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Poorly Diagnostic Findings

Myasthenia Gravis: Likelihood Ratios

Introduction: None written.

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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
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The likelihood ratios 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.)

Poorly Diagnostic Findings
Finding	Result	+LR	-LR
[ + ] 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