Medscape Medical News. Salpeter MM. CT scan of chest showing an anterior mediastinal mass (thymoma) in a patient with myasthenia gravis. Fewer thymic changes in MuSK antibody-positive than in MuSK antibody-negative MG. Díaz-Manera J, Martínez-Hernández E, Querol L, Klooster R, Rojas-García R, Suárez-Calvet X, et al.
This results in the decrement in the amplitude of the CMAP.In patients with myasthenia gravis, this decremental response usually has a maximum decrement at the fourth or fifth response, followed by a tendency toward repair (see the image below).
Courtesy of Wikimedia Commons. As a result, pharmacologic inhibition of AChE increases ACh concentration at the NMJ, improving the chance for interactions between ACh and its receptor.
Unlike anti-AChR-abs, there appears to be a correlation between anti-MuSK titers, disease severity, and the application of immunomodulatory therapy. A 2 mg (0.2 mL) test dose is administered initially, as some patients are extremely sensitive to low doses. About 70% of those who test negative for MG antibodies, but have generalized MG symptoms, will test positive on this blood test. Less than 1% of MG patients have anti-AChR-ab (blocking) without detectable binding or modulating antibodies, thus, this test not clinically useful.The anti–striated muscle (anti-SM) Ab refers to a class of antibodies against components of skeletal muscle including titin, the ryanodine receptor, myosin, and alpha-actin. Seropositive means that there is presence of the antibody in the blood, and seronegative means that the antibody was not detected.If a physical exam is consistent with MG, a neurologist usually orders a blood test designed to detect antibodies to the acetylcholine (Ach) receptor. In: Engel AG, Franzini-Armstrong C, eds. Management of myasthenia gravis. Brooks M. PLEX and IVIG both effective maintenance options in juvenile MG. Liew WK, Powell CA, Sloan SR, et al. It is implicated with an increased risk of thymoma.
2001
The temperature of the skin overlying the tested muscle should be at least 34°C.Administration of AChE inhibitors before testing may mask the abnormality and consequently should be avoided for at least 1 day beforehand (even longer for long-acting agents).Factors related to tetanic contraction may also affect RNS findings.
Qureshi AI, Choundry MA, Mohammad Y, et al.
Hart IK, Sathasivam S, Sharshar T. Immunosuppressive agents for myasthenia gravis. Late-Onset Myasthenia Gravis Linked to Higher Cancer Risk. Surgical and neurologic outcomes after robotic thymectomy in 100 consecutive patients with myasthenia gravis. Cleveland Clinic is a non-profit academic medical center. Randomized Trial of Thymectomy in Myasthenia Gravis. Statins may aggravate myasthenia gravis.
A tetanic contraction of muscle is followed by 2 distinct phases:Post-exercise facilitation or repair of decrement, occurring for the first 2 minutes after tetanic contractionPost-exercise exhaustion, lasting an additional 15 minutes after posttetanic potentiationDuring post-exercise facilitation, accumulation of calcium inside the terminal axon causes enhanced mobilization and release of ACh, which overcomes the reduced number of AChRs at the NMJ and thus leads to larger EPSPs with additional recruitment of muscle fibers, resulting in a larger CMAP. Edrophonium (tensilon) is a short-acting AChE inhibitor that improves muscle weakness in patients with MG.This test evaluates weakness (eg, ptosis, partial or complete ophthalmoplegia, and forced hand grip) in an involved group of muscles before and after intravenous (IV) administration of edrophonium.
The most common employed stimulation rate is 3 Hz.Patients with MG rarely have a decreased response in a clinically normal muscle. Note limited elevation of left eye, denoting superior rectus palsy (A). Treatment of autoimmune myasthenia gravis. Zinman L, Ng E, Bril V. IV immunoglobulin in patients with myasthenia gravis: a randomized controlled trial. Your doctor will review your symptoms and your medical history and conduct a physical examination. Martignago S, Fanin M, Albertini E, Pegoraro E, Angelini C. Muscle histopathology in myasthenia gravis with antibodies against MuSK and AChR. Learn about laboratory tests related to myasthenia gravis. Phillips LH 2nd, Melnick PA.
Padua L, Stalberg E, LoMonaco M, Evoli A, Batocchi A, Tonali P. SFEMG in ocular myasthenia gravis diagnosis.