Incidence and mortality rates of myasthenia gravis and myasthenic crisis in US hospitals.

OBJECTIVE: To determine the incidence and mortality rates and predictors of death in myasthenia gravis (MG) and MG crisis in a large US cohort. METHODS: Our cohort was identified from the Nationwide Inpatient Sample database for the years 2000 through 2005 using ICD-9-CM codes. MG crisis was identified by the principal diagnosis code or by the presence of respiratory failure. The incidence of MG was stratified by age, ethnicity, and gender. Multivariate logistic regression analysis was used to identify predictors of mortality in MG. For trend analyses of immune intervention, we used the Cochrane-Armitage test. RESULTS: After data cleansing, 5,502 patients with MG were included. In women, the incidence of admission was two to three times higher during the first 5 decades. In men, the incidence of admission was higher during the sixth, seventh, and eighth decades. The annual incidence rate of MG was higher in black women (0.01 per 1,000 persons/year) compared to white women and white and black men (0.009, 0.008, and 0.007 per 1,000 persons/year). The overall in-hospital mortality rate was 2.2%, being higher in MG crisis (4.47%). Older age and respiratory failure were the predictors of death, with adjusted odds ratios of 9.28 (95% confidence interval [CI], 3.31, 26.0) and 3.58 (95% CI, 2.01, 6.38). The trend of i.v. immunoglobulin utilization has increased compared to plasma exchange and thymectomy (p < 0.0001). CONCLUSION: Myasthenia gravis (MG) is still a disease of young women and old men, as reflected by the hospital admission rates. In-hospital mortality of MG is low. Hospital utilization of i.v. immunoglobulin has significantly increased compared to plasma exchange and thymectomy.

Localization of ulnar neuropathy with conduction block across the elbow.

We performed short segment incremental stimulation on 13 consecutive patients with ulnar neuropathy across the elbow (UNE) and conduction block. Conduction block occurred proximal to the medial epicondyle in 62%, at the epicondyle in 23%, and below the elbow in 15%. The ulnar nerve may be more prone to external compression above the elbow than previously recognized. Short segment incremental studies are useful to identify conduction block above the elbow in such patients.

Inflammatory myopathy in hemiatrophy resulting from linear scleroderma.

Hemiatrophy caused by linear scleroderma is a race clinical entity of unknown etiology. We present a patient with an inflammatory myopathy in hemiatrophy resulting from linear scleroderma.

Distal median neuropathies.

Distal median neuropathy is the most common entrapment neuropathy affecting the upper extremity. Although most often idiopathic, there are a large number of associated medical disorders. Electrophysiologic testing plays a central role in the diagnosis of distal median neuropathy in establishing localization; assessing severity; and excluding disorders of the proximal median nerve, plexus, and nerve roots which can mimic the clinical symptoms of distal median neuropathy. Treatment is usually successful, especially when diagnosis is established early in the course before any significant axonal loss has occurred.

Amplitude-dependent slowing of conduction in amyotrophic lateral sclerosis and polyneuropathy.

The mechanism of motor nerve conduction slowing in amyotrophic lateral sclerosis (ALS) is thought primarily to be loss of large, fast-conducting motor fibers; this is less certain in axonal polyneuropathy. We compared motor conduction studies in 64 patients with axonal polyneuropathy with 72 patients with ALS. Compound motor action potential amplitude, distal motor latency, and conduction velocity were converted to a percentage of the upper or lower limit of normal and then represented as a square root (SQRT) transformation, plotted with SQRT amplitude as the independent variable and SQRT latency or SQRT conduction velocity as the dependent variables. Regression analysis of the lower extremity nerve data showed that prolongation of latency and slowing of velocity were amplitude-dependent and were virtually identical in ALS and polyneuropathy. In the upper extremity, amplitude-dependent prolongation of latency was similar in both groups, but amplitude-dependent slowing of velocity was seen in ALS and not in axonal polyneuropathy. Our data support the hypothesis that the major mechanism of slowing is similar in both polyneuropathy and ALS and is the loss of large, fast-conducting fibers. However, the presence of distal but not proximal slowing in the upper extremity of axonal polyneuropathy suggests that additional mechanisms may be contributory.

Short-segment incremental studies to localize ulnar nerve entrapment at the wrist.

Precise electrophysiologic localization of ulnar neuropathy at the wrist (UNW) is often problematic. In the last year, we evaluated only two patients who presented clinically with UNW. Routine electrophysiologic techniques were nondiagnostic for UNW. In contrast, short-segment incremental studies (SSIS) across the wrist demonstrated conduction block and segmental slowing of nerve conduction across the wrist in both patients. We conclude that SSIS is a valuable tool for diagnosis, precise localization, and prognosis of UNW.

A case of Hodgkin's lymphoma producing neuromyotonia.

The syndrome of neuromyotonia produces muscle stiffness, cramps, and frequently, excessive sweating. Most cases are idiopathic, but some are associated with neoplasms, especially immune cell cancers. Voltage-gated potassium channels may be the target of an autoantibody attack in idiopathic generalized neuromyotonia (Isaacs' syndrome). The cases associated with neoplasms may also have an autoimmune etiology. We report the first case of neuromyotonia as the presenting feature of Hodgkin's lymphoma and propose a paraneoplastic mechanism that would link the purported autoimmune etiology in Isaacs' syndrome with the cancer-associated cases.

Myotonia in colchicine myoneuropathy.

Colchicine may induce a myoneuropathy in patients with renal insufficiency. To date, myotonia has not been described in this disorder. We recently studied 4 patients treated with routine doses of colchicine who, in the setting of renal insufficiency, developed a severe myoneuropathy characterized by prominent myotonic discharges on electromyography. In addition, 1 of the 4 patients had profound clinical myotonia. In the 3 patients in whom biopsies were performed, marked myopathic change with intracytoplasmic vacuolization was identified. All 4 patients improved rapidly with discontinuation of the medication. The patient in whom electrophysiologic studies were repeated had a complete resolution of the myotonic discharges. Colchicine myoneuropathy can present with prominent clinical and electrophysiologic myotonia that resolves completely with discontinuation of the medication.

Paraspinal muscle hematoma after electromyography.

There have been few reports of complications related to electromyography. Needle examination of certain muscles is sometimes avoided in patients taking anticoagulant agents, although no clear guidelines have been established. We describe a patient who was not receiving an anticoagulant and developed a large paraspinal muscle hematoma after routine electromyography. Subsequently, all patients who underwent paraspinal muscle electromyography and were diagnosed with radiculopathy at our institution over a 14-month period were reviewed. From this group, 17 patients were identified who had also underwent MRI of the appropriate spinal levels within 1 week after the needle examination. These images were reviewed for evidence of paraspinal muscle hematomas. Four small hematomas were identified in four different patients. None of these were radiologically significant compared with the large hematoma described in the case report. Radiologically apparent paraspinal hematomas after electromyography are an unusual complication of needle examination and do not appear to have any clinical significance. Nevertheless, the presence of these lesions justifies caution when considering electromyography of paraspinal and other deeper muscles in anticoagulated patients.

Delayed radiation-induced bulbar palsy.

We report a man with a slowly progressive bulbar palsy 14 years after radiation therapy for nasopharyngeal carcinoma. Electromyography demonstrated prominent myokymic and neuromyotonic discharges in muscles innervated by the lower cranial nerves. Late effects of radiation therapy can occur in the cranial nerve musculature that are similar to well-recognized syndromes affecting the brachial plexus and spinal cord.

Activity-dependent conduction in single motor units.

Vertebrate sensory and motor axons vary in their responses to submaximal stimuli as a function of time since prior activation. When two equal but submaximal stimuli are presented in pairs, the response to the second stimulus may be greater or less than the response to the first stimulus, depending on the interstimulus interval (ISI). We studied both the supernormal period (ISI between 6 and 25 msec) and the subnormal period (ISI between 25 and 100 msec) under conditions where only single motor axons were stimulated. Twenty single motor units from eight normal subjects were studied. The behavior of single units was very similar to that observed in compound motor action potentials, with the supernormal period lasting approximately 20 msec, followed by a subnormal period lasting at least 80 msec. Surprisingly, a supernormal period could be evoked by a stimulus that did not produce a response in the motor unit being studied; however, the presence of subnormality was dependent on an action potential being generated in response to the first stimulus. Based on these results, we conclude that the supernormal period does not require the opening of voltage-dependent ion channels, in contrast to the later occurring subnormal period.