Myasthenic syndrome and myasthenia gravis are two disorders of neuromuscular transmission that result in muscle weakness and fatigue. They differ in pathophysiology and their symptoms. Myasthenia gravis (MG) is an autoimmune disorder caused by circulating antibodies that block acetylcholine receptors at the postsynaptic neuromuscular junction (NMJ). MG has early signs of ocular and bulbar nerve involvement followed by extremity weakness. This results in a rapid decline in the number of action potentials and overall contractile strength with repeat use of said muscle. Medication that improves strength in myasthenia gravis includes anticholinesterases such as neostigmine and pyridostigmine. These patients are very sensitive to NMBAs.
Myasthenic syndrome (sometimes referred to as Lambert-Eaton syndrome) is an autoimmune disorder where there are antibodies formed against the voltage-gated calcium channels in the NMJ. Roughly 60% of patients with LEMS have an underlying malignancy, most commonly SCLC making this a “paraneoplastic syndrome.” Symptoms of LEMS improve with successful treatment of the underlying malignancy. Weakness in LEMS involves the arms and legs; with leg involvement more pronounced than in MG. Proximal musculature (hip and shoulder girdle) is predominately affected making climbing stairs difficult. Unlike MG eye muscle weakness is uncommon. 75% of LEMS patients have involvement of the autonomic nervous system resulting in ataxia, impaired sweating, orthostatic hypotension, and metallic taste in mouth. A key difference between LEMS and MG is that the weakness and reflexes on neurologic examination improves with repeated testing.