[Electrophysiologic data on myasthenic syndromes of the Lambert-Eaton type. A series of 23 cases]

Trouble at the junction: When myopathy and myasthenia overlap

Although myopathies and neuromuscular junction disorders are typically distinct, their coexistence has been reported in several inherited and acquired conditions. Affected individuals have variable clinical phenotypes but typically display both a decrement on repetitive nerve stimulation and myopathic findings on muscle biopsy. Inherited causes include myopathies related to mutations in BIN1, DES, DNM2, GMPPB, MTM1, or PLEC and congenital myasthenic syndromes due to mutations in ALG2, ALG14, COL13A1, DOK7, DPAGT1, or GFPT1. Additionally, a decrement due to muscle fiber inexcitability is observed in certain myotonic disorders. The identification of a defect of neuromuscular transmission in an inherited myopathy may assist in establishing a molecular diagnosis and in selecting patients who would benefit from pharmacological correction of this defect. Acquired cases meanwhile stem from the co-occurrence of myasthenia gravis or Lambert-Eaton myasthenic syndrome with an immune-mediated myopathy, which may be due to paraneoplastic disorders or exposure to immune checkpoint inhibitors.

Distinguishing Features of the Repetitive Nerve Stimulation Test Between Lambert-Eaton Myasthenic Syndrome and Myasthenia Gravis, 50-Year Reappraisal

OBJECTIVE

To reappraise the distinguishing features of the repetitive nerve stimulation (RNS) tests in the abductor digiti quinti muscle between myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) 50 years after the 1965's Lambert seminal paper.

METHODS

The various parameters of the RNS test were compared between 34 patients with LEMS and 140 patients with MG to assess their diagnostic sensitivity.

RESULTS

RNS test was abnormal in all (100%) patients with LEMS and 76 (54%) patients with MG. The diagnostic hallmark of LEMS, ≥60% increment at high-rate stimulation or postexercise facilitation, was present in 33 (97%) of patients. The diagnostic hallmark of MG, decrement at low-rate stimulation (LRS), was present in 65 (46%) of patients with MG. The most prominent difference in the various parameters of RNS tests was noted between LEMS versus MG and normal controls.

CONCLUSIONS

Distinguishing features of the RNS test in LEMS and MG are confirmed in this direct comparison study: the low compound muscle action potential amplitude, decrement at LRS, and increment at high-rate stimulation or postexercise facilitation for LEMS and normal compound muscle action potential amplitude and decrement at LRS for MG.

Autoimmune paraneoplastic syndromes associated to lung cancer: A systematic review of the literature Part 4: Neurological paraneoplastic syndromes, involving the peripheral nervous system and the neuromuscular junction and muscles

The development of new immune treatment in oncology and particularly for lung cancer may induce new complications, particularly activation or reactivation of auto-immune diseases. In this context, a systematic review on the auto-immune paraneoplastic syndromes that can complicate lung cancer appears useful. This article is the fourth of a series of five and deals mainly with neurological paraneoplastic syndromes involving the peripheral nervous system and the neuromuscular junction and muscles.

Respiratory failure in lambert-eaton myasthenic syndrome precipitated by calcium-channel blockers: report of a case and literature review

There have been a few reports in the literature documenting the effects of calcium-channel blockers in Lambert-Eaton myasthenic syndrome (LEMS). We report a case of a patient with LEMS who developed respiratory failure after administration of diltiazem and verapamil. A patient with LEMS, associated with small-cell carcinoma in remission, developed multifocal atrial tachycardia, necessitating the use of diltiazem and verapamil, which led to respiratory failure and intubation 5 days after admission. The patient was taken off all calcium-channel blockers and weaned off the respirator after 6 days. Oral time-release diltiazem was restarted without further exacerbation of LEMS. Most likely the combined use of two different calcium-channel blockers, verapamil and diltiazem, was responsible for this patient's respiratory failure. To date, 5 cases (2 patients with LEMS and 3 with myasthenia gravis) in which cardiac calcium-channel blockers had a definite detrimental effect have been reported.

Ten-second exercise is superior to 30-second exercise for post-exercise facilitation in diagnosing Lambert-Eaton myasthenic syndrome

An incremental response after brief exercise or high-rate stimulation on the repetitive nerve stimulation (RNS) test is a critical diagnostic criterion for Lambert-Eaton myasthenic syndrome (LEMS). This prospective study was performed to determine what duration of exercise shows the highest diagnostic sensitivity for LEMS. The compound muscle action potential amplitude in the abductor digiti quinti muscle was obtained at rest and after 5 s, 10 s, 15 s, 20 s, 25 s, and 30 s of exercise. Incremental responses were compared for the different exercise durations in 24 studies performed in nine LEMS patients. The increment was highest with 5-s and 10-s exercises (244%-243%) and lowest with 30-s exercise (84%). A gradual decrease in the increment was noted from 5- to 30-s exercise. A significant difference in the increment was noted between 5- to 10-s and 20-s to 30-s exercise. There was significantly higher diagnostic sensitivity with the 10-s exercise compared with 30-s exercise at 100% increment and 60% increment levels. Higher increment and diagnostic sensitivity were achieved with 10-s exercise than with 30-s exercise. Thus, 10-s exercise should be the standard protocol for the RNS test for LEMS.

Electrophysiological diagnostic criteria of Lambert-Eaton myasthenic syndrome

Various parameters of the repetitive nerve stimulation (RNS) test of the abductor digiti quinti muscle were analyzed statistically in 34 patients with Lambert-Eaton myasthenic syndrome (LEMS). The sensitivity and specificity of the increments after exercise and after 50-HZ stimulation for the diagnosis of LEMS were compared with reference values in 40 normal subjects and data from 538 tests in patients with myasthenia gravis (MG). When we used a 100% increment (the "gold standard") as the normal limit for the postexercise facilitation (PEF) or the high-rate stimulation (HRS) test, the diagnosis of LEMS was confirmed in 29 (85%) cases. When a 60% increment was used as the normal limit, the diagnosis of LEMS was made in 97% of cases. In MG, a 60% increment was observed in only 4 of 538 cases by HRS and in none by the exercise test. Thus, the use of a 60% increment showed a sensitivity of 97% for the diagnosis of LEMS and a specificity of 99% in excluding MG. A 60% increment in either the PEF or HRS test for the diagnosis of LEMS is a desirable alternative to the 100% increment previously considered to be the gold standard for this diagnosis.

[Lambert-Eaton syndrome and small cell cancer. Etiopathogenic considerations apropos of 2 cases]

The authors report two cases of Lambert-Eaton myasthenic syndrome associated with small cell lung carcinoma. Following the observations, the clinical diagnosis of this syndrome is considered. We discuss the autoimmune pathogenesis and the relation between paraneoplastic syndrome and small cell cancer. This syndrome is caused by autoantibodies that block the voltage-dependent calcium channels at motor nerve terminals. Small cell carcinoma cells appear to express calcium channels, suggesting that autoantibody production may be triggered by tumor calcium channels determinants. The autoimmune paraneoplastic syndrome theory refers to cross-antigenicity.