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Myasthenia Gravis: Sensitivity and Specificity

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Prevalence

Population / Calculator	Prevalence	Comments / Study / Link
Thymoma	30%	Myasthenia Gravis occurs in 30-50% of patients with a thymoma
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The sensitivity and specificity of findings for Myasthenia Gravis are listed below. See the left navigation bar to change the display.

Pre-Test Probability (Prevalence): %.	Post-Test Probability (Predictive Value): %.	Switch to display mode.
Note that calculating probabilities from more than one** finding is inherently inaccurate because findings are not independent. (For example, using two positive findings that share a common pathogenesis is likely to overestimate the true probability.)

Sensitive and Specific Findings
Finding	Result	Sensitivity	Specificity
[ + ] Sleep Test	+ 0 -	99%	98%
During the sleep test the patient is left in a quiet dark room with the eyes closed for 30 minutes. Complete or almost complete resolution of ptosis or at least a 2-mm increase in palpebral fissure width constitutes a positive response to these maneuvers. It is very important to evaluate the improvement immediately following the test, as the lids may quickly begin to droop again. NOTE: only 1 study. Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
[ + ] Ice Test	+ 0 -	85%	96%
The ice pack test is performed by placing a latex glove finger filled with crushed ice over the more ptotic eyelid for 2 minutes. Complete or almost complete resolution of ptosis or at least a 2-mm increase in palpebral fissure width constitutes a positive response to these maneuvers. It is very important to evaluate the improvement immediately following the test, as the lids may quickly begin to droop again. Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
[ + ] Anticholinesterase Test	+ 0 -	90%	94%
Edrophonium chloride is a fast- and short-acting acetylcholinesterase inhibitor that may be administered in the office setting to diagnose myasthenia gravis (Box). Its effect usually occurs within 30 seconds and lasts less than 5 minutes. Most myasthenic muscles respond to the test dose of 2 mg, but many will require more. Adverse effects are rare and usually mild (excess salivation, sweating, abdominal cramps, or fecal incontinence). Unequivocal improvement in ptosis or extraocular muscles constitutes a positive response. Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866

Poorly Diagnostic Findings
Finding	Result	Sensitivity	Specificity
[ + ] Single Fiber Electromyography		Sensitivity = 82%
Sensitivity 82-99% if test more muscles. Specificity not assessed, though anecdotally reported 70-80% - and reported to be worse than RNS. Study: Muscle Nerve. 2001 Sep;24(9):1239-47. PMID: 11494281
[ + ] Repetitive Nerve Stimulation		Sensitivity = 53%
For generalized myasthenia, sensitivity ranges 53-100%. Sensitivity is only 10% for ocular myasthenia. Specificity not assessed, though anecdotes put it at 70-80%. The test is done by repetitively stimulating a nerve and looking at the muscular response; at 2 Hz stimulation, the amplitude of response should decrease over time. At higher frequency stimulation, there is a variable response (i.e. decrease or constant amplitude) because of increased calcium in the synaptic cleft. Study: Muscle Nerve. 2001 Sep;24(9):1239-47. PMID: 11494281
[ + ] Anti-MuSK Antibodies		Sensitivity = 38%
Sensitivity is 38-50% of patients who do not have Anti-AChR antibodies! Specificity not assessed. Study: Muscle Nerve. 2007 Nov;36(5):651-8. PMID 17654558