Extraocular myotoxicity of the retrobulbar anesthetic bupivacaine hydrochloride.

Morphopathological changes induced in the extraocular muscles by the local anesthetic agent bupivacaine hydrochloride were studied in the monkey using light and electron microscopy. Retrobulbar anesthetic blocks, using 0.75% bupivacaine hydrochloride, were performed in five adult cynomolgus monkeys. Morphological alterations in extraocular muscle fiber types were examined following survival periods of 3-27 days. Bupivacaine injections produced a mild and very limited myopathic response, with changes largely restricted to the global layer singly-innervated muscle fiber type which is characterized by low mitochondrial content. For survival times beyond 3 days, this fiber type exhibited peripheral migration and swelling of mitochondria and an outside-in pattern of myofibril dissolution. Some affected fibers also exhibited the Ringbinden or ring fiber pathology. Maximal myotoxic response was observed at 14 days after injections, and pathological changes were largely resolved by 27 days. A more limited analysis of the effects of retrobulbar injection of lidocaine revealed similar morphopathological responses, thereby suggesting that these effects are a property common to the entire class of aminoacyl anesthetics. In contrast to previous observations in other skeletal musculature, the extraocular muscles proved to be unexpectedly resistant to local anesthetic treatment as only limited alterations were observed. The observed muscle fiber type specificity was interpreted to result from differences in the relative ability of muscle fiber types to adapt to anesthetic-induced elevations of intracellular free calcium levels.