Narrative literature review for intermittent exotropia
Reviewed by Fiona Rowe

This is a narrative literature review on prevalence, terminology, risk factors, natural history and clinical characteristics for intermittent exotropia. A Medline search was conducted with no date restrictions up to September 2020 and collating English language studies. Prevalence was reported of 0.1–3.7% with variance related to diagnostic criteria, strabismus classification, study design and subject characteristics. Intermittent exotropia was the most common form and there was no recent association found for exotropia prevalence and geographical location. The exotropia:esotropia ratio was higher for Asian countries / race than Western countries / non-Asian race. This was explained in part by prevalence of hypermetropia vs. myopia in these countries. A discussion of classification included intermittent exotropia, divergence excess, convergence insufficiency, infantile exotropia, exophoria and basic exotropia. Risks and associations for infantile exotropia included hypoxia at birth, being more common in females than males, family history of strabismus, maternal smoking in pregnancy, birth weight, gestational age, neurological and congenital conditions, and refractive error. Natural history reported a general deterioration of exotropia is uncommon and occurs mostly before age three years. However, there was note of marked individual variation in exotropia angle, control ability and sensory / motor aspects. These, along with poor control at the initial visit, were more likely to receive surgical treatment. Angle of deviation was most usually measured with alternate prism cover test. Deviations <20PD were classed as small and >35PD as large. Near angle was reported to be affected by target size but not distance angle. The authors discussed the role of monocular diagnostic occlusion, accommodative convergence / accommodation (AC/A) ratio, and prism adaptation test to elucidate maximum angle measurements. Control scores were discussed including the Office Control score, and Newcastle Control score as the two most commonly used. Reported symptoms included diplopia, photophobia, monocular eye closure in bright light, blinking, panoramic vision, rubbing eyes, awareness of the exo and abnormal head posture. Issues of psychological impact were reported as less for intermittent exotropia than constant. Clinical characteristics included stereopsis (importance of maintaining near stereo) and suppression (wide spread vs. scotoma); abnormal retinal correspondence as an adaptive response for some cases; AC/A ratio with typical gradient measurement; accommodation with regard to accommodative error, lag and influence of the dominant fixing eye; vergence with measurements of fusional ranges and amplitudes; and comitancy with prevalence of A and V patterns, lateral incomitance and vertical deviations.

Divergence excess and basic exotropia types of intermittent exotropia: a major review. Part 1: prevalence, classification, risk factors, natural history and clinical characteristics.
Ma MM-L, Scheiman M.
STRABISMUS
2023;31(2):97–128.