Chapter 9 The Lambert–Eaton myasthenic syndrome

Abstract

The Lambert–Eaton myasthenic syndrome (LEMS) is a rare autoimmune condition where weakness results from a presynaptic abnormality of acetylcholine (ACh) release at the neuromuscular junction. LEMS begins in later life. LEMS may be first discovered when prolonged paralysis follows the use of neuromuscular blocking agents during surgery. Small cell lung cancer (SCLC) is the most common associated cancer. The classical clinical findings of LEMS are weakness, particularly of proximal muscles, reduced or absent muscle stretch reflexes, and dry mouth. The diagnosis can be confirmed by demonstrating characteristic abnormalities on electrodiagnostic studies. Autonomic function tests are abnormal in most LEMS patients, even when there are no clinical signs of autonomic dysfunction. These tests show evidence of sympathetic or parasympathetic dysfunction. LEMS patients with SCLC may have other paraneoplastic syndromes, such as inappropriate ADH secretion (SIADH), sensorimotor neuropathy, or cerebellar degeneration. Clinical features for MG are ocular muscle weakness, limb weakness (predominating in arms), and normal muscle stretch reflexes. Therapy must be tailored for each patient, based on the severity of weakness, underlying disease(s), life expectancy and response to previous treatment. The prognosis in LEMS patients is determined by presence and type of any underlying cancer, presence and severity of any associated autoimmune disease, and severity and distribution of weakness. Overlapping features of MG and LEMS include: clinical features of one condition but facilitation, or lack thereof, typical of the other; clinical and electrodiagnostic patterns typical of one condition initially, but changing to the other later; or electrodiagnostic patterns typical of MG in one muscle, and of LEMS in another. The ultimate diagnosis in patients with mixed features of MG and LEMS may be moot because many treatments are the same for both conditions.