What is the Wartenberg sign in a patient with suspected ulnar nerve damage or neuropathy?

Wartenberg Sign in Ulnar Nerve Pathology

Wartenberg sign is the involuntary abduction of the little finger (fifth digit) that cannot be actively adducted back to the ring finger, resulting from ulnar nerve neuropathy due to weakness of the third palmar interosseous muscle and unopposed action of the extensor digiti minimi (EDM) muscle. 1, 2

Pathophysiology

The sign develops from an imbalance between:

• Weakened third palmar interosseous muscle (ulnar nerve-innervated) that normally adducts the little finger 1
• Unopposed extensor digiti minimi (EDM) muscle which pulls the little finger into abduction 1, 2
• Overactive abductor digiti minimi (ADM) muscle contributing to the persistent abduction 1

This creates the characteristic appearance of the little finger held away from the other fingers, particularly noticeable when the patient attempts to place the hand flat on a surface. 2

Clinical Presentation in Ulnar Nerve Damage

When evaluating a patient with suspected ulnar neuropathy, Wartenberg sign appears as:

• Persistent abduction of the little finger that the patient cannot voluntarily correct 1, 2
• Inability to adduct the little finger toward the ring finger during active movement 1
• Often accompanied by ulnar claw hand deformity (metacarpophalangeal hyperextension with interphalangeal flexion of the fourth and fifth digits) in more severe cases 2
• May present with Froment sign (compensatory thumb interphalangeal flexion when attempting key pinch) and Egawa sign in motor branch involvement 3

Diagnostic Confirmation

To confirm ulnar nerve pathology as the cause:

• Electrodiagnostic studies (nerve conduction studies and EMG) should demonstrate denervation changes in ulnar nerve-innervated hand muscles, particularly the interossei 1
• Sensory nerve conduction abnormalities in the ulnar distribution if the lesion is proximal 1
• MRI with T2-weighted neurography showing high signal intensity and nerve enlargement at the compression site 4, 5
• Ultrasound can assess nerve cross-sectional area with 77-79% sensitivity and 94-98% specificity 4, 5

Critical Pitfall: Pseudo-Wartenberg Sign

Not all cases of little finger abduction indicate ulnar neuropathy. The "pseudo-Wartenberg sign" (PWS) occurs in 70% of cases due to focal dystonia of specific hand muscles, with other causes including:

• Avulsion of the third palmar interosseous muscle (20% of PWS cases) 1
• Contracture of the ADM muscle (10% of PWS cases) 1
• Trauma-related dystonia (20% of PWS cases) 1

In PWS, electrodiagnostic studies show normal ulnar nerve conduction and no denervation changes in ulnar-innervated muscles. 1 This distinction is critical because treatment approaches differ completely—PWS may require treatment of dystonia or surgical repair of avulsed muscles rather than ulnar nerve decompression. 1

Clinical Context

Wartenberg sign appears in:

• Distal ulnar nerve lesions (at or below the wrist) where motor fibers to the interossei are affected 2
• During ulnar nerve regeneration as an early sign of incomplete recovery 2
• Isolated motor branch compression at the piso-hamate hiatus or adductor pollicis hiatus, where patients may have a positive Froment sign but negative sensory findings 3

The presence of Wartenberg sign without sensory deficits or with preserved sensation should prompt consideration of isolated motor branch pathology or pseudo-Wartenberg sign. 1, 3