Decrement pattern in Lambert–Eaton myasthenic syndrome is different from myasthenia gravis

• Published in: Neuromuscular disorders : NMD Vol. 16; no. 7; pp. 454 - 458
• Main Authors: Baslo, M. Baris, Deymeer, Feza, Serdaroglu, Piraye, Parman, Yesim, Ozdemir, Coskun, Cuttini, Marina
• Format: Journal Article
• Language: English
• Published: England Elsevier B.V 01.07.2006
• Subjects: Adult; Decrement; Diagnosis, Differential; Electric Stimulation; Electromyography; Female; Humans; Lambert-Eaton myasthenic syndrome; Lambert-Eaton Myasthenic Syndrome - diagnosis; Lambert-Eaton Myasthenic Syndrome - physiopathology; Male; Middle Aged; Myasthenia gravis; Myasthenia Gravis - diagnosis; Myasthenia Gravis - physiopathology; Neural Conduction; Repetitive nerve stimulation; Retrospective Studies; Lambert-Eaton myasthenic syndrome; Myasthenia gravis; Decrement; Repetitive nerve stimulation
• ISSN: 0960-8966; 1873-2364
• DOI: 10.1016/j.nmd.2006.05.009

The decrement pattern at low rates of repetitive nerve stimulation in myasthenia gravis (MG) is characterized by a decrease of compound muscle action potential size within the first 4–5 responses. With subsequent stimuli, compound muscle action potential size either increases or does not change. Following an observation that the pattern of decrement might be different in patients with Lambert–Eaton myasthenic syndrome (LEMS), we retrospectively studied traces from eight LEMS patients and 14 patients with seropositive generalized MG, calculating decrement percent from first to fourth and from first to ninth compound muscle action potential. In the LEMS patients, compound muscle action potential amplitude decreased progressively from first to ninth stimulus at 2, 3 or 5 Hz in all traces but one. In contrast, MG patients demonstrated the expected improvement after the initial decrement in all traces except one. In the evaluation of patients suspected of having myasthenia gravis, the finding of progressive decrement pattern at low rates of repetitive nerve stimulation may alert the electromyographer to the possibility of Lambert–Eaton syndrome and prompt the performance of further electrodiagnostic tests.