Tinnitus is the perception of sound, such as ringing, buzzing, or hissing, when no external source is present. This condition is common, affecting many adults. When tinnitus occurs in both ears (bilateral tinnitus), it is frequently linked to general causes like age-related hearing loss or widespread noise exposure. Conversely, when the sound is heard strictly in one ear (unilateral tinnitus), it immediately demands a comprehensive medical investigation, suggesting a localized issue within the auditory pathway that must be identified.
Why Unilateral Tinnitus Requires Urgent Attention
Bilateral tinnitus is often idiopathic or a generalized consequence of the auditory system processing a loss of input. Its onset is typically gradual and rarely associated with an immediately serious underlying medical problem. However, sound perception in only one ear fundamentally changes the diagnostic picture, strongly suggesting a structural or localized lesion.
Unilateral tinnitus acts as a medical “red flag” because it points to an identifiable disorder affecting only one side of the head or auditory nerve. Identifying the cause allows medical professionals to intervene before any condition progresses to cause permanent neurological or hearing damage.
Peripheral Causes of One-Sided Tinnitus
The causes of one-sided tinnitus can begin in the outer, middle, or inner ear, representing localized issues that do not affect the entire auditory system. One of the most common and easily treatable reasons is cerumen impaction, or a buildup of earwax that completely blocks the ear canal. This blockage changes the pressure dynamics in the ear, which can result in the perception of sound in that single ear.
Issues in the middle ear, such as acute otitis media or Eustachian tube dysfunction, can also trigger unilateral tinnitus. Middle ear infections cause fluid accumulation and inflammation behind the eardrum, disrupting the normal transmission of sound vibrations through the tiny middle ear bones. Similarly, dysfunction of the Eustachian tube, which connects the middle ear to the back of the throat, can cause pressure imbalances that result in a temporary ringing or clicking sound in the affected ear.
Localized acoustic trauma from a loud sound source, such as a gunshot or an explosion near one side of the head, can damage the delicate sensory hair cells in only that ear’s cochlea. Less common peripheral issues include otosclerosis, which is the abnormal growth of bone in the middle ear, or a cholesteatoma, which is a benign skin growth that can erode structures in the middle ear.
Central and Neurological Concerns
The most significant concern driving the investigation of unilateral tinnitus involves conditions that affect the vestibulocochlear nerve (Cranial Nerve VIII) or the brain structures it connects to. This nerve carries both auditory and balance information from the inner ear to the brainstem. The presence of a space-occupying lesion, such as an acoustic neuroma, is the primary reason unilateral tinnitus is taken seriously.
An acoustic neuroma, also known as a vestibular schwannoma, is a non-cancerous growth that develops on the sheath of the balance portion of the vestibulocochlear nerve. As this slow-growing tumor expands, it presses on the nerve, interfering with the transmission of sound and balance signals to the brain. This mechanical compression of the auditory nerve root is what causes the onset of unilateral tinnitus and, often, asymmetrical hearing loss on the same side.
Another serious, though distinct, concern is pulsatile tinnitus, a rhythmic whooshing or throbbing sound that can be heard in one ear. This type of sound is often objective, meaning a clinician can sometimes hear it with a stethoscope, and it suggests a vascular anomaly near the ear. These anomalies can include turbulent blood flow in the carotid artery or jugular vein, or even a small glomus tumor, which is a highly vascular growth near the skull base. The localized nature of these blood flow disturbances causes the sound to be perceived exclusively in one ear.
Diagnostic Steps and Specialized Testing
The medical workup for unilateral tinnitus is a focused protocol designed specifically to rule out the serious neurological causes. The process begins with a comprehensive audiological evaluation, which is a hearing test performed by an audiologist. This test is crucial for detecting asymmetrical hearing loss, which is a strong correlative sign of a retrocochlear lesion like an acoustic neuroma.
If asymmetrical hearing loss is confirmed, or if the tinnitus is pulsatile, the next step is typically specialized imaging. Magnetic Resonance Imaging (MRI) with gadolinium contrast is the preferred neuroimaging technique for unilateral, non-pulsatile tinnitus. The MRI provides highly detailed images of the internal auditory canal and the brainstem, allowing physicians to visualize the vestibulocochlear nerve and detect even small acoustic neuromas.
For pulsatile tinnitus, a Computed Tomography (CT) scan or Magnetic Resonance Angiography (MRA) may be used to specifically evaluate the blood vessels and bone structures near the ear. The entire diagnostic process is not aimed at treating the tinnitus directly, but rather at definitively identifying or excluding any underlying pathology that could be causing pressure or interference on the auditory pathway.