Which muscles will be paralyzed if there is a lesion of the Ulnar (ulnar nerve)?

Muscles Paralyzed in Ulnar Nerve Lesion

In a lesion of the ulnar nerve, the muscles that will be paralyzed are the palmar interossei and medial two lumbricals (option B).

Ulnar Nerve Innervation and Function

The ulnar nerve provides motor innervation to several intrinsic hand muscles, which can be affected when the nerve is damaged:

• Palmar interossei muscles: These muscles are responsible for finger adduction (bringing fingers together)
• Medial (ulnar) two lumbricals: These muscles control the metacarpophalangeal joint flexion and interphalangeal joint extension of the ring and little fingers
• Hypothenar muscles: Including opponens digiti minimi, abductor digiti minimi, and flexor digiti minimi
• All dorsal interossei: These muscles abduct the fingers (spread fingers apart)
• Adductor pollicis: Controls thumb adduction
• Deep head of flexor pollicis brevis: Assists with thumb flexion

Evidence for Muscle Paralysis in Ulnar Nerve Lesions

Research specifically examining ulnar nerve damage confirms that patients with ulnar nerve paralysis experience significant weakness in the intrinsic hand muscles. Studies have shown that patients with ulnar nerve damage have only about 12% of normal strength in affected fingers due to the loss of intrinsic muscle function 1.

The palmar interossei and medial two lumbricals are specifically affected in ulnar nerve lesions, as demonstrated by electrophysiological studies comparing lumbrical-interosseous latency differences 2.

Clinical Manifestations of Ulnar Nerve Lesions

When the ulnar nerve is damaged, several clinical signs may be observed:

• Weakness in finger adduction (palmar interossei)
• Inability to properly flex the metacarpophalangeal joints of the ring and little fingers (medial two lumbricals)
• Clawing of the ring and little fingers (due to unopposed action of extrinsic finger extensors)
• Wartenberg's sign (abduction posture of the little finger) due to unopposed action of extensor digiti minimi 3
• Weakness in thumb adduction (adductor pollicis)

Common Causes of Ulnar Nerve Lesions

Ulnar nerve lesions can occur from various mechanisms:

• Compression at the elbow (cubital tunnel syndrome)
• Compression at the wrist (Guyon's canal)
• Handlebar palsy in cyclists 4
• Forearm bone fractures 5
• Prolonged pressure on the ulnar groove

Differential Diagnosis

It's important to distinguish ulnar nerve lesions from other conditions:

• Median nerve lesions affect the lateral two lumbricals and opponens pollicis (option A)
• Radial nerve lesions affect wrist and finger extensors
• C8-T1 radiculopathy may present with similar symptoms but would typically have additional findings
• Thoracic outlet syndrome may mimic ulnar nerve symptoms

Key Points for Correct Answer Selection

Option B (Palmar interossei & Medial two lumbricals) is correct because:

1. The ulnar nerve innervates all palmar interossei muscles
2. The ulnar nerve innervates the medial (ulnar) two lumbricals
3. The lateral two lumbricals are innervated by the median nerve (ruling out options C and E)
4. Opponens pollicis is innervated by the median nerve (ruling out option A)
5. The ulnar nerve innervates all dorsal interossei (not just some as suggested in option C)

Understanding these innervation patterns is crucial for accurate diagnosis and management of ulnar nerve lesions in clinical practice.