David C. Preston

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 IV 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 IV immunoglobulin has significantly increased compared to plasma exchange and thymectomy.

Iatrogenic complications and risks of nerve conduction studies and needle electromyography.

Electrodiagnostic procedures are routinely performed in patients with a variety of neuromuscular disorders. These studies are generally well tolerated and rarely thought to be associated with any significant side effects. However, needle electromyography is an invasive procedure and under certain situations has the potential to be associated with iatrogenic complications, including bleeding, infection, nerve injury, pneumothorax, and other local trauma. Similar complications are possible if needles are used for either stimulating or recording. In addition, like all other electrical devices and monitoring equipment connected to patients, electrodiagnostic testing carries the risk of stray leakage currents that under certain circumstances can result in electrical injury, especially in patients in the intensive care setting. Similarly, certain precautions are required during nerve conduction studies (NCS) in patients with pacemakers and other similar cardiac devices. In this review, we address the known and theoretical complications of NCS and needle electrode examination, and the possible methods to avoid such hazards. Muscle Nerve 27: 517–526, 2003

Entrapment and Compressive Neuropathies

Entrapment and compressive neuropathies of the upper and lower extremities are frequently encountered disorders in the office. Certain clinical clues in the history and examination, along with electrodiagnostic testing and imaging studies, often suggest the correct diagnosis. Some of the more common neuropathies are discussed, along with suggestions regarding testing and treatment.

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. NEUROLOGY 1996;46: 1604-1606

Lumbrical and interossei recording in severe carpal tunnel syndrome

Patients with severe carpal tunnel syndrome (CTS) may occasionally have absent median motor and sensory responses; in these cases, it is not possible to accurately localize the median mononeuropathy to the wrist using standard electrodiagnostic tests. We prospectively investigated the use of comparing the median motor latency to the second lumbrical and the ulnar motor latency to the interossei muscles in 28 patient hands with severe CTS and absent median motor and sensory responses. We found a prolonged latency difference in 92.8%. Along with its use in mild CTS, study of the lumbrical‐interossei latency difference is helpful in patients with severe CTS with absent median motor and sensory responses.

Electrodiagnosis of ulnar neuropathy at the wrist: Conduction block versus traditional tests

BackgroundCompared to ulnar neuropathy at the elbow (UNE), ulnar neuropathy at the wrist (UNW) is rarer and more difficult to localize with routine electrophysiologic studies. MethodsBy stimulating the ulnar nerve at the wrist and palm, and recording from first dorsal interosseous (FDI), the sensitivity and specificity of conduction block (CB) and slow conduction velocity (CV) of FDI fibers across the wrist was compared to traditional electrodiagnostic techniques for localization of UNW. Twenty patients with clinically defined UNW (due mainly to wrist trauma), 30 normal controls, and 20 disease controls with severe (n = 10) and mild (n = 10) UNE were evaluated prospectively. The upper (mean +2.5 SD) and lower (mean −2.5 SD) limits for all measurements were derived from the normal controls. ResultsThe UNW patients showed: slow wrist-palm FDI CV (<37 m/s) in 16 (80%); definite or probable CB in 14 (70%); prolonged distal latency (DL) to FDI (>4.5 milliseconds) in 12 (60%), to ulnar-innervated palmar interosseous (PI) versus median-innervated lumbrical (L) in 12 (60%), and to abductor digiti minimi (ADM) in 11 (55%). However, only CB and slow wrist-palm FDI CV (<37 m/s) were specific for UNW; prolonged DL to FDI was found in 4 patients (40%), to ADM in 4 patients (40%), and to PI in 1 patient (10%) with severe UNE. Overall, CB or slow wrist-palm FDI CV was present in 19 patients with UNW (95%). EMG failed to differentiate UNW from UNE, because forearm ulnar-innervated muscles were typically normal in UNW, but also often normal in mild UNE. ConclusionsIn UNW, an additional palmar stimulation site improves electrodiagnostic yield, and demonstrates that CB is an important cause of muscle weakness.

Isolated necrotizing myopathy associated with ANTI-PL12 antibody.

Immune‐mediated myopathies are a heterogeneous group of chronic autoimmune disorders. Autoantibodies associated with this disease complex are classified into myositis‐associated and myositis‐specific. Anti‐tRNA synthetase antibodies are the most well known of the myositis‐specific antibodies. Previous reports have revealed an association of tRNA synthetase autoantibodies with systemic connective tissue disorders.

Optimal recording electrode placement in the lumbrical-interossei comparison study.

The lumbrical–interossei comparison study is commonly employed in the electrodiagnosis of carpal tunnel syndrome. Placement of the recording electrodes relies on anatomic landmarks as the muscles being recorded cannot be seen or palpated. To determine the optimal active electrode location, 15 controls and 5 patients were studied using a grid of 12 electrodes placed over the lateral palm. Amplitudes, rise‐times, and latencies of the responses at each location were measured. The lowest control latency difference was located in the lateral mid‐proximal palm (mean 0 ms, upper range of 0.5 ms). This location also had the highest lumbrical amplitude and rise‐time. More distal in the palm, the mean latency difference increased to 0.5 ms with an upper range of 0.9 ms. When performing the lumbrical–interossei comparison study, it is essential to place the active recording electrode in the optimal location. Failure to do so will result in an increased number of false‐positive studies. Muscle Nerve, 2006

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.

Repetitive nerve stimulation and exercise testing

Repetitive nerve stimulation and exercise testing are useful in the evaluation of patients with suspected disorders of the NMJ and muscle membrane excitability when performed with close attention to technical factors. They can be very helpful in the diagnosis of myasthenia gravis. Lambert-Eaton myasthenic syndrome, and botulism, as well as rare disorders of skeletal muscle membrane excitability, including paramyotonia congenita, myotonia congenita, myotonic dystrophy, and the periodic paralyses.