How to differentiate between a stroke and ulnar nerve (UN) palsy due to positioning in a patient who developed numbness and weakness in the 4th and 5th fingers after a 5-hour surgery with an intraoperative watershed stroke?

Differentiating Watershed Stroke from Ulnar Nerve Palsy in Postoperative Hand Weakness

The key distinguishing feature is that ulnar nerve palsy affects only the ulnar distribution (4th and 5th fingers with ulnar-sided sensory loss), while cortical hand stroke affects multiple peripheral nerve territories and typically includes the radial side of the 4th finger. 1

Critical Clinical Examination Points

Sensory Distribution Pattern

• Ulnar nerve palsy: Sensory loss strictly limited to the ulnar side of the 5th finger and ulnar side of the 4th finger 2
• Cortical hand stroke: Sensory disturbance extends to BOTH the ulnar AND radial sides of the 4th finger—this is the pathognomonic difference 3
• Examine specifically whether the radial (thumb) side of the ring finger has sensation loss; if yes, this indicates cortical origin 3

Motor Involvement Pattern

• Ulnar nerve palsy: Weakness isolated to ulnar-innervated intrinsic hand muscles (interossei, medial lumbricals, hypothenar muscles, adductor pollicis) with characteristic "claw hand" deformity of 4th and 5th digits 4
• Cortical hand stroke: Weakness typically involves digits 3-5 but may show subtle weakness in other fingers or even the entire arm, often with sparing of shoulder girdle muscles 1, 4
• Test for weakness beyond the ulnar distribution—any involvement of thumb, index, or middle finger points to central etiology 1

Associated Neurological Signs

• Presence of ANY of the following mandates urgent brain imaging 5:
  • Facial asymmetry or lower facial palsy (even subtle) 4
  • Dysarthria or speech difficulties 5
  • Visual field defects 5
  • Hemispatial neglect 5
  • Any contralateral limb weakness 4

Timing and Evolution

• Ulnar nerve palsy: Symptoms present immediately upon emergence from anesthesia and remain stable 5
• Cortical hand stroke: May show progression over hours to days, with evolving weakness spreading to involve more of the arm 4
• Watershed strokes from intraoperative hypoperfusion comprise 20-30% of intraprocedural strokes and are diagnosed when patients emerge from anesthesia 5

Diagnostic Algorithm

Immediate Bedside Assessment

1. Map the exact sensory territory: Use pinprick to determine if the radial side of the 4th finger is affected 3
2. Test motor function in ALL fingers: Weakness beyond ulnar distribution (thumb abduction, index/middle finger extension) indicates cortical lesion 1
3. Examine for subtle facial asymmetry: Ask patient to smile and show teeth 4
4. Assess speech: Listen for dysarthria or word-finding difficulty 5

Risk Stratification for Stroke

High-risk features requiring immediate CT head 1:

• Multiple stroke risk factors (age >60, hypertension, diabetes, hyperlipidemia, carotid disease) 1, 4
• Prolonged surgery (5+ hours increases hypoperfusion risk) 5
• Intraoperative hypotension or hemodynamic instability 5
• Deficits in multiple peripheral nerve territories 1
• Any progression of symptoms 4

Imaging Decision

• Obtain urgent CT head (non-contrast) if: Radial side of 4th finger involved, any weakness beyond ulnar distribution, facial asymmetry, speech changes, or stroke risk factors present 1, 4
• Consider peripheral nerve etiology only if: Strictly ulnar distribution, no stroke risk factors, appropriate positioning injury mechanism (prolonged elbow flexion >90°, direct pressure on ulnar groove) 5

Common Diagnostic Pitfalls

Pitfall #1: Assuming Positioning Injury Without Adequate Examination

• Perioperative stroke symptoms are frequently masked and misattributed to positioning injuries 5, 1
• The cortical hand syndrome specifically mimics peripheral neuropathy and has been misdiagnosed in surgical patients 1
• Always examine the radial side of the 4th finger and test for facial weakness 3, 1

Pitfall #2: Delayed Imaging in High-Risk Patients

• A 68-year-old patient with stroke risk factors and hand numbness after surgery was initially presumed to have positioning injury; CT the following day revealed cortical infarct 1
• Watershed strokes from intraoperative hypoperfusion are associated with 2.4-fold higher mortality 5
• Image immediately if any uncertainty exists in patients with vascular risk factors 1

Pitfall #3: Missing Subtle Central Signs

• Lower facial palsy may be subtle and only apparent on careful examination 4
• Mild contralateral limb weakness may be overlooked if focus is solely on the hand 4
• Perform a complete neurological examination including face, speech, and all extremities 5, 1

When Ulnar Nerve Palsy is Confirmed

If imaging excludes stroke and clinical examination confirms isolated ulnar distribution:

• Apply appropriate padding at the elbow (foam or gel pads, not too tight) 2
• Avoid elbow flexion beyond 90° 2, 6
• Obtain electrodiagnostic studies to confirm diagnosis and establish baseline severity 2, 6
• Most positioning-related ulnar neuropathies improve with conservative management 5

Documentation Considerations

Document specifically:

• Exact sensory boundaries (including radial vs ulnar side of 4th finger) 3
• All motor territories tested 1
• Presence or absence of facial asymmetry 4
• Intraoperative positioning details and duration 5
• Hemodynamic stability during surgery 5