The head-impulse test, commonly abbreviated as HIT, is a rapid, bedside clinical examination used to assess the function of the vestibular system. It is one of the most valuable tools available to clinicians evaluating a patient who presents with dizziness, vertigo, or imbalance. The test is designed to detect a deficiency in the vestibulo-ocular reflex, or VOR, which is the reflex that keeps the eyes stable on a target while the head moves.
Understanding the Vestibular System
The vestibular system, located in the inner ear, is responsible for sensing head position and movement. It works in concert with vision and proprioception to maintain balance and a stable visual field. When the vestibular system functions normally, tiny movements of the head are detected by the semicircular canals, which send signals to the brainstem. The brainstem then directs the eye muscles to move the eyes in the opposite direction and at the same speed as the head. This is the vestibulo-ocular reflex. When the head moves to the right, the eyes move to the left, keeping the world in focus.
How the Head-Impulse Test Is Performed
The head-impulse test is performed by a clinician who sits facing the patient. The patient is asked to fixate their gaze on a stationary target, such as the clinician's nose. The clinician then holds the patient's head firmly and delivers a small, rapid, and unpredictable rotation of the head to one side. This quick movement is the "impulse." The impulse is brief and high-velocity, designed to challenge the vestibular system on that side. The clinician observes the patient's eyes carefully throughout the movement and immediately after it.
The test is then repeated to the opposite side. It is important that the head movement is sudden and that the patient's head is not allowed to move with the trunk. The amplitude is small, usually about 10 to 20 degrees, but the acceleration is very high. The unpredictable direction of the impulse prevents the patient from using a voluntary eye movement, a strategy known as a corrective saccade, to mask an underlying deficiency.
Interpreting the Results
In a healthy individual, the eyes remain locked on the target throughout the head impulse. The vestibulo-ocular reflex produces an eye movement of equal and opposite velocity to the head movement, so the gaze stays perfectly stable. This is a negative or normal result.
In a patient with a peripheral vestibular deficit on the side being tested, the vestibulo-ocular reflex is weakened or absent. As the head is rapidly turned, the eyes are carried with the head, moving off the target. The patient will then make a rapid, corrective saccade, a quick flick of the eyes, to bring the gaze back to the clinician's nose. This corrective saccade is the hallmark of a positive or abnormal head-impulse test. It indicates a loss of vestibular function on the side of the impulse.
The Role of HIT in the Diagnosis of Dizziness
The head-impulse test is a critical component of the HINTS examination, a set of three bedside tests used to differentiate central causes of dizziness, such as stroke, from peripheral causes, such as vestibular neuritis. A positive HIT, along with other findings, suggests a peripheral lesion. A negative HIT in a patient with continuous vertigo and other specific eye movement abnormalities can point toward a central, potentially dangerous, cause. In this context, the HIT serves as a powerful screening tool that helps guide urgent imaging and management decisions in the emergency department.
Beyond the acute setting, the HIT is also used in the assessment of chronic dizziness and balance disorders. It can identify a compensated unilateral vestibular loss, helping to explain a patient's persistent unsteadiness. The test is also useful in monitoring recovery after vestibular injury, as the corrective saccade may become smaller or be replaced by covert saccades that occur during the head movement itself.
Advantages and Limitations
One major advantage of the head-impulse test is that it can be performed quickly at the bedside without any specialized equipment. It is painless, safe, and requires only the active participation of the patient. It provides immediate information about the integrity of the high-frequency vestibulo-ocular reflex, which is exactly the range where the reflex is most sensitive to vestibular loss.
However, the test does have limitations. It requires a cooperative patient who can maintain fixation and who does not resist the head movement. It can be difficult to perform in elderly patients with cervical spine restrictions, and it is not suitable for patients with neck pain or known cervical instability. Furthermore, the test is qualitative in standard clinical practice; subtle vestibular deficits may be missed by the naked eye. For a more objective measure, video head-impulse testing uses high-speed cameras to record eye and head movement, allowing quantitative analysis of the vestibulo-ocular reflex gain.
Conclusion
The head-impulse test remains an indispensable part of the physical examination for any patient reporting dizziness. Its simplicity, speed, and diagnostic power make it a first-line vestibular test for clinicians across many specialties, including neurology, otolaryngology, audiology, and emergency medicine. By directly challenging the vestibulo-ocular reflex, the HIT provides a window into the function of the inner ear and helps separate benign peripheral conditions from dangerous central disorders. A clinician who masters the head-impulse test gains a reliable and efficient method for evaluating the dizzy patient.