The clinical evaluation begins with a history, including the age at onset of the strabismus, frequency and duration of misalignment, circumstances under which the deviation is manifest, and whether the exotropia is becoming more frequent with time. The clinician should determine whether symptoms such as diplopia, asthenopia, or difficulty with interpersonal interactions secondary to ocular misalignment are present.
Exodeviation control may be categorized as follows:
Good control: Exotropia manifests only after cover testing, and the patient resumes fusion rapidly without blinking or refixating.
Fair control: Exotropia manifests after fusion is disrupted by cover testing, and the patient resumes fusion only after blinking or refixating.
Poor control: Exotropia manifests spontaneously and may remain manifest for an extended time.
Some ophthalmologists use the Newcastle Control Score for Intermittent Exotropia to quantitatively grade the control exhibited by patients with this deviation.
Because visual acuity and alignment tests are dissociating and may adversely affect assessment of strabismus control, they should be performed after sensory tests for stereopsis and fusion. Prism and alternate cover testing should be used to evaluate the exodeviation at fixation distances of 6 m and 33 cm. A far-distance measurement at 30 m or greater (eg, at the end of a long hallway or out a window) may uncover a latent deviation or elicit an even larger one. The deviation at near fixation is often smaller than the deviation at distance fixation. This difference is usually due to tenacious proximal fusion, a slow-to-dissipate fusion mechanism at near. The difference may sometimes be due to a high accommodative convergence/accommodation (AC/A) ratio, but a high AC/A ratio occurs much less commonly in exotropia than in esotropia (see Chapter 7, which discusses measurement of AC/A ratios). The exodeviation is termed basic intermittent exotropia when the size of the deviation at distance fixation is within 10 prism diopters (Δ) of the deviation size at near fixation. Some children have a larger deviation at near than at distance; this is distinct from convergence insufficiency (discussed later in the chapter).
When the exodeviation at distance is larger than the deviation at near fixation by 10Δ or more, the near exodeviation should be remeasured after 1 eye is occluded for 30–60 minutes (the patch test). The patch test eliminates the effects of tenacious proximal fusion, helping distinguish between pseudodivergence excess and true divergence excess. A patient with pseudodivergence excess has similar distance and near measurements after the patch test. A patient with true divergence excess continues to have a significantly larger exodeviation at distance. Many patients with true divergence excess also have a high AC/A ratio. For these patients, the AC/A ratio can be determined by measuring the near deviation with and without +3.00 diopter (D) lenses (while the patient wears corrective lenses, if necessary), after the patch test is completed. The measurements are then compared. Alternatively, the distance deviation can be measured with and without −2.00 D lenses to determine the AC/A ratio.
American Academy of Ophthalmology Pediatric Ophthalmology/Strabismus Panel. Preferred Practice Pattern Guidelines. Esotropia and Exotropia. San Francisco, CA: American Academy of Ophthalmology; 2012. For the latest guidelines, go to www.aao.org/ppp.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.