From the Guidelines
Diagnosing myasthenia gravis involves several specific tests, with single-fiber electromyography (EMG) being the most sensitive and considered the gold standard for diagnosis, detecting impaired neuromuscular transmission in over 90% of patients with ocular myasthenia 1.
Key Diagnostic Tests
- Acetylcholine receptor antibody blood test: detects antibodies that block or destroy acetylcholine receptors at the neuromuscular junction, positive in about 85-90% of patients with generalized myasthenia gravis
- Anti-MuSK antibody testing and anti-LRP4 antibody testing: for patients with negative antibody tests
- Electrophysiological tests: repetitive nerve stimulation, which shows a characteristic decremental response in muscle action potentials
- Single-fiber electromyography (EMG): detects impaired neuromuscular transmission, positive in over 90% of patients with ocular myasthenia 1
- Edrophonium (Tensilon) test: a short-acting acetylcholinesterase inhibitor that temporarily improves muscle strength in myasthenia gravis patients, 95% sensitive for generalized myasthenia and 86% sensitive in cases of ocular myasthenia 1
Additional Recommendations
- Imaging studies like chest CT or MRI: recommended to check for thymoma, as approximately 10-15% of myasthenia gravis patients have this thymus gland tumor
- Complete sensorimotor examination and external examination: critical, with attention to the presence of strabismus, ptosis, and slow saccades
- The Ice Test: application of an ice pack over the closed eyes for 2 minutes in the case of ptosis and for 5 minutes in the case of strabismus, may demonstrate a reduction of ptosis and misalignment, highly specific to this disorder 1
From the Research
Diagnostic Tests for Myasthenia Gravis
- The diagnosis of myasthenia gravis (MG) can be supported by various tests, including:
Electrophysiological Abnormalities
- RNS abnormalities are observed in 86% of MuSK Ab-positive and 82% of AChR Ab-positive patients, but in only 55% of seronegative patients 4
- SFEMG of the extensor digitorum communis is abnormal in 90% of MuSK Ab-positive patients 4
- Proximal myopathic pattern is more common in MuSK Ab-positive patients (71%) compared to MuSK(-)/AChR(+) patients (23%) 5
Clinical Characteristics
- MuSK Ab-positive patients have distinct clinical phenotypes, with prevalent involvement of cranial and bulbar muscles and a high frequency of respiratory crises 2
- The course of the disease in MuSK Ab-positive patients is often characterized by periodic exacerbation phases requiring hospitalization and even assisted ventilation 2
- Double-positive antibodies against AChR and MuSK are associated with a severity of disease between that of AChR-MG and MuSK-MG patients 3