[Oculomotor forms of myasthenia]

The diagnostic difficulty in purely oculo-motor forms of myasthenia can be due to the lack of fluctuation in symptoms, the occurrence of complete spontaneous remissions, lasting several months or years, the absence of a decremental response in muscle potentials as evidenced by classical repetitive stimulation of the motor nerve, and negative responses to pharmacological testing. Thus, several investigative procedures have been developed to aid in the diagnosis of questionable cases. These include electromyography of proximal muscles and single fiber recordings, the dosage of acetylcholine receptors antibodies (specificity of nearly 100 %), and histological examination of neuro-muscular junction (invariably finding specific lesions of motor end plates). In spite of the diffuse pathological process and the possible overlapping with other types of myasthenia, many oculo-motor forms remain strictly localized throughout their course. Furthermore the prognosis of oculo-motor myasthenia is far better than other forms. Several hypotheses exist to explain the peculiarities of ocular myasthenia. Among these explanations are the theories that there are structural factors specific to oculo-motor muscles and that there exist a heterogeneity of acetylcholine receptor antibodies. On the therapeutic grounds, the frequent failure of anticholinesterase agents has been established. Corticosteroids are often very efficacious, but require high maintenance doses. The adverse side effects of such therapy have led some authors to suggest thymectomy as an alternative therapeutic modality.