Article Text

An unusual cause of asthenopia: “pseudo-accommodative insufficiency” associated with a high AC:A ratio
  1. CHARLOTTE ANNE HOUSTON
  1. Department of Orthoptics, Gartnavel General Hospital, 1053 Great Western Road, Glasgow G12 0YN, UK
  2. Department of Ophthalmology
  1. DAVID JONES,
  2. CLIFFORD R WEIR
  1. Department of Orthoptics, Gartnavel General Hospital, 1053 Great Western Road, Glasgow G12 0YN, UK
  2. Department of Ophthalmology
  1. Charlotte Anne Houston

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Editor,—Asthenopia is characterised by ocular fatigue, frontal headache, and blurred vision, particularly during periods of sustained close work.1 It can be caused by accommodative insufficiency, a condition in which the effort required to maintain accommodation for near objects produces troublesome symptoms. This may be associated with a low accommodative convergence: accommodation (AC:A) ratio, which the patient has to overcome using positive fusional reserves. When fusion is insufficient symptoms of asthenopia can occur. In contrast, a high AC:A ratio would not normally be associated with asthenopia, but rather with overconvergence, potentially resulting in a convergence excess type of esotropia.2 We report two cases in which “pseudo-accommodative insufficiency” was identified as the cause of asthenopia and paradoxically associated with a high AC:A ratio.

CASE REPORTS

Case 1

A 12 year old male patient was referred complaining of difficulty with reading. His visual acuities were 6/6, N4.5 in the right eye, and 6/5, N4.5 in the left eye. Cycloplegic refraction showed no significant refractive error. A cover test revealed a 2 prism dioptre exophoria both for distance and near, and he had a full range of ocular motility. Convergence as measured using the RAF rule was well maintained to 6 cm, but accommodative amplitude for an N5 target was only 9 dioptres. It was noted at this stage that when accommodation failed a right esotropia developed transiently. The negative fusional vergence was 4 prism dioptres base-in for near, and 6 prism dioptres base-in for distance. Positive fusional vergence was 40 prism dioptres base-out for both near and distance. The AC:A ratio, measured using the distance gradient method, was 11:1. He was treated with exercises to build up negative fusional vergence.

Case 2

A 20 year old man was referred complaining of blurred vision for near work, associated with frontal headaches. His visual acuities were 6/5, N5 in each eye. Cycloplegic refraction showed no significant refractive error. A cover test revealed a 2 prism dioptre exophoria for near and a 4 prism dioptre exophoria for distance. He had a full range of ocular motility. Convergence as measured using the RAF rule was well maintained to 6 cm, but accommodative amplitude for an N5 target was only 8 dioptres. Again it was noted that when accommodation failed a right esotropia developed transiently. The negative fusional vergence was 12 prism dioptres base-in for near, and 4 prism dioptres base-in for distance. Positive fusional vergence was 20 prism dioptres base-out for near and 14 prism dioptres base-out for distance. The AC:A ratio, measured using the distance gradient method, was 12:1. He was treated with exercises to build up negative fusional vergence.

COMMENT

We believe that we have identified a cause of asthenopia, which paradoxically, is associated with a high AC:A ratio. Although both patients had signs and symptoms, which initially were suggestive of accommodative insufficiency, the sudden transient esotropia that was observed while testing accommodation, together with the high AC:A ratio, indicates that this diagnosis was incorrect. We speculate that these patients choose to relax their accommodation in order to maintain binocular single vision, but at the expense of clarity of vision for near work. If they continued to accommodate their high AC:A ratio resulted in a greater angle of esophoria, and when their negative fusional vergence reserves were no longer sufficient to compensate then a manifest deviation developed. We have coined the term “pseudo-accommodative insufficiency” to describe this phenomenon.

When assessing patients with asthenopic symptoms it is important to distinguish between true accommodative insufficiency and “pseudo-accommodative insufficiency”. Treatment for the former using convex lenses would be inappropriate for the latter, as it does not address the underlying cause of the problem. Instead, management should be aimed at augmenting negative fusional reserves and negative relative vergence.

References

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