Peripheral nerve injuries (PNIs) are a known, albeit infrequent, risk associated with general anesthesia. They are a significant concern for both patients and healthcare providers, as they can lead to long-term sensory and motor deficits. While vigilance and careful patient management are standard practice, the unconscious nature of general anesthesia prevents the patient from communicating discomfort, making them dependent on the surgical team for protection.
The Ulnar Nerve: An Anatomical Vulnerability
The ulnar nerve is the most frequently cited peripheral nerve injured during general anesthesia. This is primarily due to its anatomical course and superficial location, which makes it prone to external pressure. The nerve travels from the brachial plexus down the arm and passes through the cubital tunnel, a tight space behind the medial epicondyle of the elbow. Here, it is covered only by skin and a thin layer of connective tissue, leaving it unprotected from compression against hard surfaces like the operating table.
Mechanisms of Injury
Nerve injury during surgery is often multifactorial and can result from one or a combination of several mechanisms:
- Compression: This is the most common cause of ulnar nerve injury. Prolonged, external pressure on the elbow, often from an inadequately padded armboard or a patient's own body weight, can compromise blood flow (ischemia) to the nerve.
- Stretching: Exaggerated positioning of the limbs can stretch nerves. Abducting the shoulder more than 90 degrees or extreme elbow flexion can place excessive tension on the ulnar nerve and the brachial plexus.
- Ischemia: A compromised blood supply to the nerve, a condition known as ischemia, is a common pathway for nerve damage. This can result from pressure, but also from systemic factors like hypotension or hypothermia.
- Direct Trauma: While less common, a nerve can be directly injured by surgical instruments or by a needle during a regional anesthetic procedure.
- Double Crush Syndrome: A pre-existing, subclinical neuropathy (common in diabetics or smokers) can make a nerve more vulnerable to a second, minor injury during surgery, leading to a more pronounced deficit.
Factors that Increase the Risk of Ulnar Nerve Injury
Both patient and procedural factors can increase the likelihood of perioperative nerve damage. Recognition of these risks is crucial for preventive care.
Patient-Related Risk Factors
- Male Gender: Men are three times more susceptible to ulnar nerve injury, potentially due to less protective adipose tissue around the elbow.
- Older Age: Patients aged 50-75 are at a higher risk.
- Extremes of Body Weight: Both very thin and very obese patients are at increased risk due to less or poorly distributed protective tissue.
- Pre-existing Conditions: Chronic diseases like diabetes, hypertension, and vascular disease can compromise nerve health and blood supply.
- History of Smoking: Nicotine can contribute to vascular disease, further hindering nerve perfusion.
Procedural Risk Factors
- Prolonged Surgery: Longer surgical times increase the duration of pressure or stretching on nerves.
- Surgical Position: Positions like the steep Trendelenburg (head-down) and lateral decubitus can increase stress on the upper limbs and brachial plexus.
- Arm Position: Abducting the arm more than 90 degrees or prolonged elbow flexion beyond 90 degrees puts strain on the ulnar nerve.
- Use of Equipment: Imprudent use of restraints, shoulder braces, or automated blood pressure cuffs can cause compression.
Comparison of Common Perioperative Nerve Injuries
Perioperative nerve injuries are not limited to the ulnar nerve, and their clinical presentation varies depending on the nerve affected.
| Feature | Ulnar Nerve Injury | Brachial Plexus Injury | Common Peroneal Nerve Injury |
|---|---|---|---|
| Most Common Mechanism | Compression at the elbow (cubital tunnel) due to pressure from the operating table or armboard. | Stretching or compression, often from prolonged and extreme arm abduction or use of shoulder braces in Trendelenburg position. | Compression at the knee (fibular head) from poorly padded stirrups or lateral positioning. |
| Typical Symptoms | Numbness/tingling in the little and ring fingers; weakness in hand muscles, potentially leading to 'claw hand'. | Sensory and/or motor deficits affecting a wider area of the arm and hand, depending on which part of the plexus is injured. | Weak ankle dorsiflexion and foot eversion, resulting in 'foot drop.' Numbness on the shin and top of the foot. |
| Contributing Positions | Arms hyper-flexed or pronated on armboards. | Steep Trendelenburg or arm abducted more than 90 degrees. | Lithotomy or lateral positions. |
Prevention Strategies
Preventing perioperative nerve injury requires meticulous attention from the surgical team. Key strategies include:
- Proper Arm Positioning: Keeping arms adducted (tucked at the patient's side) or, if extended, ensuring shoulder abduction is less than 90 degrees. Forearms should be kept in a neutral position with palms facing upward.
- Adequate Padding: All pressure points, especially the elbow at the cubital tunnel, should be generously padded. Care must be taken not to apply padding too tightly, which can cause its own compression.
- Regular Monitoring: For prolonged procedures, periodic assessment and, if necessary, repositioning of the patient are crucial. The surgical team must be vigilant for any inadvertent movement of extremities.
- Careful Equipment Use: The use of automated blood pressure cuffs, braces, and retractors should be monitored to prevent localized pressure.
- Addressing Risk Factors: Identifying and documenting patient-specific risk factors, like diabetes or pre-existing neuropathy, helps in planning individualized preventive strategies.
Conclusion
While the incidence of perioperative peripheral nerve injury is low, the ulnar nerve remains the most common casualty under general anesthesia, largely due to its exposed anatomy at the elbow. These injuries are often the result of sustained compression or stretch related to patient positioning. The majority of ulnar neuropathies are transient, with symptoms improving over weeks to months, though some cases result in permanent dysfunction. By understanding the inherent risks and implementing careful, methodical prevention strategies—such as proper padding and monitoring of patient positioning—the surgical team can significantly reduce the likelihood of this distressing complication.
For additional information on peripheral nerve injuries and other anesthetic risks, consult professional medical guidelines such as those provided by the Royal College of Anaesthetists in the UK.
