Subconjunctival gentamicin induction of extraocular toxic muscle myopathy

Subconjunctival injection of commercially available gentamicin, delivered in one of several protocols, caused an acute toxic reaction myopathy of the extraocular muscles. The response began as a small focal infiltrate of polymorphonuclear leukocytes, and as the lesion progressed it assumed a mononuclear cell predominance accompanied by muscle fiber degeneration. The injection of individual components of the commercial preparation (gentamicin, methylparaben, ethylenediaminetetraacetic acid (EDTA) and sodium bisulfite), each at the same concentration as in the parent solution, gave no response except for gentamicin alone. Gentamicin caused acute toxic myopathy when injected away from the muscles and when given as an isotonic solution in saline compared to the hypotonic commercial solution. Solutions at acidic pH not containing gentamicin did not initiate myopathy. Several factors that have been inferred as participating in the toxic response to gentamicin have been eliminated. Gentamicin alone is responsible for the induction of the extraocular muscle myopathy.