Exotropia

Outward deviation of the eye. Common types:

• Intermittent exotropia (most common form, onset 6 months to 4 years, affects about 1 percent of children, prevalence higher in Asian populations). Divided by distance and near measurement: basic, divergence excess (true or simulated), convergence insufficiency.
• Constant exotropia: infantile (rare, usually with neurological disease or craniofacial syndromes), sensory (poor vision in one eye), consecutive (after esotropia surgery), and decompensated.
• Paralytic: third nerve palsy, internuclear ophthalmoplegia, myasthenia, and Duane syndrome type 2.

• Risk factors: hypermetropia is not typical; myopia, anisometropia, family history, prematurity, neurological impairment, craniofacial syndromes.

• Intermittent exotropia: the deviation appears with fatigue, daydreaming, illness, bright light (children close one eye in sunshine), and at distance; diplopia is rare because of suppression; asthenopia in older children; control worsens over time.
• Control scale: from good (deviation only on dissociation) to poor (manifest most of the time); control measured by office control score (0 to 5) and home observation.
• Constant exotropia: obvious outward deviation, amblyopia in sensory cases, no fusion.
• Adults: diplopia, asthenopia, convergence insufficiency, and social stigma.
• Features of A and V patterns, lateral incomitance, and dissociated vertical deviation.

• Cycloplegic refraction and fundus; visual acuity and stereoacuity (Titmus fly with and without correction at distance and near).
• Prism cover test at 6 m, 33 cm and with 30 minutes' monocular occlusion (patch test) to unmask the full deviation and to define true versus simulated divergence excess.
• Versions and ductions; convergence near point; amplitudes of fusional convergence; Worth four dot test at distance.
• Exclude neurological cause, third nerve palsy, thyroid disease, myasthenia, orbital mass (MRI or CT) in acquired or constant adult onset.

• Observation with repeated assessment every 3 to 6 months when control is good and angle is small (below 15 prism dioptres) and vision normal.
• Overminus lens therapy: -1.00 to -3.00 dioptres over the refractive correction, to induce accommodative convergence; used in children aged 2 to 8 years.
• Occlusion therapy: part time patching of the dominant eye 2 to 6 hours per day for 3 to 6 months, particularly in mild to moderate control.
• Prism therapy: base in prisms for small deviations; and orthoptic convergence exercises when exophoria with convergence insufficiency (office based vergence therapy 12 weeks, with home exercises 15 minutes daily).
• Surgery: indicated for deteriorating control, constant exotropia, or large angles. Bilateral lateral rectus recession 5 to 8 mm (for distance greater than near), unilateral recess resect (if near greater than distance), or medial rectus resection with lateral rectus recession; aim for slight overcorrection in the early postoperative period (esotropia of 5 to 10 prism dioptres), which reduces recurrence.
• Sensory exotropia: surgery for cosmetic improvement after treatment of the cause.
• Botulinum toxin into the lateral rectus (2.5 to 5 units) in selected adults or diagnostic use.
• Follow up: monitor for diplopia and recurrence; treat amblyopia.