What lesions can cause unilateral finger drop?

Unilateral Finger Drop: Anatomical Lesions

Unilateral finger drop can result from lesions at multiple levels of the neuraxis, most commonly from peripheral radial nerve injury, but also from central lesions affecting the motor cortex (hand knob area), cerebral peduncle, or cervical spinal cord at C6-C8 levels.

Peripheral Nerve Lesions

Radial Nerve (Posterior Interosseous Nerve)

• Posterior interosseous nerve palsy is the most common peripheral cause of isolated finger drop, affecting finger extension while typically sparing wrist extension if the lesion is distal to the radial nerve branching 1
• The key clinical finding is inability to extend the metacarpophalangeal joints of the fingers 1
• Electromyography showing abnormalities isolated to radial-innervated muscles distal to the triceps confirms peripheral radial nerve pathology 1

Ulnar Nerve

• High ulnar nerve lesions (above the elbow) cause loss of ring and small finger distal interphalangeal flexion, resulting in inability to flex these digits properly 2
• Clinical findings include clawing deformity, loss of key pinch, and abduction of the small finger 2
• Low ulnar nerve palsy presents with intrinsic muscle weakness and sensory loss but preserved flexor digitorum profundus function 2

Central Nervous System Lesions

Cortical Lesions (Motor Cortex)

• Acute infarction of the contralateral hand knob area in the precentral gyrus can present as isolated finger/wrist drop 3, 4
• Bilateral hand knob infarcts can cause bilateral wrist drop, typically with one side more affected than the other 3
• The presence of Wartenberg sign (spontaneous abduction of the small finger) indicates upper motor neuron pathology and helps differentiate central from peripheral causes 3
• Normal nerve conduction studies with clinical finger drop should prompt brain imaging 5, 4

Cerebral Peduncle

• Infarction of the medial cerebral peduncle can cause contralateral wrist and finger drop as the corticospinal tract descends through this structure 5
• This represents a "cortical" wrist drop despite the subcortical location because it affects descending motor pathways 5
• MRI brain is diagnostic, showing acute infarction in the cerebral peduncle 5

Cervical Spinal Cord Lesions

• Cervical spondylosis at C6/7 can cause unilateral finger drop through combined C7 radiculopathy and C8 segmental spinal cord compression 1
• MRI shows high T2 signal intensity in the spinal cord at the affected level 1
• Critical differentiating features from peripheral nerve palsy include electromyographic abnormalities in the triceps and first dorsal interosseous muscles, which are not affected by posterior interosseous nerve lesions 1
• Evoked spinal cord potentials demonstrate attenuation at the compression level 1

Diagnostic Approach

Initial Clinical Differentiation

• Examine for upper motor neuron signs (increased tone, hyperreflexia, Wartenberg sign) versus lower motor neuron signs (atrophy, fasciculations, hyporeflexia) 3
• Test triceps strength and first dorsal interosseous function—weakness here excludes isolated posterior interosseous nerve palsy 1
• Assess sensory distribution: radial nerve territory versus dermatomal pattern 1

Electrodiagnostic Testing

• Normal nerve conduction studies of the radial nerve mandate central nervous system imaging 5, 4
• Electromyography showing triceps or first dorsal interosseous involvement indicates cervical cord or root pathology rather than peripheral radial nerve lesion 1

Imaging Strategy

• For suspected peripheral lesions: MRI of the affected limb to evaluate nerve compression or injury 1
• For suspected central lesions: MRI brain with diffusion-weighted imaging to detect acute infarction in motor cortex, cerebral peduncle, or internal capsule 5, 3, 4
• For suspected cervical pathology: MRI cervical spine to evaluate for cord compression, myelopathy, or radiculopathy 1

Common Pitfalls

• Assuming all finger drops are peripheral radial nerve injuries—this delays diagnosis of stroke or spinal cord pathology 5, 4
• Failing to perform nerve conduction studies when clinical presentation is atypical 1, 5
• Not recognizing that bilateral presentation strongly suggests central pathology, particularly embolic stroke affecting both hemispheres 3
• Missing cervical cord lesions by not testing muscles outside the radial nerve distribution 1