Simian hand

Palmar abduction moves the thumb perpendicularly away from the plane of the palm. Opposition rotates the thumb so that its tip approximates the tips of the fingers. With the thumb in its normal resting position, the thumbnail is at right angles to the fingernails. Paralysis of abduction and opposition leave the thumb resting in adduction, rotated parallel to the plane of the palm, with the thumbnail lying approximately in line with the fingernails. This degree of paralysis is usually accompanied by marked atrophy of the thenar muscles. The appearance of a paralyzed, atrophic thumb lying parallel instead of perpendicular to the fingers resembles an ape’s hand, and has been called the simian hand, ape hand or monkey paw deformity.

The simian hand deformity can occur with severe median neuropathy, combined median and ulnar nerve lesions, lower brachial plexopathy, C8 radiculopathy, motor neuron disease or severe peripheral polyneuropathy, in this case due to long-standing CIDP. When the disease process also involves ulnar innervated muscles, there is severe atrophy of the entire hand and the fingers lie with the MCP joints slightly extended and the IP joints slightly flexed (claw hand deformity). The term clawing refers to the position of the fingers, the term simian hand refers to the position of the thumb.