Thymoxamine hydrochloride: an alpha-adrenergic blocker

Pigment dispersion syndrome and pigmentary glaucoma: a review and update

INTRODUCTION

Pigment dispersion syndrome (PDS) is a condition where anomalous iridozonular contact leads to pigment dispersion throughout the anterior segment and the released pigment is abnormally deposited on various ocular structures.

CLINICAL PRESENTATION

The clinical presentation of PDS is defined by the presence of pigmented cells on the corneal endothelium, an increase of pigmentation of the trabecular meshwork, and mid-periphery transillumination defects of the iris. This syndrome, more common in myopes, is usually bilateral and can be associated with ocular hypertension or glaucoma. Secondary open-angle pigmentary glaucoma (PG) can develop due to reduction of the outflow of aqueous humour and consequent increase in intraocular pressure leading to glaucomatous optic neuropathy. Diagnosis of PG is commonly between 40 and 50 years of age, occurring more frequently in men. The advent of ultrasound biomicroscopy and anterior segment optical coherence tomography has contributed to enhancing our knowledge on the condition. Typical alterations of the anterior segment are the posterior insertion of the iris and iris concavity. Treatment of PG should be initiated early to hinder disease progression, glaucomatous damage, and vision loss. Management is based on medical therapy, laser iridotomy, selective laser trabeculoplasty, and filtration procedures.

CONCLUSIONS

The differential diagnosis of PDS with other disorders can be challenging and awareness of the condition together with meticulous ophthalmologic examination allows early diagnosis followed by appropriate management strategies. The present review is a comprehensive report on the clinical characteristics, pathogenesis, current management, and status quo of PDS and PG.

Characterization of alpha(1)-adrenoceptor subtypes in the eye

The presence of the alpha(1)-adrenoceptor subtypes in various parts of the pig and rabbit eyes was investigated using [(3)H]-prazosin radioligand binding. The characterization of the subtypes was achieved by performing competition experiments with various subtype selective drugs. In the pig retina, both alpha(1A)- and alpha(1B)-adrenoceptors were detected and the proportion of sites was 70% alpha(1A)- and 30% alpha(1B)-adrenoceptors, respectively. In the pig iris, ciliary body and choroid, which are melanin-rich tissues, the non-specific binding of [(3)H]-prazosin was too high to detect any of the alpha(1)-adrenoceptor subtypes. However, in the albino rabbit iris, ciliary body and retina both alpha(1A)- and alpha(1B)-adrenoceptors were detected. The proportion of sites in the iris was 60 % alpha(1A)- and 40% alpha(1B)-adrenoceptors, respectively. In the ciliary body and rabbit retina the proportion of sites were 70% alpha(1A)- and 30% alpha(1B)-adrenoceptors. Only the alpha(1A)-adrenoceptor subtype was detected in the rabbit choroid.

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.

Current concepts in pigmentary glaucoma

Since its initial description over 50 years ago as a rare clinical entity, pigmentary glaucoma has become recognized as one of the most common forms of secondary open-angle glaucoma. Pigmentary glaucoma affects a much younger patient population than most other forms of open-angle glaucoma, and has a predilection for Caucasian males with myopia. Hallmarks of this disease include midperipheral iris transillumination defects, Krukenberg spindles and a heavily pigmented trabecular meshwork. The mechanism of pigment dispersion appears to be a rubbing between iris pigment epithelium and packets of lens zonules, possibly associated with an inherent abnormality of the pigment epithelium, and the mechanism of aqueous outflow obstruction is believed to involve accumulation of the pigment granules in the trabecular meshwork, followed by denudation, collapse, and sclerosis of the trabecular beams. Current management includes standard antiglaucoma drugs, laser trabeculoplasty, and filtering surgery, although research suggests the possibility of earlier intervention with medication or surgery to arrest the pigment dispersion and reverse or prevent the secondary glaucoma.

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.

Ocular pharmacokinetics of dapiprazole

Dapiprazole is a drug having specific alpha 1 adrenergic blocking properties. Following topical instillation on the eye, it crosses the corneal epithelium reaching high concentrations in the ocular tissue and producing a prompt miotic and hypotensive effect. The high concentration ratio between ciliary bodies and iris versus aqueous humor suggests a peculiar affinity for these structures containing adrenoceptors of the alpha type. The very low concentrations in the plasma, as compared to those after systemic administration, and in the fellow eye indicate that the systemic absorption is negligible.

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.

The effect of corynanthine on intraocular pressure in clinical trials

A single drop, dose-response, double-masked study of the effect corynanthine, a selective alpha 1 adrenergic antagonist, on intraocular pressure (IOP) was carried out in 10 symmetrically ocular hypertensive patients. Corynanthine 1% had no significant pressure lowering effect. Topical application of a 2% solution significantly (P less than 0.05) reduced IOP for at least eight hours; at five hours, mean IOP (+/- SEM) was 20.6 +/- 2.0 mmHg and 26.0 +/- 4.9 mmHg, comparing treated and control eyes, respectively. The 5% solution caused a significant (P less than 0.05) bilateral reduction in IOP, comparing treated and control eyes to baseline IOP respectively. Two percent corynanthine applied topically two or three times daily for one, two, or three weeks to seven patients with symmetrical ocular hypertension did not reduce IOP. Alpha adrenergic antagonists may have a role in the treatment of glaucoma.

Ectopia lentis

Ectopia lentis was first described more than 200 years ago, but its value as a significant diagnostic clue to the presence of other ocular and systemic disorders has only recently been appreciated. Ectopia lentis may cause a marked reduction in visual acuity, which varies with the type and degree of dislocation and the presence of other ocular abnormalities. An approach to the diagnostic evaluation of the patient with ectopia lentis is outlined. The differential diagnosis is reviewed in detail both clinically and histologically. The complications of ectopia lentis and the appropriate management of affected patients are discussed.