Inner Ear Decompression Sickness
- Article Author:
- Katharine Boyd
- Article Editor:
- Anton Wray
- Updated:
- 5/17/2020 11:48:29 AM
- For CME on this topic:
- Inner Ear Decompression Sickness CME
- PubMed Link:
- Inner Ear Decompression Sickness
Introduction
Recreational scuba diving is becoming an increasingly popular sport throughout the world, with approximately 1.2 million divers worldwide. Originally, divers concentrated most of their dives in warmer coastal regions. However, recently they have been extending their dives in other bodies of water, including local lakes, quarries, etc. This means that diving-related disorders can present to any hospital, whether inland or coastal, and all clinicians should be aware of signs and symptoms of decompression sickness.[1]
The inner ear consists of the vestibulocochlear organ, which is involved in hearing and one's sense of position and balance. This organ is surrounded by a bony exterior and contains a fluid called endolymph, which is responsible for the conduction of sound and changes in position. The cochlea is the portion responsible for the conversion of mechanical sound waves into action potentials in neurons, while the vestibular system modulates the sense of position and balance.[2]
Inner ear decompression sickness (IEDCS) is an incompletely understood condition observed in compressed-gas divers likely resulting from precipitation of gas bubbles in the endolymphatic and perilymphatic spaces during a quick ascent. Another suggested mechanism of IEDCS is that patients may also have a right-to-left shunt, suggesting arterial gas embolism (AGE) as a contributing factor if it enters the labyrinthine artery.[2][3] The classic common presenting symptom of inner ear DCS is a sudden onset of vertigo. However, tinnitus and acute sensorineural hearing loss may also be present alone or in any combination.[2][3]
Etiology
In general, decompression sickness (DCS) is caused by the generation of gas bubbles within the vasculature and organ tissues.[4] These bubbles are caused by excess dissolved gas in body fluids, which are from breathing compressed gas while subjected to high pressures during dives. During appropriate decompression, this dissolved gas is ventilated out of the body through the lungs without significant bubble formation. In DCS, excess dissolved gas is subjected to pressures low enough to precipitate bubbles in blood and tissues, causing obstruction and inflammation. This usually occurs during the ascent of a dive when there is a reduction of ambient pressures.[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). 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 to avoid changing diluents when tissues are close to critical supersaturation.[3][5][6][7]
Epidemiology
Inner ear decompression sickness is extremely rare. Although the total number and frequency of IEDCS in divers is unknown, a source reports an incidence of 0.2-0.3% per recreational dive.[8] It has been estimated that there is a 0.01-0.095% risk of any form of DCS, depending on the diving population.[4]
Approximately one-quarter of recreational divers that present 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 to develop inner ear DCS in patients with venous-arterial shunts.
Studies have shown that the frequency of IEDCS represents between 24-34% of all patients studied.[10][11]
Pathophysiology
The pathophysiology of inner ear decompression sickness (IEDCS) remains incompletely understood. General causes of IEDCS are similar to those that cause DCS in other organ systems, as described above. 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, thereby reducing the risk.[4]
In addition to these factors, reports also show 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, then 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 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 in relation 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 origin such as vertigo, ataxia, or 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 difficult 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 exam 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. This should be started as soon as possible. This shrinks existing bubble diameter and increases oxygen delivery to ischemic tissues, as well as enhances dissolved gas offloading. 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 (regardless if 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]
Other initial treatment 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.
Differential Diagnosis
The top 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 as well as the clinical features of the presenting illness. It is important 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 required.[1]
Prognosis
While complete recovery from DCS generally is quite high, especially with appropriate treatment, complete recovery from IEDCS appears to be less common. Studies have reported residual deficits from between 32-91% of patients, with 3 out of 5 studies reporting residual deficits in >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, pneumothorax, pulmonary edema, reversible myopia, and oxygen toxicity. There is a risk of fire and explosion, so established safety precautions should be followed closely.
Deterrence and Patient Education
Due to improvements in technology and established safety protocols, DCS in all forms is rare. Divers, high-pressure workers, and aviators should be familiar with and follow appropriate safety measures in their area. Following decompression schedules is particularly important for divers, as the majority of patients who experience IEDCS were found to have violations of these schedules.[13]
Enhancing Healthcare Team Outcomes
Distinguishing between IEDCS and barotrauma can be difficult. An accurate report from the dive site including dive profile, the onset of symptoms, untoward events, or sudden changes of depth or gas mixture can all provide clues to distinguish DCS from barotrauma. Therefore input from diving companions, a divemaster, EMS personnel, and others in contact with the patient is important to elicit. Use of other allied health professionals such as otologists, audiologists, neurologists may also help to delineate DCS from barotrauma as well as to pick up on other organ systems affected.