Vestibule of the Ear

The vestibule of the ear is best described as the area of the inner ear between the tympanic cavity and posterior to the cochlea that contains the otolith organs. Lateral to the vestibule is the oval window and stapes footplate.

The vestibule of the ear and the otolith organs, called the utricle and saccule, are part of the vestibular system which controls our equilibrium and balance. Diseases affecting this area of the middle ear typically result in vertigo.

Anatomy

The vestibule of the ear encompasses a portion of the inner ear roughly 4 millimeters (mm) in size. It is a bony cavity within the temporal bone that contains organs and nerves associated with the vestibular system. It is posterior to the cochlea and anterior to the semicircular canals.

Otolith Organs

The utricle and the saccule are the two otolith organs that make up the vestibule. In both the utricle and the saccule, there is sensory epithelium (a type of tissue) and a macula, which is a receptor.

The utricle is located within the posterior part of the vestibule of the ear. The macula in the utricle is oriented in a horizontal position, which is important, as movement responds along the horizontal plane.

The saccule is the smaller of the two otolith organs. It lies anterior to the utricle. It is similar in composition to the utricle but the macula within the saccule is oriented vertically. This orientation makes the saccule sensitive to movement of the head along the sagittal (longitudinal) plane.

Macula

The macula contains sensory hair cell bundles which contain two types of hair-like structures:

• A single kinocilium which is a true cilia (hair-like structure), is the longest portion of the hair cell bundle, and remains erect.
• Multiple stereocilia, which are actually not cilia but an actin filament (protein thread-like fiber), are shorter and flexible allowing the group of stereocilia to either bend toward or away from the adjacent kinocilium.

The hair cell bundles are split into two halves separated by a midline, which is known as the striola. The striola is actually an overlying otolithic membrane that also is separated from the hair cell bundles by a gelatinous layer.

Embedded within the otolithic membrane are specialized crystals called otoconia. The otoconia are made up of calcium carbonate. In particular, the otoliths function in two ways:

• Sense acceleration from a shearing motion as the otolithic membrane lags behind the macula.
• Sense gravity from a shift in the otolithic membrane in relation to the sensory epithelium.

Function

The vestibule's primary purpose is related to your position in the world. Each part of the vestibule continuously sends a signal to your brain and changes in velocity or position either strengthens or weakens the signal sent. This in turn is translated in the brain into your sense of equilibrium.

The Utricle

The horizontal orientation of the utricle is important to how it functions. To understand this, imagine a sheet that divides the body between top (superior) and bottom (inferior). When you tilt you head while standing, your utricle is more likely to be driving your sense of balance compared to the saccule.

The tilting motion activates hair cell bundles in the macula of the utricle which strengthens the signal to the brainstem via the topmost (superior) portion of the vestibular nerve.

Unique to the utricle, sensory hair cells are slightly tipped toward the striola. The utricle also communicates with other parts of the inner ear. On one end, the utricle communicates with the semicircular canals, while on the opposite end the utriculosaccular duct allows communication with the other otolith organ, the saccule.

The Saccule

Unliike the utricle, the saccule is oriented vertically. You can imagine this plane by dividing the body into equal right and left halves. This orientation makes the saccule more sensitive to head movements when you are lying down and your head is in the horizontal position.

Also different in comparison to the utricle, in the saccule, the kinocilia are positioned further from the striola than the stereocilia. So bending in the opposite direction (away from the striola) causes increased signal strength to the brainstem. The saccule communicates with the cochlea via the ductus reuniens.

The Macula of the Utricle and Saccule

In relation to each striola (midline) the hair cell bundles are arranged in mirror-like fashion. When hair cells bend toward the kinocilium on one side of the striola, the bend away from the kinocilium on othe opposite side.

Bending towards the kinocilium strengthens the signal being sent to the brainstem (depolarization), while bending away from the kinocilium causes hyperpolarization of the receptor which decreases signal activity. Increasing or decreasing the signal strength changes your perception of balance and orientation.

Associated Conditions

Conditions that may occur in relation to the vestibules of your ear include disorders that can affect your sense of balance. Conditions may affect different parts of the vestibule and may include:

• Benign paroxysmal positional vertigo (BPPV): Condition that affects the otoconia in the macula of either the utricle or the saccule
• Migraine associated with vertigo (MAV): Not well understood, but thought to be related to the brainstems' interpretation of signals from the utricle or saccule

Other disorders not directly in relation to the vestibule, but closely related to the functionality of these otolithic organs include:

• Meniere's disease
• Labryinthitis
• Vestibular neuritis
• Superior semicircular canal dehiscence (SSCD)

Tests

Many things can alter your sense of balance and identifying the root cause may involve several different types of testing following a thorough health history.

ENG and VNG

Electronystagmography (ENG) and videonystagmography (VNG) measure a series of eye movements to determine a level of nystagmus. ENG is the most common test to begin differentiating causes of vertigo.

During ENG testing you will have electrodes placed to determine eye movement, whereas during VNG, you will wear googles with infrared sensors to measure eye movements.

Rotary Testing

Rotary testing using a similar strategy as ENG or VNG, employing the same electrodes, however you with either be instructed to rotate/tilt your head at certain intervals, or to sit in a chair that will rotate for you.

When in a chair that is rotating, you will be asked to wear a harness to secure your head to the chair's headrest. You will be placed in a dark room for this test and will hear instructions over an intercom system.

VEMP

There are two types of vestibular evoked myogenic potential (VEMP) testing to evaluate for any lesions in the vestibule of the ear:

• oVEMP: Occular vestibular evoked myogenic potential evaluates for lesions in the utricle
• cVEMP: cervical vestibular evoked myogenic potential evaluates for lesions in the saccule

During either of these tests, you will wear headphones and have electrodes placed according to the VEMP test being performed. To evaluate for dysfunction in the utricle, electrodes will be placed on your face and measure muscle movement associated with sounds heard in the headphones.

Electrodes alternatively will be placed on your neck to measure muscle movement associate with any sounds to evaluate for dysfunction related to the saccule.

CDP

Computerized dynamic posturography (CDP) tests three different systems to evaluate any disorder of equilibrium. For this test, you will stand on a platform and be secured for safety in a harness.

You will follow directions related to a visual target and maintain balance on a shifting plate. This test evaluates sensory input from your vision, vestibular system, and muscles.