Autonomic dysfunction in the Lambert-Eaton myasthenic syndrome: serologic and clinical correlates

Neurophysiological studies of thin myelinated (A delta) and unmyelinated (C) fibers: application to peripheral neuropathies

Dysfunction of small fibers may appear in isolation or associated with large fiber lesions. In some acute neuropathies, such as pandysautonomia, small-fiber impairment is relatively pure but it may also appear in disorders with prominent somatic damage, such as Guillain-Barré syndrome, in which autonomic failure worsens the prognosis. At the present time, chronic idiopathic distal small-fiber neuropathy is diagnosed more frequently, and in some prevalent disorders, such as diabetic or amyloidotic polyneuropathies, small-fiber dysfunction is very noticeable. In pure autonomic failure, a peripheral autonomic failure exists, distinguishing it from multiple-system atrophy. Complex regional pain syndrome is a severe condition in which small fibers are responsible for disabling signs and symptoms, and only instrumental recordings lead to the proper treatment. Standard neurophysiological techniques evaluate large myelinated fibers exclusively. Small-fiber polyneuropathy has been considered as a type of somatic neuropathy, but thin myelinated and unmyelinated fibers are responsible not only for temperature and pain perception but also autonomic function. For instance, full autonomic evaluation is needed in some clinical situations such as autonomic failure in the elderly or orthostatic intolerance syndrome. To evaluate small-fiber impairment we need a battery of sensitive, reproducible, specific and noninvasive tests covering somatic and autonomic systems. In this review, we describe and analyze a number of neurophysiological techniques used to diagnose and characterize small-fiber dysfunction in humans. These include cardiovascular monitoring, sudomotor testing, pupillary responses and quantitative sensory tests, and also to some extent thermography and laser evoked potentials. The use of such techniques has proven useful not only for diagnosis, but also to guide adequate therapy and optimize follow-up.

Neuromuscular disease and calcium channels

Calcium channels that are regulated by voltage have an important role in linking cellular stimulation to physiological responses in the nervous system. In addition, a number of autoimmune or genetic disorders that affect calcium channels (channelopathies) have been identified, including several that affect neuromuscular function. These include the Lambert-Eaton myasthenic syndrome, which is associated with autoantibodies directed against neuronal calcium channels, as well as at least two inherited neuromuscular diseases-hypokalemic periodic paralysis and some varieties of malignant hyperthermia-that affect calcium channels in skeletal muscle. Preliminary progress has been achieved in understanding the relationship between these immunological or genetic abnormalities and the alterations in channel function that they produce. A major challenge that remains is to determine how calcium channelopathies lead to the curious assortment of paroxysmal and progressive disorders that are observed clinically.

Radioimmunoassay for glutamic acid decarboxylase (GAD65) autoantibodies as a diagnostic aid for stiff-man syndrome and a correlate of susceptibility to type 1 diabetes mellitus

OBJECTIVE

To establish and validate a double-antibody radioimmunoassay (RIA) for detecting serum auto-antibodies against glutamic acid decarboxylase (GAD65). This enzyme catalyzes synthesis of the neurotransmitter gamma-aminobutyric acid in neurons and pancreatic islet cells.

MATERIAL AND METHODS

We compared the frequency of GAD65 and other "thyrogastric" autoantibodies in adult patients with stiff-man (Moersch-Woltman) syndrome, type 1 diabetes, or polyendocrine disorders and in healthy subjects. The frequency of pancreatic islet cell antibody (ICA) detection was also assessed. The GAD65 RIA was validated by testing blinded samples, by confirming the specificity of low-titered positive results by "cold" antigen inhibition, and by comparing the RIA results with results of a kit assay incorporating staphylococcal protein A as immunoprecipitant. Recombinant GAD65 protein labeled with 125I was used as antigen, and a combination of anti-human IgG and IgM was used as immunoprecipitant. Seropositivity was determined for ICA and gastric parietal cell antibodies by indirect immunofluorescence assays and for thyroid peroxidase (microsome) and thyroglobulin antibodies by agglutination assays.

RESULTS

We detected GAD65-specific antibodies in all but 1 of 46 local patients with stiff-man syndrome (98%); 16 had evidence of diabetes. Positive values exceeded 20 nmol/L in 96%, and 89% were ICA-positive; 76% had additional thyrogastric antibodies. Of 41 patients with type 1 diabetes (17 local and 24 workshop serum specimens), 33 were GAD65 antibody-positive (80%); 85% of these positive values were 20 nmol/L or lower. Only 18% of sera from patients with type 1 diabetes were ICA-positive, but 59% had other thyrogastric autoantibodies. Of 20 patients with autoimmune endocrinopathies without diabetes or stiff-man syndrome, 35% were GAD65 antibody-positive, 5% were ICA-positive, and 90% were thyrogastric antibody-positive. Of 117 healthy control subjects, 8% were GAD65 antibody-positive, and a third of those had other thyrogastric antibodies (14% overall); none was ICA-positive.

CONCLUSION

Seropositivity in the double-antibody RIA for GAD65 autoantibody is a sensitive and specific marker of predisposition to type 1 diabetes and related organ-specific autoimmune disorders. As such, this RIA is complemented by assays for thyroid and gastric parietal cell autoantibodies.