Anaesthesia for Major Middle Ear Surgery
Highlights
- The mechanical arrangement of the middle ear system allows a gain of up to 27 dB. This mainly results from the relative difference in the surface areas of the tympanic membrane and the oval window (Page 2)
- A person with healthy hearing can detect sounds with frequencies between 20 and 20,000 Hz, although the auditory system is most sensitive in the frequency range of 500e4000 Hz. (Page 2)
- In general terms, a 30 dB hearing loss indicates mild hearing loss, whereas a 100 dB hearing loss indicates profound hearing loss (Page 2)
- The incidence of facial nerve injury during middle ear surgery is low, at around 0.1%, but can have a profound psychosocial impact on the patient when it does occur.2 Besides sharp and blunt direct trauma, heat generated from high speed burrs can also damage the nerve, without direct contact being made. (Page 3)
- Tracheal intubation is often undertaken on account of limited access to the patient’s head, long procedure times, frequent use of controlled ventilation to ensure normocapnoea, and occasional need for intraoperative repositioning of the head. In suitable patients though, a supraglottic airway device (SAD) can be an acceptable alternative. SADs have the advantage that neuromuscular blocking agents can be easily omitted and they also minimise the chances of coughing during emergence, which can affect the surgical repair. (Page 5)
- Nitrous oxide is typically avoided because of its ability to diffuse into the non-compliant middle ear cavity faster than nitrogen diffuses out. This is of little relevance during surgery, as the cavity will be open to air, but once closed a positive pressure will result. The ability of the Eustachian tubes to equalise pressure differences between the middle ear and atmosphere appears limited during GA, even in those without middle ear pathology. (Page 5)
- When nitrous oxide is discontinued, diffusion back into the blood can result in sub-atmospheric middle ear pressures; these pressure changes can potentially compromise an ossicular chain repair or tympanoplasty. (Page 5)
- pressure changes also occur to a lesser extent when volatile anaesthetics are used without nitrous oxide, with desflurane causing greater changes compared with isoflurane.16 (Page 5)
- Middle ear surgery is associated with a high incidence of postoperative nausea and vomiting (PONV). Contributory factors include surgery in younger patients; longer procedure times compared with other surgeries; direct stimulation of the vestibular system by drilling adjacent to the inner ear; and suctioneirrigation (a caloric vestibular stimulant). (Page 7)
- Avoidance of PONV is desirable, as it is unpleasant for patients and can potentially affect the surgical repair if there is ongoing retching or vomiting. Ondansetron and dexamethasone are both effective prophylactic agents, and efficacy is increased when they are given in combination.23 TIVA also reduces early PONV.22 Prescription of a rescue antiemetic of a different class, such as cyclizine, is recommended for the postoperative period. In patients suffering from vestibular symptoms, which are not uncommon after cochlear implantation or middle ear surgery, betahistine (a structural analogue of histamine with H1 agonist and H3 antagonist activity) may also be useful (Page 7)
Anaesthesia for Major Middle Ear Surgery
Highlights
- The mechanical arrangement of the middle ear system allows a gain of up to 27 dB. This mainly results from the relative difference in the surface areas of the tympanic membrane and the oval window (Page 2)
- A person with healthy hearing can detect sounds with frequencies between 20 and 20,000 Hz, although the auditory system is most sensitive in the frequency range of 500e4000 Hz. (Page 2)
- In general terms, a 30 dB hearing loss indicates mild hearing loss, whereas a 100 dB hearing loss indicates profound hearing loss (Page 2)
- The incidence of facial nerve injury during middle ear surgery is low, at around 0.1%, but can have a profound psychosocial impact on the patient when it does occur.2 Besides sharp and blunt direct trauma, heat generated from high speed burrs can also damage the nerve, without direct contact being made. (Page 3)
- Tracheal intubation is often undertaken on account of limited access to the patient’s head, long procedure times, frequent use of controlled ventilation to ensure normocapnoea, and occasional need for intraoperative repositioning of the head. In suitable patients though, a supraglottic airway device (SAD) can be an acceptable alternative. SADs have the advantage that neuromuscular blocking agents can be easily omitted and they also minimise the chances of coughing during emergence, which can affect the surgical repair. (Page 5)
- Nitrous oxide is typically avoided because of its ability to diffuse into the non-compliant middle ear cavity faster than nitrogen diffuses out. This is of little relevance during surgery, as the cavity will be open to air, but once closed a positive pressure will result. The ability of the Eustachian tubes to equalise pressure differences between the middle ear and atmosphere appears limited during GA, even in those without middle ear pathology. (Page 5)
- When nitrous oxide is discontinued, diffusion back into the blood can result in sub-atmospheric middle ear pressures; these pressure changes can potentially compromise an ossicular chain repair or tympanoplasty. (Page 5)
- pressure changes also occur to a lesser extent when volatile anaesthetics are used without nitrous oxide, with desflurane causing greater changes compared with isoflurane.16 (Page 5)
- Middle ear surgery is associated with a high incidence of postoperative nausea and vomiting (PONV). Contributory factors include surgery in younger patients; longer procedure times compared with other surgeries; direct stimulation of the vestibular system by drilling adjacent to the inner ear; and suctioneirrigation (a caloric vestibular stimulant). (Page 7)
- Avoidance of PONV is desirable, as it is unpleasant for patients and can potentially affect the surgical repair if there is ongoing retching or vomiting. Ondansetron and dexamethasone are both effective prophylactic agents, and efficacy is increased when they are given in combination.23 TIVA also reduces early PONV.22 Prescription of a rescue antiemetic of a different class, such as cyclizine, is recommended for the postoperative period. In patients suffering from vestibular symptoms, which are not uncommon after cochlear implantation or middle ear surgery, betahistine (a structural analogue of histamine with H1 agonist and H3 antagonist activity) may also be useful (Page 7)