Myasthenia gravis is a chronic autoimmune neuromuscular junction (NMJ) disease characterized by fatigable, fluctuating weakness of skeletal muscles — weakness that worsens with activity and improves with rest, is typically worst at the end of the day, and is painless. In the vast majority of cases (~85%), the body produces IgG1 and IgG3 autoantibodies against nicotinic acetylcholine receptors (AChR) on the postsynaptic muscle membrane; these antibodies block ACh binding, accelerate receptor degradation via complement-mediated destruction, and reduce the density of functional AChRs — all of which result in unreliable neuromuscular transmission and clinically apparent weakness. In ~10% of seronegative cases, antibodies target muscle-specific kinase (MuSK), a receptor tyrosine kinase critical to AChR clustering, producing a predominantly bulbar phenotype; a small minority carry LRP4 antibodies. The hallmark of MG is its distribution pattern: weakness preferentially affects ocular muscles (ptosis, diplopia — often the first symptom), bulbar muscles (dysarthria, dysphagia, dysphonia), and proximal limb/respiratory muscles in generalized disease; the most feared complication is myasthenic crisis — respiratory muscle failure requiring intubation and mechanical ventilation, triggered by infection, surgery, medications, or rapid steroid taper. The thymus gland plays a central pathophysiologic role: ~10-15% of MG patients harbor a thymoma, and thymic hyperplasia is present in ~65%, making thymectomy both diagnostic and therapeutic.
“Weakness, lack of strength” (from a- = without + sthenos = strength)
gravis
Latin gravis
”Heavy, serious, severe”
The PIE root behind sthenos is segh- — “to hold, to have power” — also seen in isosthenic and neurasthenia. The word asthenia entered English medical vocabulary in the 18th century as a general term for weakness or debility. The full term myasthenia gravis translates literally as “serious muscle weakness” — a clinically accurate name that reflects the life-threatening potential of the disease when respiratory muscles are involved. The term was codified in the medical literature in the late 19th century; Samuel Wilks described the first clear case in 1877, and Wilhelm Erb reported the first clinical series in 1879. The autoimmune etiology was not confirmed until 1973, when Patrick and Lindstrom demonstrated AChR antibodies.
Ventilation assist and management, hospital inpatient; each subsequent day
⚠️ Coding Note:G70.00 vs G70.01 is the most critical MG code distinction — G70.01 (“with acute exacerbation”) requires explicit provider documentation of an acute exacerbation or myasthenic crisis; do not infer it from treatment escalation alone — query the provider if documentation is ambiguous. For inpatient profee, MG crisis (G70.01) combined with acute respiratory failure (J96.0x) drives significantly higher DRG weight. Sequencing: when MG occurs with a thymoma, the neoplasm (C37 or D15.0) is coded first per ICD-10-CM neoplasm sequencing guidelines, with G70.00/G70.01 as secondary. Transient neonatal MG (P94.0) is a completely separate code family — it is NOT coded with G70.x. MuSK-positive seronegative MG still codes to G70.00/G70.01 — there is no distinct ICD-10 code for antibody subtype. For plasmapheresis (CPT 36522), documentation must support the medical necessity connection to the MG diagnosis. For thymectomy, CPT selection (60520 vs 60521 vs 60522) depends entirely on the surgical approach and extent of dissection — the op note must clearly document the approach. IVIG (96365+): payer prior auth is common; ensure G70.01 or documented treatment-refractory G70.00 is clearly supported in the clinical record.
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