High concentration of dexamethasone in aqueous and vitreous after subconjunctival injection

PURPOSE: To determine the dexamethasone concentration in aqueous, vitreous, and serum of patients after a subconjunctival injection with dexamethasone disodium phosphate and to compare the effectiveness of a subconjunctival injection as a method of delivering dexamethasone into the vitreous with that of two previously tested routes: peribulbar injection and oral administration.

METHODS: In a prospective study, 50 phakic patients who underwent a pars plana vitrectomy received a single subconjunctival injection with 2.5 mg of dexamethasone disodium phosphate, aqueous solution (after topical anesthesia and a subconjunctival injection with lidocaine) at varied intervals before surgery. An aqueous and a vitreous sample were taken from each patient, and serum samples were collected at multiple time points from nine of 50 patients. Dexamethasone concentrations were measured by radioimmunoassay.

RESULTS: The estimated maximum dexamethasone concentration in the aqueous was 858 ng per ml at 2.5 hours after injection, and in the vitreous, 72.5 ng per ml at 3 hours. In serum, a mean maximum concentration of 32.4 ng per ml was measured at approximately 30 minutes after injection.

CONCLUSIONS: Subconjunctival injection of 2.5 mg of dexamethasone disodium phosphate resulted in an estimated vitreous dexamethasone peak concentration three and 12 times higher, respectively, than after a peribulbar injection of 5 mg of dexamethasone disodium phosphate and an oral dose of 7.5 mg of dexamethasone. Thus, a subconjunctival injection is the most effective method of delivering dexamethasone into both the anterior and posterior segments of the eye. Systemic drug absorption is considerable and is of the same order of magnitude as after peribulbar injection.