Wiley and colleagues (1992) investigated the effects of PB on a variety of measures of visual performance. Four healthy male pilots-in-training (mean age 23) were treated with 30 mg oral PB every 8 hours for 3 days. On the day prior to beginning treatment (day 1), subjects underwent measurements of spatial resolution ability, dark adaptation, refractive error, and oculomotor function. Testing was repeated on each of 3 days of treatment (days 2, 3, and 4) and then again on day 5 during which no drug was administered. Mean contrast sensitivity thresholds showed no significant change during PB treatment. Six tests of oculomotor function were performed, and PB did not affect four of these tests. Fusional divergence, however, did show an effect of PB with test distance (6 m versus 70 cm), a finding the authors thought could not be due to PB (although no alternative explanation was offered). Pupillary diameter and refractive error both demonstrated significant PB-associated changes with relative miosis and myopia, respectively. The authors concluded that at the study dosage, PB would not adversely affect an aviator’s visual ability. The investigators do acknowledge that longer periods of treatment might have increased effects on vision.

Alhalel and colleagues (1995) studied the use of 30 mg PB three times a day as an agent to prevent the ocular anticholinergic effects of double-dose transdermal hyoscine patches. The hyoscine (scopolamine) patch has been commonly used against motion sickness, but its use is limited by the anticholinergic symptoms of impairment of near vision and accommodation as well as mydriasis. Investigators studied 47 healthy men (ages 18 to 21) in a placebo-controlled double-blind fashion. Subjects were treated with either double-strength or placebo hyoscine patches and PB tablets or placebo. Treatment duration was 2 days. PB significantly ameliorated hyoscine-induced impairment of near vision and accommodation but did not prevent the mydriatic effect of the patch. The authors concluded that PB at this dose was an effective antagonist of some ocular anticholinergic effects (impaired near vision and accommodation) of the hyoscine anti-motion sickness patch.

In summary, studies of the effects of PB on ocular or visual function in healthy volunteers reveal at least mild effects on visual function, however, the specific effects, their mechanism of action, and their significance are unclear.

No severe PB-related changes in stationary visual acuity were observed in most studies, although some findings are suggestive of such an effect. Graham and Cook (1984) noted that visual acuity decreased as the percentage of plasma AChE inhibition increased, while an opposite relationship occurred with depth perception, which improved as AChE inhibition was decreased by PB treatment. Therefore, PB treatment seemed to augment depth perception while having a negative effect on visual acuity. The latter effect might be due to drug-related induction of mild myopia (nearsightedness), which was indirectly observed by Wiley and colleagues (1992), who found that PB prevented the hyperopia (farsightedness) caused by the hyoscine motion sickness patch.

With the usual CW pre-exposure dose and schedule, some studies found miosis (i.e., decrease in pupillary diameter) (Wiley et al., 1992). PB might also

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