The Vestibular Evoked Myogenic Potential (VEMP) test is a non-invasive diagnostic tool that assesses specific parts of the inner ear responsible for balance and spatial orientation. When a patient reports symptoms like dizziness, vertigo, or unsteadiness, VEMP testing evaluates the function of the otolith organs—the saccule and the utricle—which sense linear acceleration and gravity. An abnormal VEMP result means the inner ear structure or its connected neural pathway is not responding as expected to sound or vibration stimulation. Understanding these results provides clarity on the specific nature of the vestibular dysfunction.
The Two Types of VEMP Testing
The inner ear’s balance system is complex, and VEMP testing distinguishes between two specific pathways, each linked to a different otolith organ. The cervical VEMP (cVEMP) primarily assesses the function of the saccule, which senses vertical movement, and its connection to the inferior vestibular nerve. The electrical response is recorded from the sternocleidomastoid muscle in the neck, which contracts in response to the acoustic stimulus.
The ocular VEMP (oVEMP) evaluates the utricle, which senses horizontal movement, and its corresponding superior vestibular nerve pathway. This reflex is measured by electrodes placed beneath the eyes, recording the response from the inferior oblique eye muscle. This distinction is crucial for diagnosing the location of the problem; a dysfunction isolated to one VEMP type helps localize the issue to either the saccule/inferior nerve or the utricle/superior nerve. For instance, an abnormal cVEMP but a normal oVEMP suggests a problem localized to the inferior branch of the vestibular nerve.
Defining Abnormal VEMP Metrics
An abnormal VEMP result is determined by analyzing several quantitative measurements from the electrical response. One primary metric is latency, which measures the time delay between the stimulus and the muscle responding. A significantly prolonged latency, meaning the response is delayed, often suggests a conduction issue along the neural pathway, such as demyelination or nerve compression.
The amplitude measures the strength of the muscle’s contraction, representing the functional integrity of the end organ. Reduced or absent amplitude suggests hypofunction or damage to the saccule or utricle, while an enhanced amplitude points toward an over-sensitive system. The third core metric is the asymmetry ratio (AR), which mathematically compares the amplitude of the VEMP response between the two ears. An AR exceeding established thresholds is considered a significant finding, indicating a difference in vestibular function between the left and right sides.
Conditions Indicated by Abnormal VEMP
The specific pattern of VEMP abnormalities helps connect the test metrics to recognizable vestibular disorders. For Vestibular Neuritis, an inflammation of the vestibular nerve, VEMP results identify which nerve branch is affected. Inferior vestibular neuritis often shows a significantly reduced or absent cVEMP response, reflecting dysfunction of the saccule and the inferior nerve. Conversely, if the superior nerve is affected, the oVEMP will be abnormal while the cVEMP remains normal.
Ménière’s Disease, an inner ear disorder characterized by fluctuating hearing loss and episodes of vertigo, often presents with reduced cVEMP responses, especially during symptomatic periods. The cVEMP threshold (the quietest sound needed to elicit a response) is often elevated, and the amplitude may be reduced in the affected ear. This is thought to be related to the buildup of fluid pressure, or endolymphatic hydrops, within the inner ear.
A hypersensitive response is the hallmark of Superior Canal Dehiscence Syndrome (SCDS), a condition where a portion of the bone covering the superior semicircular canal is missing. This “third window” effect causes the inner ear to become abnormally sensitive to sound and pressure, resulting in VEMP responses that are abnormally large in amplitude for both cVEMP and oVEMP. The VEMP threshold is also notably lowered, meaning a much quieter sound can trigger a response.
Finally, a tumor on the vestibulocochlear nerve, such as an Acoustic Neuroma (Vestibular Schwannoma), can cause absent or delayed VEMP responses. This typically manifests as a reduced or absent cVEMP, depending on which part of the nerve the tumor is compressing.
Clinical Management Following Abnormal Results
Receiving abnormal VEMP results necessitates further diagnostic steps. VEMP testing is rarely interpreted in isolation and serves as a piece of a larger puzzle, often requiring confirmatory testing. The VEMP findings guide the clinician on which specific area of the inner ear is affected, directing the choice of subsequent testing.
Confirmatory tests frequently used to establish a definitive diagnosis include:
- Audiometry to assess hearing.
- Videonystagmography (VNG) to evaluate the semicircular canals.
- High-resolution CT scan to visualize the temporal bone.
- Consultation with a specialist, such as an otolaryngologist or a neurologist.
The ultimate treatment pathway is determined by the underlying condition identified through the combination of VEMP results and other clinical data. Management for acute nerve damage, like vestibular neuritis, often includes physical therapy focused on vestibular rehabilitation. A diagnosis of Superior Canal Dehiscence may lead to a referral for surgical consultation. Patients with Ménière’s Disease may be managed with dietary changes and medication to regulate inner ear fluid pressure.