Trauma Akustik Akut

01Pathophysiology

Acute acoustic trauma results from a single, brief exposure to a dangerously loud sound, typically exceeding 140 dB SPL. This is distinct from chronic noise-induced hearing loss, which develops gradually over months to years of repeated exposure. The distinction matters on exams: acute acoustic trauma has a sudden, identifiable inciting event.

The cochlea is the primary site of injury. Sound waves are transmitted through the ossicular chain to the oval window, generating a traveling wave along the basilar membrane. At extreme intensities, this wave produces both mechanical shearing of the outer hair cells (OHCs) in the organ of Corti and metabolic exhaustion from excessive glutamate release at the inner hair cell synapses. The outer hair cells, which are responsible for amplifying and fine-tuning frequency selectivity, are the most vulnerable structures. Their stereocilia are physically disrupted, and the resulting oxidative stress from reactive oxygen species (ROS) triggers apoptotic cell death within hours to days.

The reason the 4 kHz region is preferentially damaged relates to the resonance properties of the external auditory canal (which amplifies frequencies around 2 to 4 kHz) and the tonotopic anatomy of the cochlea. The basal turn, which encodes high frequencies, sits at the point of greatest hydraulic energy transfer from the stapes. This explains why audiometry shows a characteristic notch at 4 kHz with partial recovery at 8 kHz, a pattern that is nearly pathognomonic on exam vignettes.

If the insult is purely acoustic (no pressure wave), the tympanic membrane and middle ear remain intact, and the hearing loss is purely sensorineural. In blast injuries, the pressure wave may also rupture the tympanic membrane, adding a conductive component and creating a mixed hearing loss pattern.

02Classification and Clinical Manifestation

Clinically, the patient reports immediate onset of hearing loss, tinnitus (often high-pitched), and a sensation of aural fullness after the inciting event. Hyperacusis (painful sensitivity to normal sounds) may also be present. In blast-related cases, look for additional findings such as bloody otorrhea, vertigo, or nystagmus suggesting concurrent labyrinthine injury.

03Diagnostic Workup

Pure tone audiometry is the cornerstone of diagnosis. The vignette will describe a patient with sudden hearing loss after noise exposure, and the answer is audiometry. The 4 kHz notch on the audiogram is the finding that confirms the diagnosis. This notch distinguishes noise-induced damage from presbycusis (age-related loss), which shows a gradual downsloping pattern without a discrete dip.

Tuning fork tests at the bedside help categorize the type of hearing loss before formal audiometry is available. A Weber test lateralizing to the unaffected ear confirms sensorineural loss on the affected side, because the cochlea on the damaged side cannot perceive bone-conducted sound as well.

OAE testing is useful as an objective confirmation, particularly in patients who cannot cooperate with standard audiometry (young children, malingerers). Absent OAEs at the affected frequencies confirm outer hair cell dysfunction.

ABR is not routinely ordered for straightforward bilateral acoustic trauma. However, if the hearing loss is unilateral and asymmetric, ABR or MRI of the internal auditory canals is indicated to exclude a vestibular schwannoma (acoustic neuroma), which is a classic exam trap.

04Management and Treatment

The first and most important step is always removal from the noise source. This is a common "next best step" question: before any pharmacotherapy, the patient must be taken out of the offending environment.

Systemic corticosteroids are the mainstay of acute treatment. The rationale is to reduce cochlear inflammation and limit oxidative damage during the critical window when hair cells are undergoing apoptosis. Prednisone at 1 mg/kg/day (capped at 60 mg) is the standard regimen, given for 10 to 14 days followed by a gradual taper. The key teaching point is the time window: treatment should begin within 72 hours of onset. Delay beyond this window reduces the probability of recovery.

Intratympanic corticosteroid injection is the rescue option when systemic steroids fail or when systemic steroids are contraindicated (e.g., uncontrolled diabetes, active peptic ulcer disease, immunocompromised state). Dexamethasone or methylprednisolone is injected directly through the tympanic membrane under local anesthesia. This delivers a high local concentration to the round window membrane while minimizing systemic side effects.

Hyperbaric oxygen therapy works by increasing oxygen delivery to the cochlea, counteracting the ischemia and oxidative stress that follow acute noise injury. It is most effective when started within the first 2 weeks and combined with corticosteroids. While not universally available, it is a recognized adjunctive treatment.

For patients with permanent hearing loss that does not recover after treatment, hearing rehabilitation is the long-term plan. Hearing aids are fitted for moderate loss, and cochlear implant candidacy is evaluated for profound bilateral sensorineural hearing loss.

If a tympanic membrane perforation from blast injury coexists, most small perforations heal spontaneously within 6 to 8 weeks. Keep the ear dry and avoid instrumentation. Surgical repair (tympanoplasty) is reserved for perforations that fail to close after 3 months.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get this topic wrong is by confusing acute acoustic trauma with chronic noise-induced hearing loss. Both produce a 4 kHz notch on audiometry, but the vignette will clearly describe either a single loud event (acute) or prolonged occupational exposure (chronic). The management differs: acute acoustic trauma has a treatment window for corticosteroids, while chronic noise-induced hearing loss is irreversible and managed only with hearing protection and rehabilitation.

A second classic trap involves blast injuries. When a vignette describes an explosion, students must consider that the hearing loss may be mixed (sensorineural from cochlear damage plus conductive from tympanic membrane perforation). A pure sensorineural pattern on tuning fork testing rules out significant middle ear involvement, but if Weber lateralizes to the affected ear, a conductive component is present and tympanic membrane integrity must be evaluated.

The third trap is unilateral sensorineural hearing loss without a clear noise exposure history. When the vignette lacks an obvious acoustic insult, the answer is not acoustic trauma. Instead, think vestibular schwannoma or idiopathic SSNHL and order an MRI or ABR accordingly.

Finally, remember the time-sensitive nature of treatment. A vignette may present a patient 5 days after noise exposure asking about treatment options. The tested concept is whether corticosteroids are still beneficial (ideally within 72 hours, but treatment up to 2 weeks may still be attempted) and whether intratympanic rescue therapy should be offered. The core competency here is recognizing the urgency of intervention and the correct sequencing: remove from exposure first, start systemic steroids early, and escalate to intratympanic injection if there is no improvement.