An 11-year-old girl with intermittent exotropia holds her chin up ‘all the time’, which her parents find odd. On examination the outward drift is modest looking up, moderate looking straight ahead, and dramatically worse looking down — she avoids downgaze, which is why the chin goes up. Version testing shows both eyes over-depressing when adducted. She has good vision in each eye and no double vision when the deviation is controlled. The family was told she needs surgery for the exotropia and asks whether one operation fixes everything.
Examination Findings
Intermittent exotropia measured at distance: about 20 prism dioptres in upgaze, 30 in primary position, and 45 in downgaze — the deviation grows markedly looking down, the definition of an A-pattern. Version testing shows bilateral superior oblique overaction: each eye over-depresses in adduction. There is a compensatory chin-up head posture to avoid the downgaze position where the deviation is worst. Visual acuity 6/6 each eye, equal refraction. Anterior segment, pupils, and fundus are normal.
Investigations
Prism cover testing in upgaze, primary position, and downgaze (at least 25 degrees up and down) to quantify the pattern — a significant A or V pattern is typically defined by a marked upgaze–downgaze disparity. Version testing grades the oblique overaction. Assessment of the compensatory head posture and binocular function. Cycloplegic refraction as baseline.
Questions to Think About
- What defines an ‘A-pattern’, and which muscles cause it?
- Why would horizontal surgery alone leave her chin-up posture unchanged?
- Why is measuring in upgaze and downgaze essential, not optional?
Diagnosis
A-pattern exotropia with superior oblique overaction — the deviation is more divergent in downgaze; management is horizontal surgery combined with superior oblique weakening.
Reasoning
Reasoning: the upgaze–downgaze disparity is the diagnosis — the exotropia grows from modest in upgaze to marked in downgaze, forming an A-pattern, driven by bilateral superior oblique overaction (the eyes over-depress in adduction). The chin-up posture is her self-management: she keeps her eyes out of downgaze, where the deviation is worst. Horizontal surgery alone would straighten the primary position but leave the pattern and the posture, because the oblique overaction is a separate mechanical component. Diagnosis: A-pattern intermittent exotropia with bilateral superior oblique overaction. Management: typical management is combined surgery — bilateral lateral rectus recession for the horizontal deviation plus bilateral superior oblique weakening to collapse the A-pattern — planned by a strabismus surgeon, with post-operative follow-up confirming both the primary-position alignment and the resolution of the pattern and head posture.
Differential Diagnosis
- V-pattern esotropia — ruled out: that pattern (existing in another case) is more convergent in downgaze with inferior oblique overaction; here the pattern is reversed — divergent in downgaze.
- Basic intermittent exotropia without pattern — ruled out: the distance–near measurements alone miss it; the upgaze–downgaze disparity defines the A-pattern.
- Bilateral inferior oblique overaction — ruled out: the overacting muscles here are the superior obliques (over-depression in adduction), the mirror image.
- Dissociated vertical deviation — ruled out: DVD is an upward drift violating Hering’s law, not a horizontal alphabet pattern.
Management
Referral to a strabismus surgeon: typical plan combines horizontal surgery (bilateral lateral rectus recession for the exotropia) with bilateral superior oblique weakening (e.g. posterior tenectomy) to collapse the A-pattern. Counselling that both components are treated together; post-operative orthoptic follow-up of the pattern as well as the primary-position alignment.
Key Learning Points
- Alphabet patterns describe how the horizontal deviation changes between upgaze and downgaze: A-pattern is worse (more divergent) in downgaze; V-pattern is worse in upgaze.
- The oblique muscles drive the pattern: superior oblique overaction produces an A-pattern; inferior oblique overaction produces a V-pattern.
- Horizontal muscle surgery alone collapses the primary-position deviation but leaves the pattern — the oblique overaction must be addressed in the same plan.
- The compensatory head posture (chin up in A-pattern exotropia) is the patient’s own pattern management; surgery aims to remove the need for it.
- Measuring only in primary position misses alphabet patterns entirely — upgaze and downgaze measurements are mandatory in the strabismus workup.
Red Flags
- A pattern that appears after previous strabismus surgery — iatrogenic oblique dysfunction; the plan must account for altered anatomy.
- Marked asymmetry of the overaction — consider a unilateral rather than bilateral oblique procedure.
- Associated vertical deviation in primary position — the surgical plan must address it, not just the pattern.
- Ignoring the pattern and operating horizontally only — the classic cause of ‘successful’ surgery with persistent abnormal head posture.
Educational content only — not medical advice. Clinical decisions must be made by a qualified professional for the individual patient.
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