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274 THE EAR
of a peripheral lesion (otolith organ); or non-fatiguable, associated with a
lesion involving central vestibular connections.
Rotation Tests. The principle of rotation tests has been described, but these
tests now have a very limited place, e.g. where there is a contra-indication to
douching the ears, and in children. A more delicate turning test employing
minimal stimuli (cupulometry) has been introduced. A subthreshold accelera-
tion is given to the turning-chair until a certain angular velocity is attained and
this is maintained for a certain time until every reaction has ceased. The chair
is then suddenly stopped. This is repeated at different velocities and the
duration of after-sensation and after-nystagmus are recorded on a logarithmic
scale or 'cupulogram*. Rotation tests at present play a relatively subsidiary
role in vestibular investigation.
Caloric Test. The main advantage of the caloric test is that each ear can be
tested separately. Syringeing the ear with cold or hot water induces convection
currents within the semicircular canals and therefore stimulates them with
resulting vertigo and nystagmus. In the caloric test this is done under controlled
conditions. The patient lies with the head 30° above the horizontal (Fig. 152).
This brings the horizontal semicircular canals into a vertical plane. Cold
syringeing cools the most superficial part of the semicircular canal and gives
rise to a current hi the endolymph away from the ampulla, e.g. cold syringeing
of the left ear will produce a current away from the ampulla of the left
semicircular canal and induce nystagmus with a slow phase towards the left
and a quick phase to the right. There will be a sensation of rotation to the
left and if the stimulus is great enough nausea and vomiting. Douching with
warm water produces exactly the opposite effect. The tests are carried out on
each ear with water at 30 and 44 °C. The ear is freely irrigated for 40 seconds.
During testing the eyes are fixed in the straight ahead position upon an
object on the ceiling (Fig. 152). The nystagmus elicited is thus of second
degree. The measure of response is the number of seconds between the
commencement of the stimulus and the cessation of nystagmus. The results
are recorded as shown in Fig. 153. The two continuous lines represent a
3-minute period subdivided into intervals of minutes and 20 and 10 seconds.
The interrupted lines denote response durations. Normal variations occur in
response duration, the average being 1J-2J minutes, the hot reactions being
slightly shorter than the cold. Where there is depression of function on one
side, the response to hot and cold douching on that side will be depressed—
canal paresis (Fig. 153, CT). In some cases nystagmus in one direction, e.g.
nystagmus to the left, may be more readily induced and this is known as
directional preponderance of nystagmus (Fig. 153, B). Directional preponder-
ance of nystagmus may be evidence of labyrinthine imbalance. In many cases
there is a combination of canal paresis and directional preponderance.
Etectronystagmography. Eye movements may be recorded by utilizing the
standing electrical potential between the cornea and retina. This can be picked
up by electrodes placed close to the eyes and the difference in signal between
the two eyes, when amplified, gives an accurate record of eye deviation. This
method may be used for detecting more accurately the presence of spontaneous
or positional nystagmus and may also replace direct observation of nystagmus
during caloric and rotation tests. Eye movements can be recorded with
the eyes closed, making the detection of nystagmus more sensitive