An optometrist refers a 9-year-old boy with infantile nystagmus because ‘the acuity keeps changing’ — 6/24 on one visit, 6/15 on the next, 6/30 when he is tired. The parents are confused about his true vision and what it means for school support. The eyes are otherwise healthy.
Examination Findings
Idiopathic infantile nystagmus with a 15-degree left null zone. Acuity varies with head position, fatigue, and chart type: best 6/15 sustained in the null with single optotypes, dropping to 6/30 when tired or rushed. Near acuity N6 with the null. Refraction normal, fundi normal.
Investigations
The ‘investigation’ is technique: acuity measured in the null position, with the head turn allowed, using single-optotype or crowded-optotype comparisons, without time pressure, and at consistent times of day. Documenting the null and the best sustained acuity standardises the record.
Questions to Think About
- Why does his acuity change between visits?
- What is the correct technique for a meaningful acuity in nystagmus?
- Which number should go on the school report?
Diagnosis
Idiopathic infantile nystagmus; acuity variability is physiological and technique-dependent.
Reasoning
Reasoning: Nystagmus acuity depends on foveation time — how long the eyes rest on target each cycle — which varies with null-zone use, fatigue, stress, and chart crowding. Forcing the head straight or rushing the test measures the worst, not the typical, vision. The meaningful number is the best sustained acuity in the habitual null position, measured without time pressure. Diagnosis: idiopathic infantile nystagmus — acuity variability is physiological. Management: typical management is standardised testing technique — allow the head turn, test in the null, use age-appropriate crowded and single optotypes, avoid fatigue, and record the conditions. The school report carries the best sustained acuity with a note on variability and fatigue, plus practical recommendations: extra time in exams, seating favouring the null side, and large-print options when tired. Parents are reassured that variability is expected and does not mean deterioration.
Differential Diagnosis
- Variable measured acuity from nystagmus technique — confirmed: the variation follows null use, fatigue, and chart crowding, not pathology.
- Progressive visual loss — ruled out: best sustained acuity is stable across visits; variation is methodological.
- Malingering or inattention — ruled out: the pattern is physiological and reproducible.
Management
Standardised null-position testing without time pressure; report best sustained acuity with variability noted; school accommodations (extra time, seating, print size).
Key Learning Points
- Nystagmus acuity varies with null use, fatigue, and crowding — standardise the technique.
- Allow the head turn and test in the null: that is the patient’s real-world vision.
- Report the best sustained acuity with a variability note, not a single rushed number.
Red Flags
- True decline in best sustained acuity — that is progression or new pathology, not nystagmus variability.
- Testing with the head forced straight — measures the worst acuity and misleads everyone.
- A single rushed acuity defining a child’s support — always confirm with proper technique.
Educational content only — not medical advice. Clinical decisions must be made by a qualified professional for the individual patient.
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