What was the cause of hearing loss after anesthesia in these 4 cases?

The focus of anesthesia is not limited to the vital signs on the monitor. There are many hidden risks……
At present, common anesthesia methods in clinic include general anesthesia, spinal anesthesia and nerve block anesthesia, and each anesthesia scheme is associated with different risks.
Some common post-anesthesia complications are well known to anesthesiologists. However, some complications remain unknown due to unclear theory and extremely rare incidence, including post-anesthesia hearing loss.
The formation of hearing is a complex mechanism composed of multiple structures, which can be divided into the outer ear, the middle ear and the inner ear according to its anatomical characteristics.
The outer ear comprises the auricle and the outer auditory canal; The middle ear consists of tympanum, eustachian tube, tympanic sinus, and mastoid process. The inner ear, also known as labyrinthine, is located in the temporal bone petrogeny and is divided into bony labyrinthine and membranous labyrinthine. When any one of these links is damaged, it may lead to hearing impairment.
Through literature review, it is found that there have been cases of hearing loss after anesthesia no matter what kind of anesthesia program in the past. Relatively speaking, there are more reports of hearing loss after spinal anesthesia. The following examples are given.
Case 1: Intraspinal anesthesia
The mother, 28 years old, 165cm, 90kg, was admitted to hospital in emergency due to “pregnancy complicated with giant baby and premature rupture of membranes”. The puerpera underwent caesarean section in the lower uterine segment under combined lumbo-epidural anesthesia. The vital signs of the mother were stable during the operation. The operation lasted for 1 hour. After the operation, the epidural catheter was pulled out, and the patient returned to the ward to be in supine position with the removal of the pillow.
Headache and tinnitus appeared 12 hours after the operation, and tinnitus was more severe on the left side. 10 days later, the maternal headache was relieved, and the left ear hearing was basically restored.
▌ Analysis of the Causes:
Changes in cerebrospinal fluid pressure are rapidly transmitted to the outer inner ear lymphatic through the open cochlear aqueduct. The relative balance between endolymphatic pressure and exolymphatic pressure maintains the normal structure of the inner ear. This balance can cause semicircular canal dysfunction and hearing impairment.
The drop in cerebrospinal fluid pressure caused by perforating the dura leads to a rapid drop in exolymph pressure and a slower response in the endolymphatic system. The pressure regulation of endolymph is mainly through changing the formation of the endolymph in the vasculature and the absorption of the endolymph sac.
Therefore, a sharp drop in cerebrospinal fluid pressure makes the endolymphatic pressure significantly greater than the epolymphatic pressure, resulting in deformation of the vestibular membrane and basement membrane, rupture of the hair cells and thus hearing loss.
Another possible reason is that when the abdomen is pressed during cesarean section, some amniotic fluid may enter the systemic circulation and reach the branch of the inner ear artery through the basilar artery. Because the branch of the inner ear artery has no collateral blood supply, when it is embolized, it is easy to cause ischemia and thus lead to hearing damage.
In this case, the prognosis is generally poor, with partial recovery of hearing but not complete recovery. In this case, there were no clinical manifestations of embolism during and after the operation of obstetrics and gynecology, and hearing was completely recovered after 10 days without sequelae, so this possibility can be ruled out.
Case 2: Nerve block anesthesia
A 48-year-old female patient was admitted to the hospital for “right hand trauma for 2 hours.” The patient underwent open reduction and internal fixation for a right radial fracture under brachial plexus block. The puncture point was located near the omohyoid muscle through the intermuscular sulci approach. After accurate positioning, the patient was repeatedly drawn back without blood and cerebrospinal fluid and slowly pushed into 5ml mixture of 1% lidocaine and 0.25% ropivacaine mesylate. The patient was asked if he had vertigo and tinnitus, but no abnormalities were reported, and slowly pushed into 25ml local anesthetic.
Eight minutes later, the patient developed right nasal congestion, drooping eyelids, redness, no tinnitus, dizziness, nausea, vomiting and other symptoms.
The patient’s vital signs were stable and there was no obvious discomfort. The operation time was 1 hour. One hour after surgery, the patient felt hearing loss on the right side, and a hearing test was conducted. According to the average hearing threshold of speech frequency (500-4000 Hz), there was a 60dB loss on the right side of the patient’s hearing threshold, and the left side was normal. The right side of the patient recovered 8 hours after surgery, without obvious sequelae.
▌ Analysis of the Causes:
It may be due to local sympathetic nerve block that causes vasodilation of eustachian tube and middle ear, resulting in blocked sound conduction from the middle ear to the inner ear, indirectly affecting the spiral apparatus of cochlea and leading to ipsilateral hearing loss.
The patient also presented with typical Horner syndrome, but it does not indicate that Horner syndrome is related to hearing impairment. Horner’s syndrome is mainly caused by blocking the cervical sympathetic ganglia, which are separate from the superior cervical ganglia that supply the eyes and ears.
In addition, local anesthetic poisoning can cause tinnitus and affect hearing, but this patient did not show symptoms of tinnitus or local anesthetic poisoning.
Case 3: General Anesthesia (Non-Extracorporeal Circulation)
A 72-year-old male patient underwent a L2-L5 laminectomy under general anesthesia for “severe lumbar spinal stenosis.” Prior medical history did not include ear disease, and preoperative medication was not ototoxic. General anesthesia drugs used during surgery include midazolam, fentanyl, propofol, vecuronium bromide, and nitrous oxide.
Cefazolin sodium was used for anti-infection during the operation. Postoperatively, the patient complained of ear plugs and bilateral hearing loss, but had no other signs or symptoms of ear dysfunction.
▌ Analysis of the Causes:
The mechanism of hearing impairment after general anesthesia is still unclear, but it may be related to the changes of middle ear pressure, the pathological changes of auditory organ blood vessels, the changes of cerebrospinal fluid pressure, embolism and the use of ototoxic drugs. Excessive or sudden changes in middle ear pressure can perforate the eardrum, resulting in significant hearing loss.
During nitrous oxide anesthesia, middle ear pressure fluctuations can occur, and the tympanic membrane shifts inward or outward or even perforates during nitrous oxide uptake and elimination, disrupting the reconstruction of middle ear conduction structure.
The amplitude of pressure fluctuation in the tympanic membrane is related to the inhalation concentration of nitrous oxide and the rate at which it changes. Too high or too fast inhalation concentration of nitrous oxide may lead to hearing impairment resulting from perforation of the tympanic membrane.
Case 4: General Anesthesia (Extracorporeal Circulation)
A 71-year-old male patient underwent coronary artery bypass grafting (CABG) under extracorporeal circulation. In the first few hours after surgery, patients have low cardiac output and need adrenaline and dopamine to maintain stable circulation. Immediately after awakening, the patient experienced swelling in the left ear, tinnitus, and hearing loss, but no dizziness.
Audiometry at 6 weeks and 4 months postoperatively revealed a low frequency sensory hearing loss of 65dB. Middle ear pressure is normal, and CT scans of the middle and inner ears are normal.
▌ Analysis of the Causes:
So combined with this information, can you deduce what caused the patient’s hearing loss in case four?