudoexotropia), dragged macula (e.g., retinopathy of prematurity, toxocariasis).
Evaluation
- •
Complete ophthalmic history with attention to age of onset, direction and frequency of deviation, family history of strabismus, neurologic symptoms, trauma, and systemic disease.
- •
Complete eye exam with attention to visual acuity, refraction, cycloplegic refraction, pupils, motility (versions, ductions, cover and alternate cover test), measure deviation (Hirschberg, Krimsky, or prism cover tests), measure torsional component (double Maddox rod test), Parks–Bielschowsky three-step test (identifies isolated cyclovertical muscle palsy [see Fourth Cranial Nerve Palsy section]), head posture, stereopsis (Titmus stereoacuity test, Randot stereotest), suppression/anomalous retinal correspondence (Worth 4-dot, 4 prism diopter [PD] base-out prism, Maddox rod, red glass, Bagolini’s striated lens, or after-image tests), fusion (amblyoscope, Hess screen tests), forced ductions, and ophthalmoscopy.
- •
Lab tests : Thyroid function tests (triiodothyronine [T3], thyroxine [T4], thyrotropin [TSH]) in cases of muscle restriction, and antiacetylcholine (anti-ACh) receptor antibody titers if myasthenia gravis is suspected. Electrocardiogram in patients with chronic progressive external ophthalmoplegia (CPEO) to rule out heart block in Kearns–Sayre syndrome. Consider edrophonium chloride (Tensilon) test, ice pack test, or anti-ACh receptor antibody test to rule out myasthenia gravis.
- •
Orbital computed tomography (CT) or magnetic resonance imaging (MRI) in select cases.
- •
Neurology consultation and brain MRI if cranial nerves are involved.
- •
Medical consultation for dysthyroid and myasthenia patients.
- •
Treat underlying etiology.
- •
Correct any refractive component.
- •
In children, patching or occlusion therapy for amblyopia (see Chapter 12 ); patch better seeing or fixating eye; part-time patching can be used at any age. If full-time patching is used (during all waking hours, then taper as amblyopia improves), do not occlude one eye for more than 1 week per year of age. Atropine penalization, one drop in the better-seeing eye every day or twice a week, is a good substitute to patching in children who are not compliant with occlusion.
- •
Timing of re-examination is based on patient’s age (1 week per each year of age; e.g., 2 weeks for 2 year olds, 4 weeks for 4 year olds).
- •
Consider muscle surgery depending on the type and degree of strabismus (see below).
Prognosis
Usually good; depends on etiology.
Horizontal Strabismus
Esotropia
Eye turns inward; most common ocular deviation (> 50%).
Infantile Esotropia
Appears by age 6 months with a large, constant angle of deviation (80% > 35 PD); often cross-fixate; normal refractive error; positive family history is common; may be associated with inferior oblique overaction (70%), dissociated vertical deviation (DVD) (70%), latent nystagmus, and persistent smooth pursuit asymmetry.
- •
Treat amblyopia with occlusive therapy of fixating or dominant eye before performing surgery.
- •
Correct any hyperopia greater than + 2.00 diopters (D).
- •
Muscle surgery should be performed early (6 months to 2 years): bilateral medial rectus recession or unilateral recession of medial rectus and resection of lateral rectus; additional surgery for inferior oblique overaction, dissociated vertical deviation (DVD), and overcorrection and undercorrection is necessary in a large percentage of cases.
Accommodative Esotropia
Develops between age 6 months and 6 years, usually around age 2 years, with variable angle of deviation (eyes usually straight as infant); initially intermittent when child is tired or sick. There are three types:
Refractive
Usually hyperopic (average + 4.75D), normal accommodative convergence-to-accommodation (AC / A) ratio (3 : 1PD to 5 : 1PD per diopter of accommodation), esotropia at distance (ET) similar to that at near (ET’).
Nonrefractive
High AC / A ratio; esotropia at near greater than at distance (ET’ > ET).
- •
Methods of calculating AC / A ratio
- (1)
Heterophoria method : AC / A = IPD + [(N − D)/ Diopter]
IPD, interpupillary distance (cm); N, near deviation; D, distance deviation; Diopter, accommodative demand at fixation distance
- (2)
Lens gradient method : AC / A = (WL − NL)/ D
WL, deviation with lens in front of eye; NL, deviation with no lens in front of eye; D, dioptric power of lens used.
- (1)
Mixed
Not completely correctable with single-vision or bifocal glasses.
- •
Give full cycloplegic refraction if child is under 6 years old, and as much as tolerated if over 6 years old; if esotropia corrects to within 8PD, then no further treatment is necessary.
Figure 2-3
(A) Accommodative esotropia that improves when the child wears full hyperopic correction (B).
- •
With high AC / A ratio and residual ET’, prescribe executive-style, flat-top bifocal segment that bisects the pupil (+ 2.50D to + 3.00D) or try miotic agents, especially in infants too young for glasses (echothiophate iodide 0.125% qd; be careful not to use with succinylcholine for general anesthesia); a combination of both can be used in refractory cases.
- •
Muscle surgery as above should be performed if residual esotropia is greater than 10PD.
Acquired Nonaccommodative Esotropia and Other Forms of Esotropia
Due to stress, sensory deprivation, divergence insufficiency (ET ≥ ET’), spasm of near reflex, consecutive (after exotropia surgery), or cranial nerve (CN) VI palsy.
- •
Muscle surgery can be considered either in symptomatic cases or if angle of deviation is large ( Table 2-1 ).
Table 2-1
Surgical Numbers for Horizontal Strabismus Surgery – Esotropia
Recession & Resection (R & R)
Prism Diopters (PD)
Bilateral MR Recession (mm)
Bilateral LR Resection (mm)
MR Recess (mm)
LR Resect (mm)
15
3
3.5
3
3.5
20
3.5
4.5
3.5
4.5
25
4
5.5
4
5.5
30
4.5
6
4.5
6
35
5
6.5
5
6.5
40
5.5
7
5.5
7
50
6
8
6
7.5
60
6.5
—
6.5
8
70
7
—
—
—
MR, Medial rectus; LR, lateral rectus.
- •
If no evident cause, MRI indicated to rule out Arnold–Chiari malformation.
Cyclical esotropia
Very rare form of nonaccommodative ET (1 : 3000); occurs between 2 and 6 years of age; child is usually orthophoric (eyes straight) but develops esotropia for 24-hour to 48-hour periods; can progress to constant esotropia.
- •
Correct any hyperopia greater than + 3.00D.
- •
Muscle surgery as above can be performed when deviation stabilizes.
Exotropia
Eye turns outward; may be intermittent (usually at age 2 years, amblyopia rare) or constant (rarely congenital, consecutive [after esotropia surgery], due to decompensated intermittent exotropia [XT] or from sensory deprivation [in children > 5 years of age]); amblyopia rare due to alternate fixation or formation of anomalous retinal correspondence.
Basic Exotropia
Exotropia at distance (XT) equal to that at near (XT’); normal AC / A ratio, normal fusional convergence.
Convergence Insufficiency
Inability to adequately converge while fixating on an object as it moves from distance to near (increased near point of convergence); exotropia at near greater than at distance (XT’ > XT); reduced fusional convergence amplitudes. Rare before 10 years of age; slight female predilection. Symptoms often begin during teen years with asthenopia, difficulty reading, blurred near vision, diplopia, and fatigue. Common in neurodegenerative disorders, such as Parkinson’s disease, and with traumatic brain injury; may rarely be associated with accommodative insufficiency and ciliary body dysfunction.
- •
Treatment consists of prismatic correction (often ineffective) or monocular occlusion. Bifocals can be more difficult for patients with convergence insufficiency to use and should probably be avoided.
- •
Consider muscle surgery: bilateral medial rectus resection should be used with caution owing to the risk of disrupting alignment at distance.
- •
Orthoptic exercises are controversial: near-point pencil push-ups (bring pencil in slowly from distance until breakpoint reached, then repeat 10–15 times) is the most common but is minimally effective; saccadic exercises or prism convergence exercises (increase amount of base-out prism until breakpoint reached, then repeat starting with low prism power 10–15 times) are more effective.
Pseudodivergence Excess
XT > XT’ except after prolonged patching (patch test) when near deviation increases (full latent deviation); near deviation also increases with + 3.00D lens; may have high AC / A ratio.
True Divergence Excess
XT > XT’ even after patch test; may have high AC / A ratio.
- •
Correct any refractive error and give additional minus (to stimulate convergence), especially with high AC / A ratio.
- •
Consider base-in prism lenses to help with convergence.
- •
Muscle surgery if the patient manifests exotropia over 50% of the time and is older than 4 years old: bilateral lateral rectus recession; consecutive esotropia (postoperative diplopia) can be managed with prisms or miotics unless it lasts more than 8 weeks, then reoperate ( Table 2-2 ).
Table 2-2
Surgical Numbers for Horizontal Strabismus Surgery – Exotropia
Recession & Resection (R & R)
Prism Diopters (PD)
Bilateral LR Recession (mm)
Bilateral MR Resection (mm)
LR Recess (mm)
MR Resect (mm)
15
4
3
4
3
20
5
4
5
4
25
6
5
6
5
30
7
5.5
7
5.5
35
7.5
6
7.5
6
40
8
6.5
8
6.5
50
9
—
9
7
MR, Medial rectus; LR, lateral rectus.
A-, V-, and X-Patterns
Amount of horizontal deviation varies from upgaze to downgaze; occurs in up to 50% of strabismus.
A-pattern
Amount of horizontal deviation changes between upgaze (larger esotropia in upgaze) and downgaze (larger exotropia in downgaze); more common with exotropia; clinically significant if difference is 10PD or greater; associated with superior oblique muscle overaction; patients may have chin-up position.
- •
Muscle surgery if deviation is clinically significant: weakening of oblique muscles if overaction exists or transposition of horizontal muscles (medial recti moved up, lateral recti moved down) if no oblique overaction.
- •
Absolutely no superior oblique tenotomies if patient is bifixator with 40 seconds of arc of stereoacuity.
V-pattern
Amount of horizontal deviation changes between upgaze (larger exotropia in upgaze) and downgaze (larger esotropia in downgaze); more common with esotropia; clinically significant if difference is 15PD or greater; associated with inferior oblique muscle overaction, increased lateral rectus muscle innervation, underaction of superior rectus muscle, Apert’s syndrome, and Crouzon’s syndrome; patients may have chin-down position.
- •
Muscle surgery if deviation is clinically significant: weakening of oblique muscles if overaction exists or transposition of horizontal muscles (medial recti moved down, lateral recti moved up) if no oblique overaction.
X-pattern
Larger exotropia in upgaze and downgaze than in primary position; due to secondary contracture of the oblique muscles or the lateral recti, causing a tethering effect in upgaze and downgaze.
- •
Muscle surgery if deviation is clinically significant: staged surgery if due to oblique muscles or lateral rectus recessions if due to tether effect.
Vertical Strabismus
Brown’s Syndrome (Superior Oblique Tendon Sheath Syndrome)
Congenital or acquired anomaly of the superior oblique tendon sheath, causing an inability to elevate the affected eye especially in adduction. Elevation in abduction is normal or slightly decreased; may have hypotropia in primary gaze causing a chin-up position, positive forced duction testing that is worse on retropulsion and when moving eye up and in (differentiates from inferior rectus restriction, which is worse with proptosing eye); V-pattern (differentiates from superior oblique overaction, which is A-pattern), no superior oblique overaction, down-shoot in adduction, and widened palpebral fissures on adduction in severe cases. Ten percent bilateral, female predilection (3 : 2) affects right eye more often than left eye. Acquired forms are associated with rheumatoid arthritis, juvenile rheumatoid arthritis, sinusitis, surgery (sinus, orbital, strabismus, or retinal detachment), scleroderma, hypogammaglobulinemia, postpartum, and trauma.
- •
No treatment usually required, especially for acquired forms, which may improve spontaneously depending on the etiology.
- •
Consider injection of steroids near trochlea or oral steroids if inflammatory etiology exists.
- •
Muscle surgery for abnormal head position or large hypotropia in primary position: superior oblique muscle tenotomy or tenectomy or silicon band expander, with or without ipsilateral inferior oblique muscle recession.
Dissociated Strabismus Complex: Dissociated Vertical Deviation, Dissociated Horizontal Deviation, Dissociated Torsional Deviation
Updrift, horizontal, oblique, or torsional movement of nonfixating eye with occlusion or visual inattention. Often bilateral, asymmetric, and asymptomatic; does not obey Hering’s law (equal innervation to yoke muscles). Demonstrates Bielschowsky phenomenon (downdrift of occluded eye as increasing neutral density filters are placed over fixating eye), red lens phenomenon (when red lens held over either eye while fixating on light and the red image is always seen below the white image); associated with congenital esotropia (75%), latent nystagmus, inferior oblique overaction, and after esotropia surgery.
- •
No treatment usually required.
- •
Muscle surgery for abnormal head position or if deviation is large and constant or very frequent: bilateral superior rectus recession, inferior rectus resection, or inferior oblique weakening ( Table 2-3 ).
Table 2-3
Surgical Numbers for Vertical Strabismus Surgery
Dissociated Vertical Deviation
Magnitude
Recess SR
Mild
5 mm
Moderate
7 mm
Severe
10 mm
Inferior Oblique Overaction
Magnitude
Recess IO
Mild
10 mm
Moderate
15 mm
Severe
Myectomy
SR, superior rectus; IO, inferior oblique.
Monocular Elevation Deficiency (Double Elevator Palsy)
Sporadic, unilateral defect causing total inability to elevate one eye (may have good Bell’s reflex); hypotropia in primary position and increases on upgaze, ipsilateral ptosis common, may have chin-up head position to fuse; may be supranuclear, congenital, or acquired (due to cerebrovascular disease, tumor, or infection). Three types:
Type 1
Inferior rectus muscle restriction–unilateral fibrosis syndrome.
Type 2
Elevator weakness (superior rectus, inferior oblique)–true double elevator palsy.
Type 3
Combination (inferior rectus restriction and weak elevators).
- •
Muscle surgery for chin-up head position, large hypotropia in primary position, or poor fusion: inferior rectus recession for inferior rectus muscle restriction; Knapp procedure (elevation and transposition of medial and lateral recti to the side of superior rectus) for superior rectus muscle weakness.
- •
May require surgery to correct residual ptosis.
Miscellaneous Strabismus
Duane’s Retraction Syndrome
Congenital agenesis or hypoplasia of the abducens nerve (CN VI) with variable dysinnervation of the lateral rectus muscle; 20% bilateral, female predilection (3 : 2), affects left eye more often than right eye (3:1). Three types (type 1 more common than type 3, and type 3 more common than type 2):
Type 1
Limited abduction; esotropia in primary position.
Type 2
Limited adduction; exotropia in primary position.
Type 3
Limited abduction and adduction.
Co-contraction of medial and lateral rectus muscles causes globe retraction and narrowing of palpebral fissure; narrowing of palpebral fissure on adduction and widening on abduction in all three types; upshoots and downshoots (leash phenomenon) common; may have head turn to fuse; amblyopia rare; rarely associated with deafness and less commonly with other ocular or systemic conditions.
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