The Vestibular Evoked Myogenic Potential (VEMP) is a non-invasive diagnostic procedure used to assess the function of the inner ear’s balance organs. This test specifically evaluates the health and responsiveness of the otolith organs: the saccule and the utricle. These structures sense linear acceleration, gravity, and maintain spatial orientation. VEMP testing provides objective data about the integrity of the peripheral vestibular system, complementing other balance assessments.
How the VEMP Test Works
The VEMP test relies on stimulating the otolith organs with a loud sound or a mechanical vibration delivered to the ear. This activates specialized hair cells within the saccule and utricle, triggering an involuntary reflex arc. The resulting electrical potential is a short-latency electromyographic signal that travels to specific muscles.
This diagnostic tool is divided into two primary types, named for the muscle group where the response is recorded. For the Cervical VEMP (C-VEMP), surface electrodes are placed over the sternocleidomastoid muscle in the neck. The patient is asked to keep this muscle contracted, usually by lifting or turning their head, to enhance the recorded response.
The Ocular VEMP (O-VEMP) uses electrodes placed beneath the eyes, typically monitoring the inferior oblique muscle. Both C-VEMP and O-VEMP measure the muscle response to inner ear stimulation, reflecting different reflex pathways. The C-VEMP evaluates the vestibulocollic reflex (head and neck stability), while the O-VEMP assesses the vestibulo-ocular reflex (stabilizing vision during head movement).
Identifying Specific Inner Ear Function
The utility of VEMP testing comes from its ability to isolate and evaluate the two distinct otolith organs. These organs detect movement in a straight line, such as riding in a car or feeling gravity when tilting the head.
The C-VEMP response is directly linked to the function of the saccule, which senses vertical linear acceleration. The neural signal for this reflex travels along the inferior branch of the vestibular nerve before reaching the neck muscles. An abnormal C-VEMP response therefore points toward dysfunction in the saccule or the inferior vestibular nerve pathway.
In contrast, the O-VEMP primarily evaluates the function of the utricle, which is sensitive to horizontal linear acceleration and static head tilt. This response is mediated by the superior branch of the vestibular nerve. By comparing the results of both the C-VEMP and O-VEMP, clinicians can determine whether a balance disorder involves the superior or inferior vestibular nerve branches and localize the specific otolith organ affected.
Clinical Conditions Diagnosed by VEMP
VEMP testing is a valuable tool for confirming the diagnosis of several balance and inner ear disorders.
Superior Semicircular Canal Dehiscence (SSCD)
One of the most specific applications is in the diagnosis of Superior Semicircular Canal Dehiscence (SSCD). In SSCD, a missing piece of bone creates a “third window” in the inner ear, causing the vestibular system to become hyper-responsive to sound and pressure. This hyper-responsiveness results in a pathologically lowered VEMP threshold, meaning a response can be triggered at an intensity much quieter than normal. The amplitude of the VEMP response is also often significantly increased in ears affected by SSCD.
Meniere’s Disease
For Meniere’s Disease, VEMP findings help confirm the diagnosis and track the disease’s progression. Early in Meniere’s Disease, the buildup of fluid, known as endolymphatic hydrops, often causes the saccule to distend. This can lead to an abnormally enlarged C-VEMP response due to the increased pressure on the saccule’s sensory cells. In later stages of the disease, when the otolith organs may have suffered damage, the VEMP response might become reduced or entirely absent.
Vestibular Neuritis
The test is also instrumental in distinguishing between types of Vestibular Neuritis, an inflammation of the vestibular nerve. If the inflammation affects the inferior vestibular nerve branch, the C-VEMP will be abnormal while the O-VEMP remains normal, indicating an isolated problem with the saccule pathway. Conversely, if the superior branch is affected, the O-VEMP will be abnormal, providing specific anatomical localization of the nerve damage.
Vestibular Schwannoma
VEMP testing also plays a supporting role in the assessment of Vestibular Schwannoma, a benign tumor on the eighth cranial nerve, often called an acoustic neuroma. Since this tumor grows on the nerve that contains both the superior and inferior vestibular branches, VEMPs can help assess which nerve branch is being compressed or damaged. The presence or absence of C-VEMP and O-VEMP responses provides objective evidence of the functional status of the nerve prior to treatment planning.