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Surgical positioning and nerve injuries
Every surgical position presses on or stretches a different structure: the ulnar nerve at the elbow, the brachial plexus with an abducted arm, the peroneal nerve in lithotomy, and the eye when prone — prevention is padding, correct angles and repeated checks.
Why it matters in the exam
The exam asks which nerve injury is the most common in anaesthesia (ulnar), what causes foot drop after lithotomy (the peroneal nerve), how far the arm may be abducted, what causes visual loss after prone spine surgery, and why blood pressure must be corrected in the sitting (beach chair) position. It also asks what to do when a patient wakes with weakness or numbness.
Ulnar injury sometimes appears only days after surgery, is more common in men and at the extremes of BMI, and is prevented by padding the elbow and keeping the forearm neutral or supinated. The brachial plexus is stretched when the arm is abducted beyond 90 degrees or when the head is turned to the other side. In lithotomy, pressure on the fibular head injures the peroneal nerve, and long surgery raises the risk of compartment syndrome in the legs. In the lateral position a roll is placed under the chest, not in the axilla itself, to protect the dependent brachial plexus.
Visual loss after long prone spine surgery is most often caused by ischaemic optic neuropathy, linked to long surgery, heavy blood loss, anaemia, hypotension and obesity — not by direct pressure. Direct pressure on the globe causes central retinal artery occlusion, usually in one eye. The most common eye injury under anaesthesia is a corneal abrasion, so the eyes are closed. In the sitting position the brain is higher than the heart, so cerebral pressure is lower than the arm reading — about 0.75 mmHg for each cm of height difference.
Key points
- The most common nerve injury: the ulnar nerve.
- Lithotomy — the peroneal nerve and foot drop.
- Arm abduction up to 90 degrees.
- Prone — visual loss, usually from ischaemic neuropathy; corneal abrasion is the most common eye injury.
- Sitting — the pressure at the brain is lower than the arm reading.
Summary table
| Position | Structure at risk | Prevention |
|---|---|---|
| Supine, arms on boards | The ulnar nerve; the brachial plexus | Pad the elbow, forearm supinated, abduction up to 90 degrees |
| Lithotomy | The peroneal nerve (foot drop); compartment syndrome | Pad the fibular head, shorten time in the position |
| Prone | Eyes (retinal artery occlusion, ischaemic neuropathy); face, chest, abdomen | Neutral head, no pressure on the eye, maintain blood pressure and haemoglobin |
| Lateral | The dependent brachial plexus; the dependent ear and eye | A roll under the chest, not in the axilla |
| Sitting (beach chair) / steep Trendelenburg | Cerebral perfusion; air embolism / facial and airway oedema | Correct blood pressure to brain level; check a cuff leak before extubation |
A free exam question
איזה מהבאים לא ?מגדיל את הסיכון לאובדן ראיה אחרי ניתוח (the question as examined, in Hebrew)
- א. מטופל ממין זכר
- ב. אובדן דם מוגבר במהלך ניתוח
- ג. ( בצקת בלחמית conjunctiva )
- ד. שימוש ב- wilson bed frame
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Sources
- ASA Practice Advisory for the Prevention of Perioperative Peripheral Neuropathies, 2018 — Positioning recommendations
- ASA Practice Advisory for Perioperative Visual Loss Associated with Spine Surgery, 2019 — Risk factors and prevention
- Miller's Anesthesia, 9th edition (2020) — Patient Positioning and Associated Risks
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.