Effect of melanin on traumatic hyphema in rabbits.

OBJECTIVE: To investigate the role of melanin in influencing the clearance of traumatic hyphema and in the incidence of rebleeds following the hyphemas.

METHODS: Hyphemas were induced in 30 eyes of New Zealand white albino rabbits using an Nd:YAG laser. A total of 3.75 mg of synthetic melanin suspended in 0.1 mL of balanced salt solution was introduced into the anterior chambers of 16 animals. A total of 0.1 mL of balanced salt solution was injected into 14 control eyes. Hyphema levels were measured by a masked observer (V.D.B.) daily for 15 days. Pairs of animals were sacrificed at 1, 3, 5, 10, and 15 days and the eyes studied histologically.

RESULTS: Hyphemas were consistently produced in all eyes with mean+/-SD levels of 1.44+/-0.22 mm and 1.57+/-0.24 mm in the melanin-treated and control eyes, respectively. The clearance of hyphemas in the melanin-treated eyes was significantly prolonged throughout the study (P<.001). The rate of rebleed in the melanin-treated group was 18.8% and in the control group was 7.1% (P<.01). Histologically, both groups showed variable degrees of blood in the anterior chambers and trabecular meshwork. In addition, the melanin-treated eyes showed free melanin, melanin-laden macrophages, and an inflammatory response in the anterior chamber and trabecular meshwork that was greater than that in the control eyes. Melanin-treated eyes with rebleeds showed organized hemorrhage with neovascularization.

CONCLUSIONS: The presence of melanin results in a significantly prolonged course of hyphemas and may influence the rate of rebleeds. Occlusion of the trabecular meshwork with melanin-laden macrophages and inflammation may be the mechanisms responsible for these effects.

CLINICAL RELEVANCE: The release of melanin into the anterior chamber during ocular trauma may be partly responsible for the susceptibility of darker-pigmented individuals to more serious complications following a traumatic hyphema.