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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.

Sensitive and Specific Findings

Finding	Sensitivity	Specificity	Comments, Study
Sleep Test	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	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	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

Specific Findings

Finding	Sensitivity	Specificity	Comments, Study
Anti-AChR Binding Antibodies	80%	100%	Sensitivity is >80% for generalized myasthenia but down to 50+% in ocular myasthenia. Sensitivity is as high as 99% for patients with associated thymoma. In addition, some number of patients become seropositive over time so numbers can be 92% seropositive. Specificity is per UpToDate. Study: Semin Neurol 2004 Mar;24(1):31-9. PMID 15229790 Muscle Nerve. 2007 Nov;36(5):651-8. PMID 17654558 UpToDate http://www.utdol.com/online/content/topic.do?topicKey=muscle/11770
Peek Sign	12%	100%	After complete initial apposition of the lid margins, they quickly (within 30 seconds) start to separate and the sclera starts to show (ie, a positive peek sign). Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
Food in mouth after swallowing	32%	98%	Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
Rest Test	50%	97%	During the rest test the patient places a glove filled with cotton (a placebo) over the more ptotic eyelid while holding the eyes closed 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. NOTE: only 1 study. Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
Quiver Eye Movements	23%	95%	Quiver eye movements are very fast, small-twitch, "lightning-" or "jerk"-like movements of the eyes upon changing direction of gaze. They are said to occur even in the setting of profound ophthalmoplegia. Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
Dysarthria after long period of speaking	45%	90%	Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866

Poorly Diagnostic Findings

Finding	Sensitivity	Specificity	Comments, Study
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