Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.
Myasthenia gravis means fatigable muscle weakness caused by autoantibodies against the muscle endplate, the postsynaptic side of the neuromuscular junction. Strength fades with repetition and returns with rest, often fluctuating through the day. Ocular and bulbar muscles are usually affected first, and reflexes and sensation stay normal throughout.
The notes below belong to one family. They cover how the diagnosis is provoked and confirmed, how treatment escalates from symptom control to immune therapy and thymectomy, and how the disease differs from its presynaptic counterpart, Lambert-Eaton myasthenic syndrome.
The junction in brief
A motor nerve ending releases acetylcholine into the synaptic gap, and receptors on the folded muscle membrane catch it and trigger contraction. In myasthenia gravis the release is normal but fewer receptors answer, because antibodies destroy, block, or internalise them and complement damages the folded membrane itself. Each repeated signal therefore finds fewer working receptors, so weakness grows with use and eases with rest. That single picture explains the defining symptom.
Antibody subtypes shape the disease
Most generalised disease is driven by antibodies against the acetylcholine receptor itself, found in roughly 85% of generalised cases and about half of purely ocular cases (Gilhus et al., 2019).
The remaining subtypes cluster among patients negative for receptor antibodies. Antibodies against muscle-specific tyrosine kinase (MuSK), a protein that clusters receptors at the membrane, produce a distinctive pattern: predominantly bulbar weakness with minimal ocular involvement, sometimes isolated head drop, a poor response to pyridostigmine and thymectomy, and a good response to rituximab. Antibodies against LRP4, another organiser of the junction, account for a fraction of double-negative cases and usually behave mildly. About 5–10% of patients are seronegative for all three and are diagnosed on clinical grounds plus electrodiagnosis.
Antibody levels do not track severity in a strict dose–response way, so a modest titre never excludes severe disease. A falling titre during treatment generally parallels clinical improvement.
The thymus connection
The thymus is where the autoimmune response is organised: thymic tissue expresses receptor-like proteins that activate T cells, which in turn drive B cells to produce the circulating antibodies. About 10–15% of patients have a thymoma, which can invade locally and must be removed regardless of its apparent contribution to the weakness.
Around 70% of patients without thymoma show lymphoid follicular hyperplasia of the thymus instead. Thymectomy lowers subsequent immunosuppressive requirements, with benefit most firmly established in acetylcholine-receptor-positive generalised disease without thymoma (Wolfe et al., 2016). It is generally not recommended in MuSK-positive disease, where the pathology is not thymus-driven.
The clinical pattern
Weakness usually appears in a descending order: ocular muscles first, then bulbar muscles, then proximal limbs. Ptosis is often asymmetric and diplopia comes from extraocular fatigue. Speech and swallowing fade through sustained use, so counting aloud or finishing a meal exposes what a brief examination misses. Limb weakness is proximal more than distal, with finger extensors disproportionately affected. Respiratory muscles are the life-threatening dimension, and infections commonly precipitate decline.
Three findings stay normal and help separate the disease from its mimics: muscle bulk is preserved early, tendon reflexes remain intact, and sensation is entirely unaffected. Symptoms are typically mildest in the morning and worst in the evening.
Severity framework
The Myasthenia Gravis Foundation of America classification grades the disease from purely ocular (class I) through mild, moderate, and severe generalised weakness (classes II–IV, with class IVb marking predominant bulbar or respiratory involvement) to crisis requiring intubation (class V). Ocular disease that has not generalised within about 2 years is statistically unlikely to do so later, but late generalisation remains possible, so the 2-year mark is a tendency rather than a guarantee.
Choose a route through the topic
- Myasthenia Gravis — Diagnosis: how fatigable weakness is provoked at the bedside, confirmed with electrodiagnosis and antibody testing, and separated from mimics, with chest imaging for thymic disease.
- Myasthenia Gravis — Treatment: symptom control, immunosuppression, thymectomy, targeted biologics, crisis management, and the medicines that can worsen weakness.
- MG versus LEMS: the postsynaptic-versus-presynaptic distinction across reflexes, exercise effect, autonomic features, cancer associations, electrodiagnosis, and first-line treatment.
Read the diagnosis note for the workup sequence, then treatment for the escalation logic, with the comparison alongside whenever proximal weakness could sit on either side of the junction.
Evidence anchors
- Gilhus NE, Tzartos S, Evoli A, Palazzo J, Burns TM, Verschuuren JJGM. Myasthenia gravis. Nat Rev Dis Primers. 2019;5(1):30. doi:10.1038/s41572-019-0079-y
- Sanders DB, Wolfe GI, Benatar M, et al. International consensus guidance for management of myasthenia gravis. Neurology. 2016;87(4):419-425. doi:10.1212/WNL.0000000000002790
- Wolfe GI, Kaminski HJ, Aban IB, et al. Randomized trial of thymectomy in myasthenia gravis. N Engl J Med. 2016;375(6):511-522. doi:10.1056/NEJMoa1600403
- National Institute of Neurological Disorders and Stroke. Myasthenia gravis: https://www.ninds.nih.gov/health-information/disorders/myasthenia-gravis