Observation
Strong evidence supports Thenar atrophy is strongly associated with ruling-in carpal tunnel syndrome, but poorly associated with ruling-out carpal tunnel syndrome.

Rationale
There were two high quality (Claes, 2013; Naranjo, 2007) and two moderate quality studies (Gomes, 2006; Makanji, 2014) with strong evidence that the presence of thenar atrophy can rule in the diagnosis of CTS.  Pooling the results into a meta-analysis demonstrated a strong association with electrodiagnostic studies (EDS) that used the criteria for the diagnosis of CTS established by the American Association of Electrodiagnostic Medicine (AANEM). The individual studies, as well as the meta-analysis, showed that the absence of thenar atrophy did not rule out the diagnosis of CTS.  The meta-analysis did not include two moderate quality studies (De Krom, 1990 or Gerr, 1998) because of variations in the electrodiagnostic test methods and also because of the availability of higher quality evidence examining the utility of thenar atrophy. The study by Claes was somewhat limited by its exclusion of the patients with severe thenar atrophy.  The studies also did not clearly differentiate loss of thenar muscle bulk on a neurogenic basis versus disuse atrophy, for example in cases of trapeziometacarpal joint osteoarthritis.
 

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