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Tangent screens are still useful in the assessment of vigabatrin induced visual field defects
  1. JA Olson,
  2. AT Purdie,
  3. RJ Coleman
  1. The Eye Clinic, Aberdeen Royal Infirmary, Forester Hill, Aberdeen
  1. Correspondence to: Dr Olson; j.olson{at}arh.grampian.scot.nhs.uk

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Vigabatrin induced constriction of peripheral visual fields was first reported in 1997.1 The potential mechanisms of vigabatrin induced peripheral field constriction are many,2 but in our opinion, not enough attention has been paid to the method and appropriateness of recording this constriction. Current recommendations for patients prescribed vigabatrin are that they are screened at regular intervals by automated perimetry. Automated perimetry cannot, however, differentiate between pathological and functional (non-physiological) constriction of the visual field. Furthermore, automated perimetry, although deceptively simple for the operator to perform, is notoriously laborious and fatiguing for the patient. Although several safeguards are built into automated perimetry, in the form of reliability indices, there are traps for the unwary. This is clearly demonstrated in the recent case report where a 10 year old girl’s visual field constriction apparently reversed on cessation of vigabatrin.3 Baseline visual fields, performed with automated perimetry, showed a classic artefact cloverleaf-shaped pattern4 that was not recognised by the authors. Automated threshold perimetry involves checking the visual threshold of the retina at set intervals. To reduce the number of presented stimuli starting points for threshold determinations are made at four quadrants 9 degrees from the horizontal and vertical meridians. Not infrequently, poorly cooperative patients are only attentive during this initial stage resulting in a cloverleaf-shaped field. In this situation the reliability indices are of little help as the suprathreshold false negative reliability indices are based on already fatigued thresholded locations.4

Case report

A 30 year old woman was referred to the neuro-ophthalmology clinic in October 1998 for confirmation of vigabatrin induced constriction of visual fields. Her seizures had started at the age of 12 and consisted of sudden tonic posturing of the limbs preceded by left sided sensory symptoms. Initially she was having 12 seizures a week but by 1998 she was having 18 seizures a day. She was unable to tolerate phenytoin, valproate, carbamazepine, clobazam, lamotrogine, gabapentin, or topiramate and in 1990 had been started on vigabatrin. In 1998 she was referred for consideration of epilepsy surgery. At that time she complained of bumping into objects and she was noted on simple confrontation testing to have constricted visual fields. Automated perimetry was recommended and this was subsequently performed (Fig 1). Gross peripheral field constriction was noted but tangent screen examination at 1 and 2 metres revealed this to be non-physiological tubular visual field constriction (Fig 2).

Figure 1

Automated perimetry of a patient taking vigabatrin. Left, 1990, right, 1998.

Figure 2

The corresponding tangent fields performed at 1 metre with a 1 mm (broken line) white object and 2 metres with a 2 mm object (solid line). The 2 mm isoptre lies within the 1 mm isoptre, which is a non-physiological finding. (Top) Right eye, (bottom) left eye.

Comment

The best way to ascertain whether visual constriction is pathological or not is to test the patient at 1 and 2 metres using a wall mounted tangent screen.5 The visual field, whether constricted or not should be conical in shape and expand geometrically with increasing distances. Patients with functional visual field constriction can often be detected by the fact that on repeated testing of the visual field at an increased distance from the tangent screen they will not report this change in field diameter in an attempt to be consistent with their first field (tunnel visual field).5 This is not physiologically possible and is clear evidence of functional visual impairment.

Vigabatrin may well induce visual field constriction as a result of retinal toxicity but until studies are reported using tests of patients on vigabatrin at two viewing distances then this issue will remain open to debate.

References

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