Introduction
The functional status of the semicircular canals can be tested by the caloric test
video nystagmography/electronystagmography (VNG/ENG), dynamic visual acuity (DVA)
test, gaze stabilization test (GST), and video head impulse test (VHIT). All these
tests have certain limitations, for example the caloric test ENG/VNG as well as the
DVA and GST evaluates only the lateral canals, and it is only the VHIT that evaluates
all the six canals with precision. However, one common disadvantage is that all these
tests are pretty cumbersome and involve very costly and sophisticated instrumentation
that most clinicians cannot afford at least in the private sector. This new test called
the functional head impulse test (fHIT) is a new addition to the neurotologist's diagnostic armamentarium for evaluation
of all the six semicircular canal that addresses many of the limitations of the other
available tests. The fHIT reliably measures the functional status of all the six semicircular
canals by ascertaining the efficiency of the rotational vestibulo-ocular reflex (rVOR) in a quick, very simple, easy-to-perform way and with a perfectly physiologic
stimulus. The fHIT precisely assesses whether the rVOR of the semicircular canals
are being able to stabilize images when there is a fast head movement in the plane
of the semicircular canals. After a defective canal has been detected, the system
has inbuilt rehabilitative methods that can stimulate the canal and enhance its functionality.
In addition to its simplicity of design and affordable costs (without compromising
reliability), the fHIT system has the unique advantage of being a compact diagnostic-cum-therapeutic
device and is a boon to practioners in neurotology ([Fig. 1]).
Fig. 1 The functional head impulse test being carried out at the author's clinic.
Relevance of Functional Head Impulse Test in Modern Vestibulometry
Relevance of Functional Head Impulse Test in Modern Vestibulometry
Functional head impulse test does not ascertain the vestibulo-ocular reflex (VOR)
gain that the VHIT does or measure the percentage of canal paresis that the caloric
test ENG/VNG does, or even measure the loss of visual acuity that the DVA test does,
but it assesses whether the semicircular canals are being able to serve the function
of gaze stabilization and, if so, how efficiently. This qualitative rather than quantitative
measurement is a more practical approach and more relevant to the clinician as all
that the clinical practitioner in neurotology is interested to know is whether the
semicircular canals are serving their function of maintaining a stable gaze when there
is a fast head movement. If a stable gaze is not maintained during a fast head movement
in the plane of a particular semicircular canal then that canal is considered to be
malfunctioning. The fHIT can detect this very well by ascertaining loss of readability
during fast head movements.
Vestibulo-ocular Reflex, Visual Acuity, and Functional Head Impulse Test
Vestibulo-ocular Reflex, Visual Acuity, and Functional Head Impulse Test
The VOR keeps the visual objects in focus and maintains a stable gaze with perfect
visual acuity during fast head movement. The semicircular canals sense the head movement
very accurately, and the oculomotor centers then generate the requisite eye movement
reflexly to ensure gaze stabilization. The reflex mechanism is known as the VOR, and
as the semicircular canals detect only rotational acceleration, that is, acceleration
when the head is rotated at any angle, this VOR is called rotational VOR (rVOR).
Visual acuity is the clarity of vision and is dependent on the stability and sharpness
of the retinal focus within the eye. Readability or the faculty of recognizing a visual
object like a letter/Figure / picture depends on visual acuity. If the images of the
visual surroundings are not stable in the retina, there will naturally be a loss of
visual acuity. If a semicircular canal is defective, then the rVOR which is generated
from the inputs from that semicircular canal is poor and consequently images of the
visual surroundings slip in the retina (loss of gaze stabilization) whenever there
is a sudden fast head movement in the plane of that semicircular canal. This causes
a loss of the sharpness of the visual focus which in turn leads to a blurring of vision
and a loss of visual acuity and consequently a degradation of readability. The functional
Head Impulse Test (fHIT) ascertains if there is degradation or impairment of readability
on sudden head movement and thereby identifies any impairment of the rVOR generated
from that semicircular canal in the plane of which the head movement is taking place.
It not only detects the defective semicircular canal but also detects at which range
of accelerations is the semicircular canal malfunctioning. The fHIT informs the neurotologist
that (say) the left posterior semicircular canal is malfunctioning at accelerations
between 5,000 to 6,000 degrees/sec2 but is perfect at accelerations between 2000 to 5000 degrees /sec2. This is the precision of the fHIT, and it does so for all the six semi-circular
canals in a few minutes in a simple, nonexpensive but very reliable way.
The Vestibular Rehabilitation with V-Gym
The Vestibular Rehabilitation with V-Gym
Modern vestibular physiotherapy entails specific organ-targeted exercises for specific
disorders after modern vestibulometry has very precisely diagnosed the disorder and
localized the site of lesion. The diagnostic and therapeutic parts of the fHIT fit
into this model of modern neurotology that entails very accurate diagnosis and very
precise and specific organ- or disease-targeted therapy. With fHIT, we can diagnose
which particular semicircular canal is malfunctioning at which particular range of
frequencies and then institute rehabilitative physiotherapy by V-Gym to stimulate
that particular canal only at that particular acceleration ([Fig. 2]).
Fig. 2 (A) The VGym rehabilitation being carried out using the patient‘s mobile phone with
the requisite rehabilitation app installed in it. The patient can walk around while
performing the rehabilitation exercise. (B) The hardware for the VGym rehabilitation consists of just the headband with the
gyroscope fitted on it.
The V-Gym consists of a gyroscope and is connected to the patient's mobile phone via
Bluetooth. There are various programs of exercises to stimulate a particular semicircular
canal for rehabilitative purposes at different speeds and at different levels of difficulties
inbuilt in the fHIT and V-Gym systems. The rehabilitative programs are tailor-made
to work according to the disease identified by fHIT test. Hence, a combination of
fHIT and V-Gym provides a very easy and perfect solution for any vertigo management
if the defect is in one or more of the semicircular canals. However, it does not test
the otolith organs.
Conclusion
The fHIT as it stands today is NOT a replacement of other vestibulometric tests such
as VNG, VHIT, DVA, GST etc. but a very useful addition to the neurotologist's diagnostic
armamentarium and a boon to the practicing neurotologist in a big way. Possibly in
the very near future it will even replace some of the other very costly and cumbersome
tests. The USP is its simplicity, affordability, portability, and the negligible foot
print. A gyroscope, which is the heart of the instrument is a very cheap widely available
device that is present in all smart phones. We have been using the fHIT with its built-in
rehabilitation system in our clinic for the past 2 months, and it has been a fascinating
experience. This new device is a godsend to neurotologists because of its very simple
mechanism that practically does not ever go out of order due to its extremely simple
technology, its affordability, and, of course, its very precise diagnostic and therapeutic
functions. For further reading, refer to the blog page of vertigoclinic.in.
