Answer: Also called the rapid head impulse test, the Halmagyi-Curthoys test is a method used clinically to assess the vestibulo-ocular reflex.
The vestibulo-ocular reflex is one of the fastest reflexes in the human body. Because of the vestibulo-ocular reflex (VOR), our body automatically matches a head movement with a simultaneous turn of the eyes opposite to the direction of the head turn. For example, when the head turns to the right, the eyes will automatically move to the left to counteract the head movement. As expected, the opposite is true when the head turns to the right. This automatic reflex allows the eyes to stay focused on an object even though the head is moving.
The Halmagyi-Curthoys test is a clinical neurological examination that is used to assess whether or not the vestibulo-ocular reflex is intact in a patient. It is sometimes also called the Halmagyi head thrust maneuver, or the rapid head impulse test.
Physiology of the vestibulo-ocular reflex
The vestibulo-ocular reflex consists of three main components. The vestibular system (1) detects the head movement, and the muscular system responds by using a set of agonist (2) and antagonist (3) muscles to move the eyes.
First, the reflex relies on the vestibular system, which is the series of three semicircular canals in the inner ear that can very accurately detect head movement in any direction. These semicircular canals are filled with a fluid called lymph, and when the head moves, the lymph moves as well. The canals themselves are oriented in all three dimensions, so that the head movement can be detected in any direction.
When the head detects movement with the vestibular system, it send signals to neurons that eventually innervate the muscles that control eye movement. For example, a head turn to the left will cause an excitation (contraction) of the muscles that normally pull the eyes to the right, and a simultaneous inhibition of the muscles that pull the eyes to the left. As a result of these two muscle actions, the eyes will move towards the right, which counteracts the head movement to the left. This allows for the focus of vision to fixate on an object.
The reflex is connected by a three neuron circuit, which makes it remarkable fast.
Performing the Halmagyi-Curthoys test
In a neurological exam of a patient, the Halmagyi-Curthoys (rapid head reflex) test can be performed. The physician / neurologist performing the test should stand in front of the patient. With both hands, reach to the back of the person's lower part of the head, and place your thumbs on the chin for stability. Turn the head left and right, as if to simulate nodding their head to say no. Move the head only a few degrees from center.
The physician should look into the pupils of the patient. The patient, in turn, should be asked to look at the physician and focus on some element on their face, such as the bridge of the nose or the space between the eyes. With the hands holding the head, the physician should give a quick turn of the patient’s to one direction. With these rapid head turns, the head should turn no more than 20 degrees. Make sure the head is turned to one direction, then repeat for the other, since a brain injury that damages the vestibulo-ocular reflex could occur unilaterally.
Assessment of the Halmagyi-Curthoys test
In a normal individual, the eyes will remained fixed on the focal point in front of them immediately after the forced head movement. This indicates a correct transmission of vestibular signaling to the accompanying agonist and antagonist eye muscles.
In a person with an impaired vestibulo-ocular reflex, the eyes will not remained focused when the head is turned. Instead, the eyes will follow the head momentarily before returning back to the original site of focus. This delay may be present in both eyes, and in either set of nerves - the agonist or the antagonist muscle innervating nerve.