Anatomical Location and Borders
The abductor digiti minimi muscle (m. abductor digiti minimi) resides in the medial fascial compartment of the palm (compartimentum palmaris medialis), which forms the hypothenar eminence (hypothenar).
The borders of this fascial compartment are:
- Anteriorly and medially: Palmar fascia (deep fascia of the hand), which attaches to the 5th metacarpal bone.
- Posteriorly: The 5th metacarpal bone.
- Laterally: Medial intermuscular septum.
Other muscles located in the hypothenar compartment include the opponens digiti minimi muscle (m. opponens digiti minimi) and the flexor digiti minimi brevis muscle (m. flexor digiti minimi brevis).
Innervation and Function
The primary function of the muscle is abduction of the little finger.
It receives its nerve supply from fibers originating in spinal cord segments C8–Th1. It is one of the key intrinsic hand muscles that becomes weak or paralyzed in lesions of the ulnar nerve (n. ulnaris).
Clinical Significance: Why Do Clinicians Test This Muscle?
The abductor digiti minimi is routinely evaluated during neurological examinations to assess ulnar nerve integrity.
- Inspection and Muscle Strength: In progressive ulnar neuropathy, weakness develops in ulnar-innervated muscles—including the abductor digiti minimi and the interossei—while other muscle groups remain spared. Clinicians check for hypothenar muscle wasting (atrophy). Strength is graded using the MRC (Medical Research Council) scale. A classic manifestation of chronic ulnar palsy is the "claw hand" (main en griffe) deformity, particularly affecting digits 4 and 5.
- Special Tests: When ulnar entrapment at Guyon's canal is suspected, the superficial sensory branch of the ulnar nerve and hypothenar skin wrinkling are evaluated. Froment's sign is also tested to assess adductor pollicis and first dorsal interosseous weakness.
- Needle Electromyography (EMG): Needle EMG of the abductor digiti minimi is used to detect acute or chronic denervation-reinnervation (axonal) processes. Findings are compared with thenar muscles (median nerve). This test helps differentiate ulnar neuropathy from motor neuron disease (amyotrophic lateral sclerosis), brachial plexopathies, C8–Th1 radiculopathies, Hirayama disease, distal spinal muscular atrophies, and distal myopathies.
Differentiation: The Homonymous Muscle of the Foot
The lower limb contains a corresponding abductor digiti minimi muscle of the foot (m. abductor digiti minimi). It is located in the lateral plantar fascial compartment, which is bounded by the plantar aponeurosis, the lateral intermuscular septum, the cuboid, and the 5th metatarsal bone, forming the lateral border of the foot. This compartment also houses the flexor digiti minimi brevis of the foot and the opponens digiti minimi.