Myasthenia Gravis: Likelihood Ratios

Introduction: None written.

[Edit Diagnosis] [Merge dx] [Add prevalence]

Tags: Autoimmune Neurologic Tag this Diagnosis.

Prevalence

Population / Calculator	Prevalence	Comments / Study / Link
Thymoma	30%	Myasthenia Gravis occurs in 30-50% of patients with a thymoma
More, Edit...

The likelihood ratios of findings for Myasthenia Gravis are listed below. See the left navigation bar to change the display.

Good Positive and Negative Likelihood Ratios

Finding	+LR	-LR	Comments, Study
Sleep Test	49.5	0	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
Anticholinesterase Test	15	0.1	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
Ice Test	24	0.2	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
Anti-AChR Binding Antibodies	inf	0.2	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
Rest Test	16.7	0.5	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

Good Positive Likelihood Ratio

Finding	+LR	-LR	Comments, Study
Peek Sign	30	0.9	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	13	0.7	Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
Dysarthria after long period of speaking	4.5	0.6	Study: JAMA. 2005 Apr 20;293(15):1906-14. PMID: 15840866
Quiver Eye Movements	4.1	0.8	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

Poorly Diagnostic Findings

Finding	+LR	-LR	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