Effect of thymoxamine on the human pupil

Effects of some ophthalmic medications on pupil size: a literature review

Ophthalmological pharmacology is a rapidly expanding field aimed at achieving the safest and most effective treatment results. Physicians must be aware of the side-effect profiles, both beneficial and harmful, of medications currently used. This review highlights the available data on the effect of some ophthalmic medications on pupil size; it was limited to all reports or studies describing topical ophthalmic agents not originally designed or indicated to alter pupil diameter. This awareness will protect patients from unwanted drug-induced side effects and will improve clinical management and patient care.

Efficacy of Bunazosin Hydrochloride 0.01% as Adjunctive Therapy of Latanoprost or Timolol

PURPOSE

To evaluate the ocular hypotensive response of bunazosin hydrochloride 0.01% administered as adjunctive therapy in patients with glaucoma who were already receiving latanoprost 0.005% or timolol 0.5%.

METHODS

Patients with primary open angle glaucoma who had received latanoprost (n = 60) or timolol (n = 60) for 6 months or longer were enrolled and prospectively randomized to receive additional administration of bunazosin or placebo. One hundred twenty eyes of 120 patients were thus divided into 4 subgroups of 30 patients each. Bunazosin was administered twice daily, and timolol or latanoprost was administered per label. The patients were followed up for 3 months. Responders were defined as having a reduction in intraocular pressure of greater than 2 mm Hg from baseline.

RESULTS

Mean baseline intraocular pressure was 22.3 +/- 3.0 mm Hg in the bunazosin subgroup and 22.3 +/- 3.1 mm Hg in the placebo subgroup of the latanoprost arm, and 22.5 +/- 3.5 mm Hg in the bunazosin subgroup and 22.3 +/- 3.0 mm Hg in the placebo subgroup of the timolol arm. In the bunazosin subgroups of both arms, intraocular pressure was significantly reduced compared with baseline measurements (P < 0.05) with mean intraocular pressure measurement reductions of 2.1 +/- 2.4 mm Hg and 2.8 +/- 2.1 mm Hg in the latanoprost arm and 2.6 +/- 2.1 mm Hg and 2.8 +/- 2.1 mm Hg in the timolol arm at 6 and 12 weeks after the start of the follow-up, respectively. In the latanoprost group, bunazosin provided a further reduction of intraocular pressure (7.7%) at 12 weeks from that initially obtained at 2 weeks (P = 0.0377). In the placebo subgroups of the latanoprost and timolol arms, no significant change was found between at baseline and at any visit after the start of the follow-up. In the latanoprost and timolol arms, there was a significant difference in intraocular pressure and its change between the bunazosin subgroup and placebo subgroup at any visit after 4 weeks from the start of the follow-up (P < 0.01).

CONCLUSION

Bunazosin hydrochloride 0.01% may provide an additional intraocular pressure reduction in patients already receiving latanoprost or timolol. Since adding bunazosin to eyes treated with latanoprost caused a relatively small hypotensive response at 2 weeks and provided a further reduction from 2 weeks to 12 weeks, longer than 4 weeks may be required to evaluate a clinically meaningful response to treatment. Further investigation on more cases and longer follow-up are needed.

Evaluation of the effect of thymoxamine solution 0.5% on mydriasis induced by ibopamine solution 1%

The effects of thymoxamine 0.5% solution and of a placebo solution (mannitol) on the mydriasis induced by ibopamine 1% solution were evaluated in 8 healthy volunteers and 12 patients with eye diseases. One drop of ibopamine was instilled into each eye and 30 min later 1 drop of thymoxamine was instilled into one eye and 1 drop of placebo into the contralateral eye. Pupillary diameter was measured before and 30 min after the instillation of ibopamine, immediately before the treatment with thymoxamine and placebo and 30, 60 and 90 min after the instillation of thymoxamine or of placebo. Within 30 min of treatment, ibopamine had produced a statistically and clinically significant mydriatic effect. In eyes treated with thymoxamine, prompt reversal of mydriasis was observed, the baseline diameter being observed within 60 min. No difference in the time-course of the mydriatic effect was detected between healthy subjects and patients. The pupillary response to thymoxamine was not influenced by the colour of the iris. The tolerability of ibopamine and of thymoxamine was good. No local or systemic adverse events were seen or reported.

The trigemino-pupillary reflex: a model of sensory-vegetative integration

Trigeminal stimulation can induce pupillary changes. In vivo and in vitro studies have demonstrated that electrical impulses applied at the trigeminal level can provoke a miotic response, whose nature has been ascribed to the anti-dromic release of neuropeptides (substance P in particular). In order to better define the pupil response to trigeminal stimulation, we investigated the human pupil response to quantified (painless and painful) corneal stimuli by means of a combined (neurophysiological and pharmacological) technique. The response to corneal stimulation was bilateral, direct and consensual. It had a biphasic progression with an initial mydriasis (which directly correlated with the stimulus intensity), followed by a miotic phase. The mydriatic phase disappeared after thymoxamine application, while homatropine pre-treatment prevented occurrence of the miotic phase. The data obtained indicate that the pupillary response to corneal stimulation (trigemino-pupillary reflex) is a multisynaptic reflex with an afferent branch involving the trigeminal system, and an afferent branch involving both the sympathetic and the parasympathetic system. Other pathways, such as the SP-mediated release of acetylcholine, cannot be excluded. Thus the reflex appears to be a potentially useful tool for investigating pain/vegetative interactions in various clinical conditions. In turn, the description of its changes in pathologies characterized by a sympathetic/parasympathetic deficit or by a SP-ergic imbalance will allow us to better describe its inner mechanisms.

The influence of iris pigmentation on the miotic effect of thymoxamine

We performed a randomly assigned, double-masked, placebo-controlled study in 78 patients with varied iris pigmentation to evaluate the influence of iris pigmentation on the ability of 0.1% thymoxamine to reverse mydriasis produced by 2.5% phenylephrine. Patients were chosen so that a 1.6:1 ratio of dark to light irides was obtained. Within one-half hour after medication, thymoxamine-treated nonbrown irides constricted significantly compared to their fellow placebo-treated irides (P less than .001). Thymoxamine-treated pupils of nonbrown irides were 1.0 to 3.1 mm smaller than placebo-treated fellow eyes. Thymoxamine-treated light brown irides constricted less (0.6 to 2.0 mm) and more slowly compared to fellow placebo-treated irides. Thymoxamine did not reverse the mydriasis in eyes of patients with dark brown irides. Thymoxamine appears similar to other adrenergic agents that bind to melanin, delaying onset and strength of action. Its efficacy as presently formulated may be limited, in part, by iris color.

The effect of 0.5% thymoxamine on the pattern-onset electroretinogram

Thymoxamine constricts the pupil without significant effect on accommodation and was used in this study because of the extreme sensitivity of the pattern electroretinogram (PERG) to defocus at high spatial frequencies. Pilocarpine, used in previous studies, can cause an unpredictable spasm of accommodation. A reduction in pupil size has two effects on the PERG due to illuminance and optical degradation. Illuminance differences can be compensated by neutral density filters. This allows the effects of optical degradation on the PERG to be examined under conditions of constant adaptation. Our studies on a group of 10 subjects have shown the PERG to be proportional to contrast. As contrast degradation is reduced with a smaller pupil, a larger PERG amplitude is predicted. However the tuning of neural elements under constant adaptation should not alter. Pattern onset ERGs were recorded from a further 10 subjects in response to checkerboards of various spatial frequencies under two conditions, a) with pupils moised by thymoxamine hydrochloride 0.5% and b) with natural pupils. Adaptation level was equated by a 0.4 neutral density filter before the natural pupils. The group-averaged PERGs were analyzed into retinal illuminance and pattern-specific responses. A paired t-test showed the pattern-onset ERG amplitudes to be significantly different in the two conditions, but the computed pattern-specific responses were virtually identical. These results therefore support the theory and method used for the extraction of the pattern-specific response.

ALO 2145 reduces the intraocular pressure elevation after anterior segment laser surgery

The effect of an alpha adrenergic agonist, ALO 2145 (para-amino-clonidine [PAC]), was examined in a double-masked, multicenter study on the postoperative intraocular pressure (IOP) elevation after laser surgery in 165 patients (83 trabeculoplasty, 36 iridotomy, and 46 capsulotomy). One drop of 1.0% ALO 2145 or vehicle was instilled 1 hour before and immediately after laser surgery. The mean IOP increase in the ALO 2145-treated group was lower (P less than 0.05) than in the placebo group at each of the first three postoperative hours. Overall, 18% of placebo-treated eyes experienced IOP increases greater than or equal to 10 mmHg as compared with 4% of ALO 2145-treated eyes (P less than 0.003). Ocular and systemic side effects were minimal and did not differ between the treatment groups. ALO 2145 safely and effectively reduced the incidence and magnitude of potentially harmful IOP elevations after anterior segment laser surgery.

Thymoxamine: a miotic for intraocular use

Thymoxamine hydrochloride can be used intraocularly to reverse phenylephrine or epinephrine mydriasis during intraocular surgery (0.2-0.5 ml of 0.01% or 0.02% solutions proved to be effective). No endothelial damage was found up to a dose of 1 ml of 0.02% thymoxamine in animal experiments using a physiologically buffered solution. Since stability is low in the buffered state, the final concentration has to be prepared at the time of surgery using a 0.5% solution and a phosphate buffer. Intracameral thymoxamine is a potent miotic during operations and can be helpful in extracapsular cataract surgery, keratoplasty, and repair of perforating injuries.

Effect of clonidine on aqueous humor flow in normal human eyes

A randomized, double-blind, placebo-controlled study of the effect of topical 0.125% clonidine, a relatively specific alpha 2-adrenergic agonist, on the rate of aqueous humor flow in the eyes of 24 normal human subjects was performed with fluorophotometry. The effect of clonidine on intraocular pressure, anterior chamber volume, pupil size, pulse, and blood pressure was also studied. Clonidine-treated eyes had a 2 mmHg decrease in intraocular pressure which was statistically significant. Fellow placebo-treated eyes had a smaller but significant decrease in pressure. However, between clonidine and fellow placebo-treated eyes, no significant difference was found in intraocular pressure. Aqueous humor flow was 21% lower in clonidine-treated eyes as compared to fellow placebo-treated eyes, 1.9 microliter min-1 as compared to 2.4 microliters min-1. No significant difference was found in anterior chamber volume, blood-aqueous barrier diffusion constant, and apparent resistance between clonidine and fellow placebo-treated eyes. Clonidine produced a significant miosis of 0.4 mm in treated eyes and miosis of 0.2 mm in fellow eyes. Topical 0.125% clonidine significantly decreased systolic blood pressure by 3 mmHg, but did not affect diastolic blood pressure or pulse.