What could be causing my left-sided hand weakness, especially in digits 4 and 5, given my history of Benign Fasciculation Syndrome (BFS)?

Left-Sided Hand Weakness in Digits 4-5 with Benign Fasciculation Syndrome

You need urgent brain MRI to exclude a cortical stroke in the right precentral gyrus ("hand knob" area), as isolated unilateral hand weakness—especially affecting specific digits—can represent a central nervous system lesion masquerading as peripheral pathology, and this requires immediate evaluation to guide time-sensitive stroke treatment. 1, 2, 3

Critical Immediate Workup

Rule Out Central Causes First

• Brain MRI with diffusion-weighted imaging is the essential first test when unilateral hand weakness presents acutely, as lesions in the contralateral "hand knob" area of the precentral gyrus can cause isolated hand weakness mimicking peripheral nerve damage 2, 3
• Look specifically for the "inverted omega sign" along the precentral gyrus, which identifies the hand motor cortex area 3
• Left-sided weakness affecting digits 4-5 suggests a right hemisphere cortical lesion in the hand representation area, which can present without other neurological signs 1, 2, 3
• Stroke risk factors (diabetes, hypertension, atrial fibrillation) make cortical infarction more likely than peripheral pathology in acute unilateral hand weakness 3

Exclude Structural Brain Lesions

• Brain metastases can present as isolated hand weakness when involving the hand knob area, with lung cancer being a common primary source 2
• The first clinical manifestation of malignancy can be pseudoperipheral hand palsy from a metastatic lesion 2
• If brain imaging is normal, proceed to peripheral nerve evaluation 2, 3

Peripheral Nerve Differential for Left Hand Digits 4-5 Weakness

Ulnar Neuropathy (Most Likely Peripheral Cause)

• Digits 4 and 5 are primarily innervated by the ulnar nerve (C8-T1), making ulnar neuropathy at the elbow or wrist the most common peripheral cause of isolated weakness in these digits
• Nerve conduction studies and EMG are required to localize the lesion and exclude peripheral nerve damage 2, 3
• Test for Tinel's sign at the cubital tunnel (elbow) and Guyon's canal (wrist)
• Assess for intrinsic hand muscle atrophy, particularly first dorsal interosseous and hypothenar muscles

Cervical Radiculopathy (C8)

• C8 nerve root compression can cause weakness in digits 4-5 with sensory changes along the medial forearm
• MRI cervical spine should be obtained if nerve conduction studies suggest radiculopathy rather than peripheral nerve entrapment 1
• Look for foraminal stenosis or disc herniation at C7-T1 level 1

Lower Brachial Plexopathy

• Lesions affecting the lower trunk (C8-T1) or medial cord can produce similar weakness patterns 1
• MRI brachial plexus is indicated if electrodiagnostic studies suggest plexopathy rather than isolated nerve or root pathology 1
• Consider thoracic outlet syndrome, Pancoast tumor, or radiation-induced plexopathy in the differential 1

Relationship to Benign Fasciculation Syndrome

BFS Does Not Cause Weakness

• Benign fasciculation syndrome is defined by fasciculations WITHOUT weakness or atrophy—the presence of objective weakness means this is NOT simply BFS 1, 4
• In systematic reviews of 180 BFS patients followed over months to years, 98.3% had persistent fasciculations but ZERO developed motor neuron dysfunction 4
• Fasciculations in BFS may improve (51.7% of patients) or worsen (4.1%), but progression to motor weakness is extraordinarily rare and described only in isolated case reports 5, 6, 4

Red Flags Suggesting Motor Neuron Disease

• New-onset weakness in a patient with longstanding BFS requires immediate evaluation for motor neuron disease, as rare case reports document evolution from benign fasciculations to amyotrophic lateral sclerosis 5, 6
• Look for progressive weakness, muscle atrophy, hyperreflexia with fasciculations, or bulbar symptoms 5
• EMG showing active denervation (fibrillations, positive sharp waves) with chronic reinnervation changes would indicate motor neuron disease rather than BFS 1

Diagnostic Algorithm

1. Obtain brain MRI immediately to exclude stroke or mass lesion in the hand knob area 2, 3
2. If brain imaging is normal, perform nerve conduction studies and EMG to localize peripheral pathology 2, 3
3. If electrodiagnostics suggest radiculopathy, obtain cervical spine MRI 1
4. If electrodiagnostics suggest plexopathy, obtain brachial plexus MRI 1
5. If all imaging and electrodiagnostics are normal, consider repeat testing in 2-3 weeks as early studies can be falsely negative 1

Critical Pitfalls to Avoid

• Never assume unilateral hand weakness is peripheral without brain imaging, as cortical strokes can mimic peripheral nerve lesions and require time-sensitive treatment 2, 3
• Do not attribute new weakness to pre-existing BFS—BFS does not cause weakness, and new motor deficits require full workup for alternative diagnoses 4
• Do not delay imaging based on normal nerve conduction studies alone, as central lesions will have normal peripheral nerve studies 2
• Recognize that the absence of sensory symptoms does not exclude stroke, as pure motor cortical infarctions can occur 3