METHODS

On March 21, 2022, PubMed^® (including Medline^®) was searched using the terms “nerve palsy” and “aviation”. No restrictions on age or any other specifics were set. Surprisingly few hits were found. In addition to these findings, two new cases of apparent aviation-induced facial nerve palsy from our clinic are described herein.

RESULTS

The PubMed^® search revealed 18 relevant studies on facial nerve palsy during flight, with 23 cases all together. All the publications were case studies.

In the Aviation Medical Section of our department, we received two cases of apparent aviation facial nerve palsy.

Case 1

A 20-yr-old woman reported recurrent transient left-sided facial nerve palsy with increased duration and intensity on four subsequent flights. The onset of the palsy was recognized approximately 20 min after each ascent. On the first flight, the left corner of the mouth was hanging and the patient had an abnormal sensation on the left side of the face and tongue. The symptoms resolved after 5 min. On the subsequent two flights the patient had similar symptoms, with the addition of drooling and a swelling around the left eye region. The symptoms resolved after 25 min. On the fourth flight the patient recalled having a transient left-sided facial nerve palsy and headache for 2 wk. The following neurological examination was normal, including audiometry, tympanometry, and a magnetic resonance (MR) scan of the brain. However, naso-endoscopy revealed extensive nasal polyposis, and the patient had a medical history of chronic rhinosinusitis, which involved pain in the frontal sinuses when diving during her career as a professional swimmer. A computerized tomography (CT) scan confirmed nasal polyposis ( Fig. 1 ), and a Computer Assisted Sinus Surgery (CAS) was performed. The patient could not recall any complaints of ear pain, dizziness, fullness, hearing loss, tinnitus, or failure to perform the Valsalva maneuver during the flights and was otherwise healthy, with no prior history of middle ear disease or Eustachian tube dysfunction. The follow-up oto-neurological examinations and naso-endoscopy were normal; an additional CT scan of the temporal bone showed no signs of facial nerve canal dehiscence. After presenting to our medical facility, a pressure equalizer (PE) tube was inserted into the tympanic membrane prior to aviation. After two subsequent flights, the patient had no complaints of facial nerve palsy.

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DISCUSSION

When an aircraft ascends, the cabin pressure decreases and the gas in the middle ear expands correspondingly according to Boyle’s Law. In most aircraft, the cabin is pressurized to three-quarters of the pressure at sea level, corresponding to an altitude of 6000–8000 ft (1829–2438 m). A pronounced pressure decrease can cause free nitrogen molecule bubbles to form, leading to embolism (also known as decompression sickness) which, in seldom cases, can give rise to monosymptomatic facial nerve palsy.

Under normal conditions, the pressure difference between the middle ear and the cabin is gradually equalized with every 400 ft (122 m) of increasing altitude. This occurs by gas escaping through the Eustachian tube via either passive or active venting—which can be achieved by swallowing (Toynbee’s Maneuver), yawning, or force—during the Valsalva maneuver. Impaired ability to equalize the pressure is directly related to Eustachian tube dysfunction, which may be due to anatomical variations such as: rare congenital malformations involving the palate; a middle ear disease such as otitis media with effusion (more common); cholesteatoma obstructing the inner orificium of the Eustachian tube; or any other condition that may narrow the lumen of the Eustachian tube or impair the mucociliar clearance due to edema or increased viscosity of mucus, typically resulting from an upper respiratory tract infection.

In the presence of Eustachian tube dysfunction at takeoff, the theoretical maximum pressure difference between the gas of the middle ear and the cabin is 261 mbar, which exceeds the capillary blood pressure. ⁵ It has, therefore, been suggested that the increased gas pressure in the middle ear may impede the blood flow into the vase nervorum of the facial nerve (ischemic neuropraxia) when the dehiscence area of the osseous facial canal is present. This theory has been supported by the fact that baroparesis disappears on descent, assuming that the relative overpressure of the middle ear becomes normalized. It is also backed by the observations of divers, in whom baroparesis of the facial nerve disappears following equalization of the middle ear by yawning or swallowing. ^{4 , 11 , 12} The theory is further supported by animal experiments with cats, where reversible neuropraxia on unexposed parts of the facial canal were induced by pressure in the range of 200 to 267 mbars. ²⁰ Another study demonstrated that an elevation of middle ear pressure reduced the blood flow to the middle ear in guinea pigs. However, the reduction of blood supply to the facial nerve was not reduced significantly. ¹⁹ It has also been suggested that facial nerve palsy in aviation may be the result of elevated middle ear pressure transmitted through the fenestra of the chorda tympani to the facial nerve. ²⁵ In the case report described by Wimmer and Ali, unilateral facial nerve paralysis was seen in a military jet fighter pilot. ²⁵ Finding this diagnosis in an aviator, especially a military aviator, has important implications for flight safety, for avoid­ing unwarranted diagnostics, and for minimizing operational impact.

Based on autopsies, the incidence of facial nerve canal dehiscence is 50%, although the average dimensions of the dehiscence were measured to be less than 1 mm. ³ Studies using high resolution computer tomograms detected a facial nerve canal dehiscence in 30%, despite the well-known limitations due to high false negative and positive rates. ⁷ Overall, a dehiscent facial canal must therefore be considered a relatively normal condition.

However, facial nerve palsy in aviation is rare—despite it being well-known that many passengers have difficulty equalizing their middle ear pressure and may complain of otalgia. Moreover, neither the failure to perform the Valsalva maneuver nor the presence of otalgia are constant observations in the small population of published cases of facial nerve palsy during aviation (Table I).

Ischemic lesions are presumed to play a role in the pathology of Bell’s palsy. ¹⁵ Oxygen saturation decreases significantly during air travel in normal individuals. S_p o ₂ has been demonstrated to be 92% at a cruising altitude of 7129 ft (2173 m) ­compared to 98% at ground level. The onset of mild hypoxia after ascent may theoretically play an additional role in the threshold of facial nerve palsy in some individuals, most likely in combination with either anatomical preconditions regarding the function of the Eustachian tube, or anatomical variations of the facial canal. The buffer capacity of the mastoid may also play a role.

In conclusion, baro-palsy conditions during flight are rare, and the mechanism is still not well described or understood. However, PE tube insertion into the eardrum seems to prevent recurrent baro-induced facial nerve palsy. Further studies are needed.