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.

Neck muscle afferent projections to the brainstem of the monkey: implications for the neural control of gaze.

Brainstem projections of first-order afferent neurons that innervate the suboccipital muscles of the monkey have been determined by using the technique of transganglionic transport of wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA/HRP) and HRP. Neck muscle afferents distribute to several distinct regions located within the caudal brainstem and rostral spinal cord. Terminal labeling was heaviest within the ventral portion of the ipsilateral lateral cuneate nucleus. Muscle afferent terminals also distributed to ventrolateral portions of the pars triangularis division of the cuneate nucleus. Projections were consistent with the known somatotopic (i.e., both place and modality) organization of the cuneate nucleus. Moreover, neck muscle projections to the cuneate nucleus were, in part, coincident with those previously demonstrated for the extraocular muscles (Porter: J. Comp. Neurol. 247:133-143, '86). Sparse terminal projections were noted in the central cervical nucleus. In addition, light terminal labeling was present in group x of the vestibular complex and in an ill-defined region along the lateral margin of the brainstem. Present observations, which provide the first complete description of the central distribution of neck muscle afferent neurons in the primate, may contribute to the known substrate for eye/head coordination.