44 - A Rare View of a Rare Condition: Cadaveric Dissection of Thenar Hypoplasia Case Involving Congenital Absence of Two Thenar Muscles

Poster Board Number: 44
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Abstract Body : The thenar eminence, consisting of the opponens pollicis (OP), abductor pollicis brevis (APB), and flexor pollicis brevis (FPB) muscles, is largely responsible for actions of the thumb and critical for grip strength and fine motor movements of the hand. Abnormalities of these muscles can cause severe motor deficits in affected individuals. Thenar hypoplasia, a rare condition whereby one or more of the thenar muscles is underdeveloped or congenitally absent, is one lesser-known cause of thenar muscle dysfunction. Congenital abnormalities of these muscles present clinically as diminished grip strength and reduced fine-motor function of the thumb, indicating muscle wasting in the thenar eminence. Symptoms are often misdiagnosed as carpal tunnel syndrome (CTS), which affects millions of people and results in more than 400,000 corrective surgeries every year. Distinction from CTS is clinically significant to avoid performing unnecessary and futile surgical procedures. Descriptions of thenar hypoplasia are rare in the literature and describe clinical findings in terms of symptoms, leaving the anatomical details of the muscles involved undetermined. Here, we describe the cadaveric study of a 92-year-old female donor exhibiting unilateral congenital absence of the OP and APB muscles; a unique variation which has not been previously reported.

Cadaveric dissection was performed at the Central Michigan University College of Medicine anatomy lab. During dissection, it was noted that the left and right hands exhibited different muscular structures. The right hand retained the FPB muscle, while the OP and APB muscles were not present; the left hand exhibited the normal structure. A cautious re-evaluation of dissected tissues from the right hand failed to identify remnant muscle fibers, nor were there fibers connected to the bony attachment sites on the first ray. There was no evidence of antemortem trauma, or surgery to the region. It was concluded that these two muscles were indeed absent from a congenital defect rather than atrophied. No other components of the musculoskeletal hand and forearm appeared diminished.

A thorough literature review was conducted to identify similar cases, their prevalence, and clinical presentation. Although incredibly rare, we identified 13 previous reports detailing cases with variations of missing thenar muscles. Among the clinical cases with sufficient anatomical specificity and functional defects reported, absence of APB was associated with pain and discomfort when writing and weakness in both hands; and absence of OP was associated with an inability to oppose thumbs or grasp objects. This case, wherein both muscles were found to be absent, likely would have presented as broad functional deficits in the right thumb, e.g., general hand weakness, difficulty with grasping, and diminished fine motor skills.

Additional clinical cases have described a general reduction in the thenar eminence, and it has been hypothesized there could have been a case of bilateral absence of these same muscles, via structural imaging of a patient presenting with similar symptoms. However, this is the first time that a unilateral absence of the OP and APB muscles have been described and visualized using a cadaveric dissection.