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Vestibular system evaluation in paediatric patients
Dr T Balasubramanian
Early identification of auditory disorders in infants and children are common these days. Similarly now the focus is shifting on early diagnosis of vestibular system disorders in children. Earlier identification of audio vestibulo disorders in children leads to the development of early remedial stratergies. A reliable and complete history is must in order to accurately diagnose diseases of vestibular system. The challenges that confront the otolaryngologist in evaluating patients of pediatric age group are the inability of the patient to describe the symptoms in detail and the prohibitive cost of the equipments used to examine the vestibular system of these patients.
No equipment could replace a good physical examination. As a first step it imperative on the part of the examiner to win the confidence of the patient. Unless a child co operates physical examination is virtually impossible.
Looking for the presence / absence of spontaneous / gaze evoked nystagmus should be the preliminary step. If the child is able to follow simple verbal commands it can be asked to look straight ahead (spontaneous nystagmus will become evident now). The child may be asked to follow the fingers of the examiner in both horizontal and vertical directions. Gaze evoked nystagmus if present will become evident.
In this evaluation consideration should be given to testing “Head shake nystagmus”. This is also known as vestibuloocular reflex. This reflex is readily observed by the age of 9-12 months in normal developing infants. Movement of the eyes can be better appreciated by using Frenzel glasses / electro oculography / infra red video. In this test the child's head is gently rotated back and forth in a rhythmic horizontal manner. Nystagmus will be observed only in children with vestibular asymmetry. Head shake nystagmus evaluation will help in those situations when it is not feasible to perform bithermal caloric test or rotatory chair testing. Under normal circumstances while performing this test the eyes of the child will move smoothly on either side. Any catch up saccades of nystagmus will point towards the diagnosis of vestibular dysfunction.
Dynamic visual acuity techniques:
This test is actually a modification of Snellen's eye chart. In this test a baseline is obtained by discerning the distance from which the child can discern the smallest line of characters from a fixed distance. While the child is attempting to read the smallest line the head is rotated back and forth with the approximate frequency of 1-2 Hz. While reading loss of one line may be considered insignificant while loss of three lines should be considered to be significant. This indicates VOR deficiency.
BPPV is rather rare in children. They may rarely experience positional vertigo which can be testing by putting the child in the provoking position. Before performing positional tests like Dix Hall Pike maneuver it is of paramount importance to win the trust of the child. The examiner assists the patient in turning the head to the right or left when the child is lying on its back. The eye is observed for the presence or absence of nystagmus.
Tests for cerebellar functions:
This can be easily assessed in a child by asking it to perform marching on the spot test/ tandem walking test etc.
The equipment used has an infrared video camera mounted in eye goggles. This effectively replaces electrodes used to record eye movements. This procedure includes a battery of tests which are used to discern peripheral and central nystagmus.
The test battery include:
It should be borne in mind that the child should stop taking drugs for cough and cold two days prior to testing. The child should have eaten only lightly before the test. Video oculography is usually performed in a darkened room in order to avoid eye fixation. Children will be comfortable in dark only in the presence of their parents. Illuminated cartoon characters can be used to maximize the attention of the children.
Computerized rotatory chair:
This test complements the vestibular evaluation battery of tests. This test helps when other vestibular function tests are inconclusive. Since multiple frequencies are used in this test the accuracy is much more. The computerized rotatory chair for paediatric use has some slight modifications. This test uses sinusoidal harmonic acceleration its accuracy is enhanced. The whole enclosure is darkened and is designed in such a way that it resembles a space ship. The enclosure should be fairly big enough to accomodate the parent and the child who should sit on their lap. It should also be provided with a talk back system through which the examiner must be constantly communicating with the child. The following parameters of the test are considered for evaluation:
Gain is a measure that refers to the strength of the reflex and is calculated by dividing slow phase eye velocity by that of chair velocity (This is an indirect reference to head velocity). Children usually demonstrated higher gain values than adults at frequencies of 0.08 and 0.5Hz.
Phase refers to the timing of the response i.e eye movement in comparison with that of head turning movement. When the phase is less than zero, the eye velocity lags behind head velocity and when the phase value is greater than zero then the eye velocity leads the head velocity.
Symmetry helps in identifying unilateral vestibular disorders.
Step velocity test using computerized rotatory chair can be performed in preschool children and adolescents. The patient seated in the rotatory chair is rotated in one direction at approximately 100 degrees / second. The build up of nystagmus activity as the rotating chair accelerates is recorded. When a steady state of rotation is reached then nystagmus begins to fade. The per rotatory nystagmus beats in the same direction as the rotation (right beating nystagmus during clockwise rotation). The vestibular time constant is measured for each rotation. Vestibular time constant is defined as the amount of time necessary for the per rotatory nystagmus to diminish to 37% of its original intensity. On sudden stopping of the chair post rotatory nystagmus would return. It should also be measured in terms of vestibular time constant.
Computerized rotatory chair enclosures can be used to perform optokinetic testing.
Visual vestibular interactions: This is another test which could be performed using computerized rotatory chair. Patient views strips projected on the enclosure wall as the chair rotates. The resultant nystagmus is recorded using a tracking device.
Computerized dynamic posturography:
It helps in assessing the balance as a function. It breaks down the relative contributions of visual, proprioceptive and vestibular clues. The patient is made to stand on a platform wearing safety harnesses and is made to face a visual surround. The platform contains sensors that measure the force exerted by the feet when the centre of gravity changes. The most commonly performed test using this equipment is Sensory organization test. In this test the patient is supposed to maintain the best balance possible despite conflicting visual and proprioceptive clues.
Motor control test is another test that is performed with this equipment. In this test the platform experiences unexpected peturbations which are classified as small, medium and severe in degrees. These peturbations take place in both forward and backward directions. The patients ability to maintain balance even during these perturbations is used as an index.
Adaptation test: This is another test that can be performed with this equipment. In this test the platform moves unexpectedly upwards / downwards in such a way that the toe either points upwards or downwards. The patient's ability to regain balance is measured.
The child should be rewarded on completion of each test as this would motivate the child to take the entire battery of tests.
This stands for vestibular evoked myogenic potentials. This test is rather quick to perform and easy to interpret. The purpose of this test is to determine if the saccule, portions of otoliths, inferior vestibular nerve and its central connections are intact and working normally. The saccule which is the lower otolith organ has mild sensitivity to sound. This sensitivity can actually be measured and is considered to be a remnant of its evolutionary function. The vemp recording is usually made from an electrode placed at the level of sternomastoid muscle.
Conductive hearing losses obliterate vemp while sensorineural hearing loss doesn't affect it.
Sound stimulus is delivered as loud clicks / tone bursts atleast 90 – 100dB sound pressure level. Sound is usually presented at 200 ms intervals. Optimum frequency is between 500 – 1000 Hz. The sternum is usually used for reference electrode and the forehead for reference electrode.