Abductor Pollicis Brevis Test

The Abductor Pollicis Brevis Test is used to evaluate the carpal tunnel syndrome to determine the type of treatment (surgical vs conservative) and measure treatment outcome.

How do you test abductor pollicis brevis muscle?

There are two test to assess the function of the Abductor Pollicis Brevis muscle:

Abductor pollicis brevis muscle strength

Abductor pollicis brevis strength is tested by placing the patient’s thumb in a position of abduction and applying a force in direction of adduction at proximal phalanx.

This test is positive if strength is reduced or markedly reduced compared with contralateral extremity.

A study on 82 patients with suspected cervical radiculopathy or carpal tunnel syndrome, the sensitivity and specificity for Abductor pollicis brevis muscle strength test was: 19% and 89%, respectively.

See Also: Carpal Tunnel Syndrome

See Also: Hand muscular strength test

Abductor pollicis brevis muscle weakness

The patient is positioned in sitting with their hand resting on the table. The clinician asked the patient to touch the pads of the thumb and small finger together. After asking the patient to keep the pads of the thumb and small finger together, the clinician applies a strong force in order to resist thumb abduction.

A positive test is weakness in some abduction with resisted testing as compared to the other hand.

Studies performed to determine weakness in the APB are relatively consistent, demonstrating moderate diagnostic accuracy. For example, in a study by Gerr and Letz the test was found to have a sensitivity of 63% and a specificity of 62%, compared to a Kuhlman and Hennessy study that found the sensitivity to be 66% and the specificity to be 66%

Abductor Pollicis Brevis Anatomy

The APB arises from the flexor retinaculum and the trapezium bone and inserts on the radial aspect of the proximal phalanx of the thumb.

It is innervated by the median nerve and functions to abduct the first metacarpal and proximal phalanx of the thumb.

See Also: Hand Anatomy

References

1. Kuhlman KA, Hennessey WJ. Sensitivity and specificity of carpal tunnel syndrome signs. Am J Phys Med Rehabil. 1997 Nov-Dec;76(6):451-7. doi: 10.1097/00002060-199711000-00004. PMID: 9431262.
2. Gerr F, Letz R: The sensitivity and specificity of tests for carpal tunnel syndrome vary with the comparison subjects. J Hand Surg Br 23:151– 155, 1998
3. Wainner RS, Fritz JM, Irrgang JJ, Delitto A, Allison S, Boninger ML. Development of a clinical prediction rule for the diagnosis of carpal tunnel syndrome. Arch Phys Med Rehabil. 2005 Apr;86(4):609-18. doi: 10.1016/j.apmr.2004.11.008. PMID: 15827908.
4. Netter’s Orthopaedic Clinical Examination An Evidence-Based Approach 3rd Edition Book.
5. Dutton’s Orthopaedic Examination, Evaluation, And Intervention 3rd Edition.