The initial visual acuity was 20/80 OD, 20/600 OS, and 20/80 OU. He had a small esotropia in the left eye and a conjugate, horizontal, pendular, and jerk nystagmus. The null point was in primary position with maximum intensity on left gaze. There was no damping of the nystagmus on convergence. Slitlamp examination showed diffuse scarring and thickening of the right cornea and a clear, slightly eccentric left corneal graft. The fundi were normal.

After informed consent was obtained, he began taking 600 mg of oral gabapentin in incremental doses of 300 mg at weekly intervals until a total daily dose of 1200 mg was achieved. After 6 weeks of taking 1200 mg/d of gabapentin, he had not noticed a subjective improvement in his vision. The dose of gabapentin was increased to 2400 mg/d in divided doses. After 4 weeks of taking 2400 mg/d of gabapentin, he noticed an improvement in vision and was much more comfortable with close work. Visual acuity was 20/60 OD and 20/60 OU with no change in the left eye.

He remained at the same dose of gabapentin and his vision improved further, to 20/40^–3 OU after 8 weeks of treatment. He has maintained his visual acuity at that level with treatment for a follow-up period of 12 months. He noticed his symptoms worsening when he forgot to take the gabapentin while on vacation. He now feels much more confident crossing roads and believes that the amount of nystagmus in all directions of gaze has been reduced while receiving treatment. He did not notice any adverse effect of gabapentin.

Eye movement recorded with an infrared video pupil tracker (EyeLink eye tracker; SensoMotoric Instruments, Berlin, Germany) confirmed a reduction in the amplitude of his nystagmus compared with pretreatment (Figure). Foveation time per second was estimated using the same criteria throughout (±2°/s position window and ±4°/s velocity window), and best-corrected visual acuity was predicted using the expanded nystagmus acuity function (NAF[X]) (www.omlab.org).³ The nystagmus was most pronounced on left gaze (eg, at 20° eccentricity). The peak-to-peak amplitudes and frequencies were measured as follows: 6.0° and 5.5 Hz before treatment and 1.5° and 6.0 Hz after treatment. In right gaze (20° eccentricity), the nystagmus was 3.5° and 3.0 Hz before treatment and 1.0° and 3.0 Hz after treatment. In primary position, the nystagmus was 2.0° and 3.0 Hz before treatment and 1.0° and 3.5 Hz after treatment. The foveation time per second increased from 0.12 second to 0.92 second in primary position, 0.008 second to 0.60 second in right gaze, and 0.027 second to 0.27 second in left gaze. Using the NAF(X) to predict best-corrected visual acuity, this corresponded to improvements from less than or equal to 20/50^– to less than or equal to 20/25^– in primary position, less than or equal to 20/240^– to less than or equal to 20/25^– in right gaze, and less than or equal to 20/180^– to less than or equal to 20/85⁺ in left gaze.

View larger version (54K): [in this window] [in a new window] [as a PowerPoint slide]   Figure. Horizontal eye movement recordings of the right and left eye during saccades of 10° and 20° to the left and right. A, Before treatment. B, After treatment.

In the right (dominant) eye, the best-corrected visual acuity after gabapentin treatment was 20/40^–3, whereas the predicted NAF(X) was 20/25^–. The difference could be due to the corneal dystrophy or amblyopia acquired from early childhood nystagmus.

Comment
Gabapentin has been used in the treatment of acquired nystagmus in multiple sclerosis.⁴ It is thought to have several possible mechanisms of action, of which the most likely to be involved in nystagmus is its antiglutamatergic activity. Our study shows that gabapentin can reduce nystagmus in congenital nystagmus forms. This suggests that abnormalities in the glutamate and/or -aminobutyric acid system are involved in congenital nystagmus as well. Double-masked trials are needed to establish the effect of pharmacologic treatment on the various forms of congenital nystagmus.

AUTHOR INFORMATION
Correspondence: Dr Sarvananthan, Department of Ophthalmology, Faculty of Medicine and Biological Sciences, Robert Kilpatrick Clinical Sciences Building, The Leicester Royal Infirmary, PO Box 65, Leicester LE2 7LX, England (nagini{at}pathmanathan.com).

Financial Disclosure: None.