Balance function is the complex ability to maintain stable posture and orientation during movement and stillness. This function relies on the brain’s seamless integration of information from three main sensory systems. The visual system provides external information about the body’s position relative to its surroundings. The somatosensory system, including proprioception, relays internal data from joints, muscles, and skin regarding contact with the support surface. Finally, the vestibular system, housed in the inner ear, senses head rotation and linear movement. When individuals experience dizziness, vertigo, or instability, a balance evaluation is necessary to pinpoint which system is causing the dysfunction.
Observational Assessments and Bedside Screening
The initial phase of balance evaluation involves quick, non-instrumented tests performed by a clinician. These bedside assessments help distinguish between issues originating in the peripheral inner ear, the central nervous system, or the somatosensory system. A foundational test is the Romberg Test, requiring the patient to stand with feet together, first with eyes open and then closed. Losing balance only when eyes are closed suggests a sensory deficit, forcing the body to rely heavily on vision to compensate for compromised proprioceptive or vestibular input.
The Single-Leg Stance (SLS) test and Tandem Gait assessment further challenge the body’s ability to maintain a small base of support. During the SLS test, the patient stands on one leg, typically with arms crossed, and the time they maintain this position is recorded. An inability to hold this posture for a minimum duration often indicates a higher risk of falling.
Tandem gait, or heel-to-toe walking, requires the patient to place the heel of the forward foot directly in front of the toes of the back foot while walking a straight line. This maneuver significantly reduces the lateral base of support. It can reveal subtle unsteadiness stemming from cerebellar or proprioceptive dysfunction that might not be visible during normal walking.
A more structured observational assessment is the Clinical Test of Sensory Interaction on Balance (CTSIB), which systematically manipulates the sensory environment to isolate performance. The patient stands on a firm surface and then on a compliant foam surface, with eyes open and then eyes closed. The firm surface provides reliable somatosensory information, while the foam surface degrades this input, forcing the brain to prioritize vision and the inner ear. Observing the patient’s sway in each condition helps the clinician infer which sensory input is compromised and how the brain re-weights available information to maintain balance.
Detailed Vestibular System Testing
When bedside screening suggests an inner ear problem, specialized, instrumented tests objectively measure the function of the vestibular apparatus. One common test battery is Videonystagmography (VNG), which uses infrared video goggles to record involuntary eye movements, known as nystagmus. Nystagmus is a sign of vestibular dysfunction because the inner ear and eyes are neurologically linked via the vestibulo-ocular reflex (VOR), which stabilizes gaze during head movement.
VNG includes positional testing, where the head and body are moved into specific positions. This checks for nystagmus that indicates misplaced calcium crystals in the inner ear, a common cause of vertigo.
A key component of the VNG battery is caloric testing, which assesses the function of the horizontal semicircular canal in each ear separately. This procedure involves introducing warm and cool air or water into the ear canal, creating a temporary temperature gradient near the inner ear fluid. This thermal stimulation mimics a head movement, causing a predictable pattern of nystagmus recorded by the VNG goggles. Comparing the eye movement responses between the two ears helps clinicians identify a unilateral weakness or partial loss of function.
A more modern assessment is the Video Head Impulse Test (vHIT), a primary tool in vestibular clinics. The vHIT uses lightweight goggles with a high-speed camera to precisely measure the VOR when the examiner quickly rotates the patient’s head. This test assesses the function of all six semicircular canals individually, checking for a reduction in the VOR gain (the ratio of eye speed to head speed). A low gain indicates the VOR is not working correctly, often resulting in corrective eye movements called “covert saccades” as the brain attempts to refocus the gaze.
Dynamic Postural Control Measurement
Advanced laboratory testing evaluates how the central nervous system integrates sensory information to manage dynamic stability. Computerized Dynamic Posturography (CDP) involves the patient standing on a sensitive force plate that measures the subtle shifts in the body’s center of pressure, or sway. The most common CDP protocol is the Sensory Organization Test (SOT), which systematically challenges the patient through six different conditions that alter or remove visual and somatosensory input.
In the SOT, the support surface and a visual surround can be made to sway in response to the patient’s movement, providing inaccurate sensory feedback. The results yield a Sensory Analysis profile that objectively quantifies the patient’s reliance on or inability to use each sensory system, essentially mapping the brain’s strategy for maintaining balance. This provides a functional picture of how the central nervous system adapts to sensory conflict, complementing diagnostic information gathered from vestibular testing.
Functional mobility tests are also employed to assess how balance control translates into real-world performance, providing a metric for fall risk. The Timed Up and Go (TUG) test measures the time it takes for a person to stand up from a chair, walk three meters, turn around, walk back, and sit down again. A score greater than a specific cutoff time, such as 14 seconds in some populations, is strongly associated with an increased probability of a fall.
The Functional Gait Assessment (FGA) is a detailed, 10-item test that evaluates dynamic balance. It requires the patient to walk while performing challenging tasks, such as turning their head, stepping over obstacles, and walking backward. These mobility assessments provide clinicians with a composite score of dynamic postural control, which is highly predictive of stability during daily activities.