Sir,

The ability of adults with long-standing monocular visual loss and associated secondary strabismus to regain stereoacuity is poorly understood. Although there are reports of patients regaining a high level of binocular function many years following the onset of strabismus they all had a good level of visual acuity with no prior period of visual deprivation.1, 2, 3 We report the case of a patient who regained a high level of stereoacuity following correction of traumatic aphakia and secondary exotropia 27 years after a penetrating eye injury.

Case report

A 64-year-old female was referred for further assessment of a secondary exotropia associated with aphakia, which developed following a penetrating injury to her left eye 27 years earlier. Her acuity in this eye was hand movements only, although with an aphakic correction it improved to 6/12. However, with this correction she complained of constant horizontal diplopia and as a result she had never worn a contact lens. In addition, she had a secondary exotropia measuring 45 prism dioptres (Δ) for near and 30Δ for distance. Her visual acuity was 6/5 in her right eye with normal ophthalmic examination. There were concerns that if she underwent anterior segment reconstruction with secondary intraocular lens implantation followed by possible strabismus surgery she was at risk of intractable diplopia. She therefore had a botulinum toxin injection to her left lateral rectus muscle, which reduced her exotropia to 12Δ for near and 6Δ for distance. Following this she was encouraged to wear her contact lens as often as possible; whereas she was initially aware of intermittent diplopia this improved over a period of several weeks by which stage she was noted to have a non-specific exophoria with left suppression. These findings remained stable over a period of several months and it was therefore deemed to be safe to proceed with implantation of a secondary intraocular lens combined with iris prosthesis. Post-operatively her acuity gradually improved to 6/9 with orthoptic assessment revealing a non-specific exophoria. Although she was initially aware of diplopia this gradually settled and over the following 4 months her best corrected acuity improved to 6/6 and she regained 110 arc sec (Frisby). At her most recent clinic visit 1 year after her surgery her examination is unchanged.

Comment

Recovery of a high level of stereoacuity following such a long period of visual deprivation and longstanding strabismus is a very rare occurrence. Although there have been reports of patients regaining up to 40 arc sec of stereoacuity more than 30 years following the onset of their strabismus,1, 2 in contrast to our patient they all had acuities of at least 6/9 in the squinting eye with no reported history of visual loss. It is thought by some that the ability of patients with chronic acquired strabismus to regain binocular vision is dependent upon their previous capacity for binocularity.4 As our patient regained such a high level of stereoacuity it is reasonable to assume that this was present before her original injury. However, the fact that this occurred after such a long period of poor vision is most unusual and would indicate that the visual system has a surprising capacity for recovery.