Presentation
A 63-year-old man presented to his primary care provider with ptosis, diplopia, dysphagia, and fatigue/weakness of arms and shoulders after mild activity (eg, raking leaves in his yard, carrying groceries, housework). His ocular symptoms had been present for about 5 months but his arm/shoulder muscle weakness was recent.
Physical examination revealed weakness after repeated/sustained muscle contraction followed by improvement with rest or an ice-pack test (see "Diagnosis" below), and a tentative diagnosis of generalized myasthenia gravis (gMG) was made. The patient was referred to a neurologist for serologic testing, which was positive for anti-AChR MG antibody, confirming the diagnosis of gMG.
Treatment was initiated with pyridostigmine, with reevaluation and treatment escalation as necessary.
gMG is generally defined as a process beginning with localized manifestations of MG, typically ocular muscle involvement. In some patients it remains localized and is considered ocular MG, while in the remaining patients it becomes generalized, most often within 1 year of onset.
Clinical findings in patients presenting with gMG can include:
Extraocular muscle weakness (85% of patients) causing diplopia, ptosis, or both
Bulbar muscle weakness (15% of patients)
Difficulty chewing, dysphagia, hoarseness, dysarthria
Facial muscle involvement causing inability to show facial expressions, and neck muscle involvement impairing head posture (dropped-head syndrome)
Upper limbs more affected than lower
Proximal muscles involved more than distal
Myasthenic crisis, considered a medical emergency due to weakness of the diaphragm and intercostal muscles, secondary to a lower respiratory tract infection
Differential Diagnosis
Several potential diagnoses should be considered on the basis of this patient's presentation.
Lambert-Eaton myasthenic syndrome: An autoimmune or paraneoplastic disorder producing fluctuating muscle weakness that improves with physical activity, differentiating it from MG
Cavernous sinus thrombosis: Also called cavernous sinus syndrome, can present with persistent ocular findings, photophobia, chemosis, and headache
Brainstem gliomas: Can present with dysphagia, muscle weakness, diplopia, drooping eyelids, slurred speech, and/or difficulty breathing
Multiple sclerosis: Can present with a range of typically fluctuating clinical features, including but not limited to the classic findings of paresthesias, spinal cord and cerebellar symptoms, optic neuritis, diplopia, trigeminal neuralgia, and fatigue
Botulism: Can present with ptosis, diplopia, difficulty moving the eyes, progressive weakness, and difficulty breathing caused by a toxin produced by Clostridium botulinum
Tickborne disease: Can present with headache, fatigue, myalgia, rash, and arthralgia, which can mimic the symptoms of other diseases
Polymyositis/dermatomyositis: Characteristically present with symmetrical proximal muscle weakness, typical rash (dermatomyositis only), elevated serum muscle enzymes, anti-muscle antibodies, and myopathic changes on electromyography
Graves ophthalmopathy: Also known as thyroid eye disease, can present with photophobia, eye discomfort including gritty eye sensations, lacrimation or dry eye, proptosis, diplopia, and eyelid retraction
Thyrotoxicosis: Can present with heat intolerance, palpitations, anxiety, fatigue, weight loss, and muscle weakness
Diagnosis
On the basis of this patient's clinical presentation and serology, his diagnosis is generalized AChR MG, class III.
Table. Myasthenia Gravis Foundation of America Clinical Classification
Commonly performed tests and diagnostic criteria in patients with suspected MG include:
History/physical examination
AChR antibody is highly specific (80% positive in gMG, approximately 50% positive in ocular MG)
Anti-MUSK antibody (approximately 20% positive, typically in patients negative for AChR antibody)
Anti-LRP4 antibody, in patients negative for anti-AChR or anti-MUSK antibody
Detecting established pathogenic antibodies against some synaptic molecules in a patient with clinical features of MG is virtually diagnostic. The presence of AChR antibody confirmed the diagnosis in the case presented above. Although the titer of AChR autoantibodies does not correlate with disease severity, fluctuations in titers in an individual patient have been reported to correlate with the severity of muscle weakness and to predict exacerbations. Accordingly, serial testing for AChR autoantibodies can influence therapeutic decisions.
Electrodiagnostic studies (useful in patients with negative serology)
Repetitive nerve stimulation
Single-fiber electromyography
Tests to help confirm that ocular symptoms are due to MG in the absence of positive serology
Edrophonium (Tensilon) test: Can induce dramatic but only short-term recovery from symptoms (particularly ocular symptoms)
Ice-pack test: Used mainly in ocular MG, in which it can temporarily improve ptosis
Chest CT/MRI, to screen for thymoma in patients with MG
Laboratory tests to screen for other autoimmune diseases, including rheumatoid arthritis (rheumatoid factor), systemic lupus erythematosus (ANA), and thyroid eye disease (anti-thyroid antibodies), which may occur concomitantly with MG
Management
The most recent recommendations for management of MG were published in 2021, updating the 2016 International Consensus Guidance for Management of Myasthenia Gravis by the Myasthenia Gravis Foundation of America.
MG can be managed pharmacologically and nonpharmacologically. Pharmacologic treatment includes acetylcholinesterase inhibitors, biologics, and immunosuppressive/immunomodulatory agents. Corticosteroids are used primarily in patients with clinically significant, severe muscle weakness and/or poor response to acetylcholinesterase inhibitors (pyridostigmine).
Pharmacotherapy
Acetylcholinesterase inhibitors
Pyridostigmine, an acetylcholinesterase inhibitor used for symptomatic treatment and maintenance therapy, is the only agent in this class used routinely in the clinical setting of MG
Biologics
Rituximab, a chimeric CD20-directed cytolytic antibody that mediates lysis of B lymphocytes
Eculizumab, a humanized monoclonal antibody that specifically binds to the complement protein C5 with high affinity, preventing formation of membrane attack protein (MAC)
Rozanolixizumab, a neonatal Fc receptor blocker that decreases circulating IgG
Ravulizumab, a terminal complement inhibitor that specifically binds to complement C5, preventing MAC formation
Efgartigimod alfa injection, a neonatal Fc receptor blocker that decreases circulating IgG, with or without hyaluronidase, which increases permeability of subcutaneous tissue by depolymerizing hyaluronan
Zilucoplan, a complement protein C5 inhibitor that inhibits its cleavage to C5a and C5b, preventing the generation of the terminal complement complex, C5b-9
Immunosuppressive/immunomodulatory agents
Tacrolimus, a calcineurin inhibitor
Methotrexate, a dihydrofolate reductase inhibitor
Cyclosporine, a P-glycoprotein inhibitor and calcineurin inhibitor that also inhibits cytochrome P450 3A4
Nonpharmacologic therapy
Thymectomy, to eliminate a major source of B and T lymphocytes and plasma cells, which produce anti-AChR antibody
PLEX (plasmapheresis; plasma exchange), to remove autoantibodies from the circulation
IVIg (intravenous immune globulin), recommended perioperatively to stabilize a patient and for management of myasthenic crises because of its rapid onset of action
Prognosis
In patients with gMG, the time to maximal weakness usually is within the first 3 years of disease onset. Accordingly, half of the disease-related mortality also occurs during this period, after which a steady state or improvement occurs. Younger age at onset (< 40 years), early thymectomy, and treatment with corticosteroids have been found to be associated with reduced risk for relapse, and thymectomy results in complete remission of the disease in some patients.
Most affected individuals have a normal lifespan. Morbidity includes quality-of-life issues resulting from muscle weakness, side effects from treatment (long-term effects of corticosteroids used for immunosuppression), and myasthenic crisis (mortality rate, 4.47%). Prognostic factors to be assessed at diagnosis may include:
Risk for secondary generalization: associated with late age of onset, high AChR antibody titers, thymoma, and presence of both ptosis and diplopia
Risk for MG relapse: reduced risk for relapse at age < 40 years at onset, early thymectomy, and prednisolone use. Increased risk for relapse with anti-Kv1.4 antibodies and concomitant autoimmune disease.
Morbidity results from fluctuating impairment of muscle strength, which may result in falls, aspiration, pneumonia, and ventilatory failure.
Principle risk factors for mortality include age of onset > 40 years, rapid progression of symptoms, and thymoma.
Clinical Takeaway
gMG is an autoimmune disease caused by an antibody-mediated postsynaptic blockade of neuromuscular transmission affecting the acetylcholine receptor. It presents as fatigable muscle weakness, which must be differentiated from other conditions with similar clinical presentations. Decreased muscle strength in patients with gMG can affect quality of life. In severe cases, untreated gMG can lead to myasthenic crisis, a potentially fatal complication due to pneumonia resulting from respiratory muscle weakness.
Many of the newest therapies, both approved and pending, are targeting specific autoimmune components of the immune system, which are mostly well defined in gMG.