18.1 Anaesthetic Complications: Accidental Awareness, Aspiration, Positioning Injuries, Visual Loss and Dental Damage
Key Takeaways
The Fifth National Audit Project (NAP5) estimated an incidence of reported accidental awareness of about 1 in 19,000 general anaesthetics, rising to about 1 in 670 when neuromuscular blockers were used for Caesarean section.
Risk factors for awareness include neuromuscular blockade, total intravenous anaesthesia without processed EEG monitoring, rapid sequence induction, obstetric and cardiac surgery, obesity, and gaps during transfer or difficult airway management.
Aspiration risk is reduced by fasting, gastric emptying where needed, rapid sequence induction for high-risk patients, and extubating fully awake; cricoid pressure is commonly applied at about 10 N awake and 30 N after loss of consciousness, although evidence for it is weak.
The ulnar nerve is the most commonly reported perioperative upper limb neuropathy; it is protected by avoiding pressure at the elbow and keeping the forearm supinated or neutral.
Ischaemic optic neuropathy is the commonest cause of perioperative visual loss after prone spinal and cardiac surgery; risk factors include long procedures, large blood loss, hypotension, anaemia and head-down or prone positioning.
18.1 Anaesthetic Complications: Accidental Awareness, Aspiration, Positioning Injuries, Visual Loss and Dental Damage
Accidental Awareness During General Anaesthesia (AAGA)
Accidental awareness is the explicit recall of events during general anaesthesia. It may cause distress, anxiety and post-traumatic stress disorder.
Incidence
- Studies using structured interviews (Brice interview) report about 1-2 per 1,000 anaesthetics.
- The UK Fifth National Audit Project (NAP5, 2014) recorded spontaneously reported cases and found about 1 in 19,000 overall; about 1 in 8,000 when neuromuscular blockers were used, and about 1 in 670 for Caesarean section under general anaesthesia.
- About half of NAP5 reports occurred at induction or emergence, not during maintenance.
Causes and Risk Factors
| Category | Examples |
|---|---|
| Drug-related | Neuromuscular blockade (paralysis prevents movement as a sign of awareness), low anaesthetic doses in shocked or obstetric patients, drug errors (syringe swaps) |
| Technique | Total intravenous anaesthesia without processed EEG monitoring, thiopental or propofol bolus followed by a delay before maintenance, rapid sequence induction |
| Patient | Previous awareness, obesity, young age, high anaesthetic requirements (alcohol or drug use) |
| Surgery | Obstetric, cardiac and emergency surgery |
| Organisation | Out-of-hours work, inexperience, transfers between induction room and theatre, difficult airway management |
| Equipment | Empty vaporiser, disconnected or leaking intravenous line (especially with TIVA) |
Prevention
- Check equipment and drug labelling; monitor end-tidal agent concentration with alarms (minimum alveolar concentration of at least about 0.7 MAC age-adjusted in paralysed patients).
- Use processed EEG monitoring (for example bispectral index) with TIVA when neuromuscular blockers are used.
- Ensure the intravenous cannula used for TIVA is visible and patent.
- Avoid unnecessary paralysis, and confirm the patient is asleep before giving relaxants.
- Avoid gaps in anaesthetic delivery during airway difficulty and transfers.
Management
If a patient reports awareness, take a detailed account, apologise, explain, document, offer psychological support and follow-up, and report the incident.
Pulmonary Aspiration
Aspiration of gastric contents causes chemical pneumonitis (Mendelson's syndrome), bacterial pneumonia or airway obstruction from particulate matter. The incidence is about 1 in 2,000 to 1 in 3,000 general anaesthetics, higher in emergency surgery.
Risk Factors
- Patient: full stomach (inadequate fasting, emergency, trauma, labour), delayed gastric emptying (diabetes, opioids, pain, GLP-1 receptor agonists), raised intra-abdominal pressure (obesity, pregnancy, bowel obstruction), reflux and hiatus hernia, reduced consciousness.
- Anaesthetic: light anaesthesia, difficult airway with prolonged mask ventilation, supraglottic airways in high-risk patients, lithotomy and head-down positions.
Prevention
- Fasting guidelines and gastric ultrasound when status is uncertain.
- Pharmacological: H2-receptor antagonists or proton pump inhibitors, oral sodium citrate before Caesarean section, and metoclopramide to promote emptying.
- Rapid sequence induction with preoxygenation, a rapid-onset relaxant, and cricoid pressure (10 N awake, about 30 N after loss of consciousness), released if it impedes laryngoscopy, ventilation or SAD insertion, or if the patient vomits actively.
- Nasogastric tube aspiration before induction in bowel obstruction.
- Extubation when fully awake with protective reflexes, usually in the lateral or semi-upright position for high-risk patients.
Management
- Head-down tilt and lateral position; suction the pharynx.
- Intubate if not already done and suction the trachea before ventilation when possible.
- 100% oxygen, PEEP; bronchoscopy to remove particulate matter.
- Supportive care; antibiotics only if infection develops or is likely (for example bowel obstruction contents); steroids are not recommended.
Positioning Injuries
Peripheral Nerve Injuries
| Nerve | Mechanism | Prevention |
|---|---|---|
| Ulnar (commonest in the upper limb) | Compression at the elbow (cubital tunnel), especially with the forearm pronated | Pad the elbow; supinate or keep the forearm neutral |
| Brachial plexus | Stretch with arm abduction above 90°, head rotation away, shoulder braces in head-down tilt | Limit abduction to below 90°; avoid shoulder braces |
| Radial | Compression against the humerus (arm board, screen) | Padding |
| Common peroneal | Compression at the fibular head in lithotomy (stirrups) | Pad the lateral knee |
| Sciatic | Stretch in lithotomy with hip flexion and knee extension | Avoid extreme positions |
| Femoral and obturator | Extreme hip flexion or abduction in lithotomy | Limit angles |
Risk increases with long procedures, diabetes, low body mass index, hypotension and pre-existing neuropathy. Most injuries are sensory and recover within weeks to months.
Other Positioning Problems
- Lithotomy: compartment syndrome of the lower legs after prolonged surgery (especially with head-down tilt and leg elevation); haemodynamic changes when legs are lowered.
- Prone: pressure on the eyes and face, abdominal compression (reduced venous return and increased epidural venous bleeding), breast and genital injury, tube displacement, cervical injury.
- Sitting and beach-chair: venous air embolism, cerebral hypoperfusion when blood pressure is measured at the arm (correct for the height difference: about 0.75 mmHg per cm).
- Lateral: brachial plexus injury in the dependent arm (use an axillary roll under the chest wall, not in the axilla), ear and eye pressure.
Perioperative Visual Loss
| Cause | Typical setting | Mechanism |
|---|---|---|
| Ischaemic optic neuropathy (posterior more common) | Prone spinal surgery longer than about 6 hours, cardiac surgery | Reduced optic nerve perfusion: hypotension, anaemia, large blood loss and fluid replacement, raised venous pressure in prone or head-down positions |
| Central retinal artery occlusion | Prone surgery with external pressure on the globe | Direct pressure on the eye; usually unilateral with periorbital signs |
| Cortical blindness | Cardiac surgery, cardiac arrest | Embolism or hypoperfusion of the occipital cortex |
Prevention: avoid pressure on the eyes (use foam or pin head holders), keep the head at or above heart level, maintain blood pressure and haemoglobin in long high-risk procedures, and consider staging very long spinal operations. Patients at high risk should be informed.
Corneal abrasion is the most common ocular injury; prevent it by taping the eyelids closed and using lubrication.
Dental Damage
Dental injury is one of the most common anaesthesia-related claims. Upper incisors are at greatest risk during laryngoscopy, from biting on airways during emergence, and with pre-existing caries, crowns or bridges. Document dentition before anaesthesia, use a bite block, avoid using teeth as a fulcrum, and if a tooth is avulsed, recover it (an adult permanent tooth may be reimplanted promptly) and refer to a dentist. An unexplained missing tooth or fragment requires a chest radiograph to exclude aspiration.
Other Common Complications
- Sore throat: after intubation (up to about 40%) and supraglottic airways.
- Postoperative nausea and vomiting and shivering: see the recovery and emergence section.
- Extravasation and intra-arterial injection: see the intravenous agents section.
- Hypothermia: impairs coagulation, wound healing and drug metabolism; prevent with active warming.
According to NAP5, which situation carried the highest reported incidence of accidental awareness?
General anaesthesia for Caesarean section with neuromuscular blockade, at about 1 in 670
Spinal anaesthesia for orthopaedic surgery, at about 1 in 100
Spontaneously breathing volatile anaesthesia with a supraglottic airway, at about 1 in 1,000
Sedation for endoscopy, at about 1 in 5,000
After a 5-hour laparotomy, a patient has numbness of the fourth and fifth fingers and weak finger abduction. Which positioning factor is most likely responsible?
Arm abduction of 70° on an arm board with the head turned towards the arm
Compression of the ulnar nerve at the elbow with the forearm pronated
Stretch of the radial nerve from the arm hanging over the table edge
Pressure on the median nerve at the wrist from the arterial line
After 8 hours of prone spinal fusion with 4 litres of blood loss, a patient wakes with painless bilateral visual loss and no periorbital injury. What is the most likely diagnosis?
Central retinal artery occlusion caused by external pressure on the globe
Bilateral corneal abrasions
Acute angle-closure glaucoma caused by atropine given at induction
Posterior ischaemic optic neuropathy
Sections you finish are checked off in the contents.