Anesthesiology APPLIED Exam › Neuroanesthesia and neuromuscular

Peripheral nerve injury and positioning

T1Neuroanesthesia and neuromuscular·1102 words · ~6 min read·v0.1✓ Sourced · 3DRAFT

Why examiners ask this

Positioning injury is the complication that the anesthesiologist owns in the eyes of the examiner, and nerve injury is second only to death as a cause of claims in the ASA Closed Claims database.¹ The topic tests application of knowledge (which nerve, which mechanism, which position), judgment (when to refuse a position or stop and reposition), and organization, because the post-operative question "the patient cannot extend the wrist, what now?" needs a structured answer: examine, document, refer, disclose. It also doubles as an OSCE complication-disclosure scenario.

The 60-second answer

The ulnar nerve is the most commonly injured, and ulnar and brachial plexus injuries together make up two-thirds of nerve injury claims;¹ most occur with no identifiable error, so my defense is prevention and documentation. I position the patient myself, I check every pressure point after the drapes go on and after any table change, and I document the position, padding and checks. Arms stay below 90 degrees of abduction, supinated or neutral with the elbow padded, head neutral, and no shoulder braces in Trendelenburg. In lithotomy I pad the fibular head and keep the hips below 90 degrees of flexion. In lateral decubitus I place an axillary roll under the chest, not the axilla. When a deficit appears postoperatively I examine it, distinguish a sensory from a motor loss, document it, consult neurology, and tell the patient what I know and what the plan is; most purely sensory ulnar deficits resolve within 6 months.¹

Decision points and the "why"

Arms below 90 degrees, supinated — abduction beyond 90 stretches the brachial plexus between the clavicle and first rib; pronation presses the ulnar nerve in the cubital tunnel against the arm board. The ASA practice advisory limits abduction to 90° and favors a supinated or neutral forearm, and it is the first thing I say.²
Ulnar nerve protection does not fully prevent injury — in Closed Claims reviews most ulnar injuries had no identifiable mechanism and the standard of care was usually judged met; men are about four times more likely to be affected, and symptoms can appear days later, which suggests patient factors.³ I pad anyway, because it is the standard, and I tell the patient that it is not always preventable.
Lithotomy — the common peroneal nerve wraps around the fibular head and is compressed by stirrups, so I pad it;² the femoral and obturator nerves are stretched by hip flexion beyond 90 degrees and abduction. I lift and lower both legs together, and I limit the time, because compartment syndrome of the calf appears after about 4 hours.
Lateral decubitus — the dependent brachial plexus is compressed against the ribs; the chest roll shifts the weight off the axilla.² I check the dependent ear and eye and the pulse in the dependent arm.
Prone — eyes free of pressure and checked every 30 minutes, arms either tucked or below 90 degrees with the elbows flexed, abdomen free to protect the epidural veins and the ventilation, and the neck neutral.
Steep Trendelenburg — I refuse shoulder braces, because they compress the plexus against the first rib;² I use a non-slip mattress, limit the time and the degree, and I watch for brachial plexus injury from sliding and for the cerebral and ocular edema of a long head-down case.
Regional anesthesia and neuropathy — a patient with a preexisting neuropathy, diabetes or chemotherapy carries a double-crush risk; I still offer the block but I document the baseline exam and I use ultrasound and low pressure.

The complication they will inject

On post-operative day one after a 5-hour robotic prostatectomy, the patient has numbness of the fourth and fifth fingers and cannot abduct them. I examine it myself: sensory loss in the ulnar distribution, weakness of the interossei, and I check that it is not a C8 radiculopathy or a plexus lesion by testing the median and radial territories. I document the findings and my positioning record, and I tell the patient directly what I found, that this is an ulnar neuropathy, that sensory symptoms usually recover over weeks to months but a motor deficit is more likely to persist, and that I am arranging neurology follow-up.¹ I request nerve conduction studies and electromyography in 3–4 weeks if the deficit persists, because EMG changes take that long to develop, though an early study can show a preexisting lesion. I avoid blaming and I avoid speculating about mechanism, and I report the event through the institution's process.

Pitfalls that cost points

Describing padding as a guarantee; the examiner wants to hear that injuries occur despite correct positioning, and that padding applied too tightly can itself cause injury.²
Confusing the mechanism: ulnar nerve from pronation and elbow pressure, plexus from abduction and shoulder braces, peroneal from the stirrup.
Skipping the neurologic exam and sending the patient straight to neurology without localizing the lesion.
Forgetting to disclose, document and report; the exam scores the professionalism as much as the anatomy.
Treating lithotomy as a lower-limb problem only; the hand resting beside the body is crushed when the foot of the table is raised.

Rapid-why

Q: Why are men more prone to ulnar neuropathy? A: A larger coronoid tubercle, a thicker flexor retinaculum and less fat over the cubital tunnel; the nerve is compressed more easily.¹

Q: Why does the axillary roll go under the chest? A: It lifts the chest wall off the dependent shoulder so the plexus and axillary vessels are not compressed; placed in the axilla it causes the injury it is meant to prevent.

Q: What is the earliest point to obtain an EMG? A: Early, within the first week, to localize the lesion and detect a preexisting one, and again at 3–4 weeks, when denervation changes appear and severity can be graded.¹

Q: When is a leg in lithotomy at risk of compartment syndrome? A: After about 4 hours, especially with hypotension and leg elevation reducing perfusion pressure; I ask for the legs to be lowered periodically.

Numbers to know

Item	Value
Most common injury	Ulnar nerve, then brachial plexus¹
Arm abduction limit	90 degrees or less, supinated or neutral²
Hip flexion in lithotomy	Below 90 degrees, legs moved together
Onset of ulnar symptoms	Often delayed, days after surgery
Lithotomy compartment risk	Beyond ~4 hours
EMG timing	Baseline early; localizing study at 3–4 weeks

Sources

1.Peripheral nerve injuries from positioning · OpenAnesthesia (IARS) · openanesthesia.org
2.Practice Advisory for the Prevention of Perioperative Peripheral Neuropathies: An Updated Report · ASA, 2011 · asa2.silverchair.com2011 updated report; the 2018 update could not be accessed.
3.Ulnar Nerve Pressure: Influence of Arm Position and Relationship to Somatosensory Evoked Potentials · Anesthesiology (Prielipp et al.), 1999 · asa2.silverchair.comBackground cites Closed Claims data: ulnar neuropathy one third of nerve injury claims; men about 4 times more likely.

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.