Reinnervation of extraocular muscles by facial-to-oculomotor nerve anastomosis in rats: Anatomic nuclear changes

OBJECTIVE: Oculomotor nerve palsy greatly impairs the patient's daily life. After oculomotor nerve injury, when the central nerve stump is not available, neurotization of the distal nerve stump with a donor nerve may be performed. Here, we present an experimental anatomic study in rats related to the motor nuclear organization after facial-to-oculomotor nerve anastomosis. METHODS: In adult rats, the right oculomotor nerve was transected at the skull base. Then, the ipsilateral facial nerve was exposed at the stylomastoid foramen and connected side-to-end to one extremity of a peroneal nerve autograft. The other extremity of the nerve autograft was connected end-to-end to the distal stump of the transected oculomotor nerve. Twelve weeks later, axonal regeneration in the autograft and brainstem somatotopic representation of the reinnervated extraocular muscles were investigated by use of histological and retrograde axonal tracing techniques. RESULTS: The autograft was reinnervated by a large number of small axons, 1 to 5 μm in diameter. After tracer injection into the superior rectus and medial rectus muscles, retrogradely labeled neurons were seen not only in the ipsilateral facial nucleus (16%) but also in the contralateral nucleus (8%). Labeled neurons were also seen in the ipsilateral abducens (12%), motor trigeminus (7%), trochlear (23%), and contralateral trochlear (34%) nuclei. In normal rats, the extraocular muscles are innervated by unilateral-ipsilateral brainstem motor nuclei, except for the superior rectus and superior oblique muscles, which are innervated by bilateral, primarily contralateral, nuclei. CONCLUSION: The central rearrangement of the extraocular muscle nuclei after facial-to-oculomotor nerve anastomosis represents an original example of plasticity. Functional studies are needed to demonstrate whether this procedure might serve to restore some degree of eye motility.