Inner Ear Decompression Sickness

Etiology

In general, decompression sickness (DCS) is thought to be caused by the generation of gas bubbles within the vasculature and organ tissues.[4] This is caused by the diver breathing compressed gas while subjected to high pressures during dives, resulting in super-saturation of the circulatory system with nitrogen due to increased ambient pressure and the compressed gas mix, in accordance with Boyle's law. This dissolved gas is ventilated out of the body during appropriate decompression (a slow ascent to gradually reduce surrounding atmospheric pressure and release super-concentrated nitrogen) through the lungs without significant incident; but in DCS, excess dissolved gas is subjected to rapid pressure changes that precipitate gas bubble formation in blood and tissues, causing obstruction and inflammation. This occurs during the ascent of a dive when ambient pressures are reduced at too rapid a rate for the gases to be maintained in solution and be expired via respiration.[5] 

Isolated IEDCS is recognized in deep diving with the use of helium-oxygen mixtures, particularly when breathing gas is switched from a decompression mixture containing nitrogen back to a helium-containing mixture (e.g., the diver's bottom mix). This is most frequently seen in technical/professional divers but can be seen in deep-diving recreational divers as well. Though the actual mechanism is poorly understood, it is suspected that since helium has a much higher diffusion rate than nitrogen, diffusion of helium into tissues like endolymph that are already close to critical supersaturation could result in bubbling even while at a decompression stop where ambient pressure remains constant. This phenomenon is known as isobaric counter diffusion. Breathing gas switches should be carefully planned to avoid such effects and avoid changing diluents when tissues are close to critical supersaturation. All dives should incorporate appropriate decompression stops to minimize such risks.[5][6][7]

Epidemiology

Inner ear decompression sickness is extremely rare. Although the total number and frequency of IEDCS in divers is unknown, one source reports an incidence of 0.2 to 0.3% per recreational dive, though this is higher than that described in other sources.[8] It has been estimated that there is a 0.01 to 0.095% risk of any form of DCS, depending on the diving population.[4]

Approximately one-quarter of recreational divers presenting with neurologic decompression sickness will have vestibular-cochlear involvement.[9] Right-to-left shunts were also found in 81% of patients diagnosed with inner ear DCS compared to 25% of patients that never developed DCI. This possibly suggests an increased risk of developing inner ear DCS in patients with venous-arterial shunts.[10][11]

Pathophysiology

The pathophysiology of inner ear decompression sickness (IEDCS) remains incompletely understood. As described above, the general causes of IEDCS are similar to those that cause DCS in other organ systems, but the presumed mechanism is the precipitation of gases into the endolymphatic system rather than into joint spaces or the more general circulation. Primary factors include inadequate decompression, deep-diving requiring decompression stops, and inadequate surface time to re-equilibrate nitrogen. Immersion, exercise, and warm temperatures are important factors that can increase the risk of developing DCS by increasing inert gas uptake while under pressure. In contrast, these same factors increase inert gas elimination during decompression, reducing the risk.[4]

In addition to these factors, reports also suggest an association of IEDCS with persistent foramen ovale (PFO), a type of right to left shunt (RLS). An RLS allows blood to bypass the pulmonary system. In other words, gas bubbles forming in the venous circulation can enter arterial circulation instead of being eliminated by the lungs, more properly defining it as an arterial gas embolism (AGE).[4][12][10] One study found in 77% of IEDCS cases, a large right-to-left shunt was detected with a preponderant (80%) right-sided lateralization.[3] However, this is not completely understood. If IEDCS is strictly caused by AGE, we should observe these phenomena concurrently, yet IEDCS is often observed in the absence of other cerebral symptoms.

History and Physical

A recent history of diving is the most likely scenario; however, DCS can also rarely occur in aviators (especially high-altitude aviators above commercial heights, such as certain military pilots and astronauts) and compressed air workers such as caisson workers. Exceeding the limits of dive tables can put patients at increased risk. DCS is rare in dives that are less than 10 meters in depth. Recreational dives that are deeper and longer could be associated with higher chances of DCS.[4] 

The clinician should obtain a comprehensive history from the patient, including details of the dive profile, including time and depth, gaseous mixtures used, and the time delay to onset of symptoms relative to surfacing.[11]

In a patient experiencing IEDCS, symptoms will typically arise within 2 hours of surfacing and can even rarely occur while patients are still under pressure.[8][11][13] The average onset of symptoms is thirty-six minutes after decompression.[2][9] The most common presenting complaints with IEDCS are of vestibular origins such as vertigo, ataxia, difficulty with coordination, nausea, and or vomiting.[9][11] Cochlear symptoms such as tinnitus and hearing loss are also observed in approximately 25% of cases.[11] The frequency of IEDCS symptoms, as estimated from 2346 cases reported to the Divers Alert Network, are dizziness/vertigo (19.4%), coordination (7.9%), and auditory (2.1%).[4] 

While IEDCS can occur in isolation, patients experiencing vertigo, balance issues, or hearing loss after diving should also be examined and questioned for other manifestations of decompression sickness and other diving injuries while ruling out all non-diving-related causes.

All patients should undergo comprehensive neurological examination, including testing for balance, gait, proprioception, nystagmus, and bilateral hearing. Further physical examination for pain and numbness, cutaneous lesions/rash, musculoskeletal pain/discomfort, and pulmonary function should also be performed.

Evaluation

Inner ear barotrauma in divers may be challenging to distinguish from inner ear decompression sickness and requires dive-risk stratification and careful questioning regarding diving-related ear events. The HOOYAH criteria were developed as an adjunct to history and physical exams to help determine the etiology of inner ear symptoms. These include: 1) H: hard to clear; 2) O: onset of symptoms; 3) O: otoscopic exam; 4) Y: your dive profile; 5) A: additional symptoms and 6) H: hearing.[2] Evaluation should include a detailed history and clinical assessment, including pure tone audiometry, serial audiometry, a fistula test, and electronystagmography.[14]

Treatment / Management

Hyperbaric oxygen therapy (recompression therapy) is considered the treatment of choice for DCS and IEDCS, and this should be instituted as soon as possible. This increases ambient pressure to drive gases back into solution and increases oxygen delivery to ischemic tissues. Controlled decompression then allows controlled dissolved gas offloading to prevent the re-formation of any precipitant bubbles. Once the patient improves clinically, the patient is decompressed slowly to atmospheric surface pressure. Multiple sessions of recompression therapy may be required if symptoms return.[1]

If recompression therapy is not immediately available, the patient should be placed on 100% oxygen for several hours (whether or not there is a resolution of symptoms) or until recompression is available. This establishes an oxygen washout, which is when there is a maximum inert gas gradient between the lungs and dissolved gases, resulting in rapid removal.[4]

The other initial intervention for IEDCS should include adequate fluid resuscitation with crystalloid solutions. The formation of free gas in the bloodstream can also induce platelet activation, so consideration of antiplatelet therapy (i.e., aspirin) is warranted.[1] 

If the patient is suffering from suspected IEDCS, it should be noted that inner ear barotrauma cannot be easily excluded. Therefore, bilateral myringotomy should be considered before initiating hyperbaric oxygen therapy.[2][15] However, empirically, recompression does not appear to cause harm if the diagnosis of barotrauma versus IEDCS is in question.(A1)

Differential Diagnosis

The differential diagnoses for inner ear decompression sickness include inner ear barotrauma, arterial gas embolism, oxygen toxicity, and persistent alternobaric vertigo. While treatment should not be delayed if any form of DCS is suspected, if auditory or vestibular symptoms are found in isolation, it may also be beneficial to rule out common causes of hearing loss and vertigo. This includes cerumen impaction and benign paroxysmal positional vertigo. Other CNS disorders such as a cerebrovascular accident or transient ischemic attacks can also mimic symptoms of IEDCS. This also emphasizes the importance of a detailed dive history and proper physical exam.

It can be challenging to distinguish between IEDCS and inner ear barotrauma, as both conditions present with similar cochlear and vestibular symptoms.[2] Therefore, clinicians should rely on a detailed history and the clinical features of the presenting illness. It is crucial because the decompression sickness patient requires immediate recompression therapy, whereas the barotrauma patient does not.[7] Inner ear barotrauma is initially managed medically, followed by surgery if needed.[1]

Treatment Planning

If the clinical and situational suspicion is high, the patient should be referred immediately to a hyperbaric chamber unless medical contraindications exist. If the diagnosis is delayed, the outcomes may be far worse, and treatment via decompression will not negatively impact more standard barotrauma of the ear or sinuses; a failure to initiate such therapy for true IEDCS can potentially cause lasting harm.

Prognosis

While complete recovery from DCS is generally quite high, especially with appropriate treatment, full recovery from IEDCS appears less common. Studies have reported residual deficits in between 32 and 91% of patients, with 3 out of 5 studies reporting residual deficits in more than 85% of patients.[11] Vestibular deficits are more common than cochlear deficits, consistent with vestibular deficits being initially more frequent.

One source states even with adequate recompression therapy, it is common to have residual deficits in balance and hearing.[7]

Complications

Complications of hyperbaric oxygen therapy are rare but include barotrauma (to the ear, sinuses, and lungs), pneumothorax, pulmonary edema, myopia, and oxygen toxicity.