Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5507_Библиотеки_им_академика_М_И_Перельмана.pdf
126 Theory and Practice of Squint and Orthoptics
into inches, and the third one into dioptres (for
NPA in dioptres), and the age is indicated in
years on the fourth side. The sliding target
contains targets for measuring NPA and NPC.
Procedure. For measurement of convergence, a
dot or a vertical line may be used as the target.
It is advanced towards the patient at, or slightly
below the eye level, until the patient has
converged maximally and cannot sustain single
bifoveal fixation as the target is brought closer.
At this break point, the subject's non-dominant
eye will diverge (objective test) and patient may
appreciate diplopia (subjective test). The distance
from the canthus to this point is read on the rule
and the NPC is recorded in mm or cm. Some of
the near point rules have the zero point of their
scales at the so-called spectacle point (i.e. 27 mm
in front of the baseline). Therefore, with such
instruments, 27 mm must be added to the
distance that is read off the scale.
Normal values. The normal values of NPC vary
considerably among different persons and even
in different examinations of the same person.
In normal adults, its average value is 70 mm (7
cm) with a range between 50 and 100 mm (5 to
10 cm). A distance closer than 5 cm is excessive,
however, in children it may be as close as tip of
the nose. NPC further away than 10 cm is
defective or remote. In patients with convergence insufficiency (CI), it may be as remote as
25 or 30 cm or more.
Measurement of maintenance of convergence
The ability of the eyes to maintain convergence
after the patient has been able to converge his/
her eyes to a near vision can be tested by the
inappropriately named test—the drop convergence
test. In this test, after bringing the fixation target
into reading distance, patient is asked to maintain
convergence at this point and the fixation object
is dropped suddenly. Some patients are better
able than others to keep their eyes converged in
the absence of a fixation object.
ASSESSMENT OF ACCOMMODATION
AND AC/A RATIO
ASSESSMENT OF ACCOMMODATION
As we know, accommodation is a unique
mechanism, by which our eyes can even focus
Fig. 6.19 Effect of accommodation on divergent rays
entering the eye.
the diverging rays coming from a near object
on the retina in a bid to see clearly (Fig. 6.19).
Assessment of accommodation is of great
diagnostic value in cases of incomitant
strabismus of non-paralytic origin. Assessment
of amplitude of accommodation (the difference
between the dioptric power needed to focus at
near point 'P' and far point 'R', i.e. A = P – R) in
practice can be made either by measurement of
near point of accommodation (NPA) or by use
of minus lenses as below.
Measurement of near point
of accommodation
The near point of accommodation (NPA) is the
closest point at which small objects can be seen
clearly. It is also called 'near point' or 'punctum
proximum'.
NPA is measured using a near point rule such
as RAF rule (Fig. 6.18) or Prince rule. The
description of such a rule has been given in the
discussion on the measurement of the near point
of convergence.
To determine the NPA, a sliding target with
6/9 letters, numbers or fine lines is moved from
or towards the eye until closest point is found
at which it still can be seen clearly. During the
examination, the patient has to wear his/her full
optical refractive correction. The NPA is
determined first for each eye separately and then
for both eyes together. The NPA is measured in
centimetres marked on one side of the
instrument bar. The side of bar marked in
dioptres will indicate the amplitude of
accommodation in dioptres. The third side of
the bar shows the age corresponding the
accommodation. For example, if the patient
reports that the point appears blurred at 25 cm,
the dioptric markings will show +4.0 D and the
age 40 years.
127Evaluation of a Case of Strabismus and Orthoptic Instruments
If, while measuring the NPA, the patient's
amplitude of accommodation is found so low that
his/her near point is beyond the length of the
instrument, plus lenses are added to his/her
correction until the near point is brought within
range. The dioptric power of these additional
lenses is then deducted from the measured values
of amplitude of accommodation. Conversely, in
young patients with very high accommodative
power, minus lenses may be added to his/her
distance correction to move his/her near point
away from his/her eyes. The dioptric power of
those minus lenses is then added to the measured
value of amplitude of accommodation.
Measurement of amplitude of
accommodation using minus lenses
This test is also performed first for each eye
separately and then for both eyes together and
during examination patient has to wear his/her
full refractive correction. The patient is asked
to fixate 6/60 symbol at a distance of 6 metres
and minus lenses of progressively increasing
power are added before the eye till he/she can
see the target clearly. The power of this minus
lens is equivalent to the amplitude of
accommodation in dioptres.
ASSESSMENT OF ACCOMMODATIVE
CONVERGENCE/ACCOMMODATION
(AC/A) RATIO
The AC/A ratio is the relationship between
accommodative convergence (AC), expressed in
prism dioptres () and accommodation (A),
expressed in lens dioptres (D). This relationship
is linear one and is thought to be relatively stable
throughout life. The normal AC/A ratio is about
3 to 5 prism dioptres for one dioptre of
accommodation. The concept of AC/A ratio was
first clearly defined by Fry who later with
Haines introduced the abbreviation AC/A ratio.
Methods of measurement of AC/A ratio
1. Heterophoria method. To measure AC/A
ratio, in this method, the deviation is measured
with full optical correction at 6 metres distance
and at 33 cm distance in prism dioptres, and IPD
is measured in centimetres. Then the AC/A ratio
is calculated from the following formula:
AC/A = IPD + , where;
IPD = Interpupillary distance in
centimetres
n = Deviation at 33 cm or 3 dioptres
distance in prism dioptres
d = Deviation at 6 metres distance
in prism dioptres
d = The fixation distance at near in
dioptres
Note: Esodeviations are denoted by positive
(+) and exodeviations by negative (–) sign.
For example, if IPD = 6 cm, n = 9
exophoria and d = 3 exophoria, then
AC/A = 6 +
= 4 /D
2. Gradient method. This method is based on the
fact that for a given fixation distance, minus
lenses placed before the eyes increase the
requirement for accommodation and plus lenses
relax accommodation. Further, it is assumed that
–1.0D lens produces an equivalent of 1.0D of
accommodation, whereas +1.0D lens relaxis
accommodation by 1.0D.
In practice, original deviation is found at near
while the patient wears his/her optical
correction and then with additional +3.0D lens
and the calculations for AC/A ratio are made as
follows:
AC/A= where
L = Deviation with additional
lenses.
O = Original deviation without
additional lenses.
D = Dioptric power of the
additional lenses.
For example, if original deviation (O) = 2
esophoria, deviation with additional lenses (L)
= 10 exophoria and the power of additional
lenses (D) used is +3D, then:
AC/A = = 4/D
Alternatively, the patient's original distance
phoria (O) is determined while he/she wears
full optical correction. A –3.0D lens is then
128 Theory and Practice of Squint and Orthoptics
placed before his/her eyes and the distance
deviation (L) is measured once more. The
AC/A is calculated as above.
The gradient method is inaccurate because it
does not take into account the patient's
interpupillary distance (IPD).
3. Clinical distance-near-relationship method.
This is a very simple method in which AC/A
ratio is known by substracting distance
deviation (D) at 6 metres from the near deviation
(N) measured at 33 cm; i.e: AC/A = N – D. For
examples:
i. In a patient with esotropia (ET) of 40 PD at
near and 20 PD at distance, the AC/A ratio
= 40 – 20 = 20D PD.
ii. In a patient with distance orthophoria and
near exotropia (XT) 15 PD the AC/A =
–15 – 0 = –15PD
The results are interpreted as below:
• Up to 10 PD of N – D is normal.
• >10 PD of N – D is high AC/A ratio and
• <10 PD of N – D difference is less AC/A ratio.
4. Fixation disparity method. In this method,
AC/A ratio is indirectly derived from the fixation
disparity induced either by forced convergence
by use of prism or by altering the accommodative stimulus by use of optical lenses. Because
of its complexity, this test is not performed in
routine clinical practice.
5. Haploscopic methods. In haploscopy, the
visual fields of the two eyes are differentiated
and a separate target is presented to each eye.
Hering's original instrument was designed
primarily for studying the AC/A ratio. In
practice, this method is no more used. However,
the haploscopic devices, such as the major
amblyoscope, are of fundamental importance
for the study of the sensorimotor co-operation
of the eyes.
ASSESSMENT FOR
EXTRAOCULAR MUSCLE PARESIS
When paresis of one or more extraocular
muscles is suspected as cause of squint, in
addition to the duction test and version test,
following tests should also be performed:
• Abnormal head posture examination
• Diplopia charting
• Quantitative measurement of extraocular
muscle actions
• Field of binocular fixation
• Bielschowsky phenomenon test
• Bielschowsky three-step test
I. ABNORMAL HEAD POSTURE EXAMINATION
Note the abnormal head posture, if any and
examine its components in detail (see pages
302–305).
II. DIPLOPIA CHARTING
Plotting of diplopia fields is indicated in patients
complaining of confusion or double vision. The
test is easy to perform provided the patient is
co-operative. To perform the diplopia charting,
patient is asked to wear red-green diplopia
charting goggles; red glass being in front of the
right eye and green in front of the left eye. The
patient is made to sit with his/her head straight
in a semidark room and is shown a fine linear
light from a distance of 4 ft. The light is moved
from primary position into all of the other eight
directions of gaze. For each direction, patient is
asked to comment on the position, brightness
and separation between the red and green
images. From the patient's comments, the
examiner notes the following points:
• Whether horizontal diplopia is homonymous
or heteronymous.
• Whether the image seen by right eye (red
image) is higher or lower than the image seen
by the left eye (green image) or vice versa.
• In which direction of gaze, separation between
red and green images is greatest.
• Whether there are any directions in which
fusion is present.
In a modified test of Franchchetti, instead of
red green goggles, a red Maddox rod is placed
in front of right eye and white Maddox rod in
front of the left eye and the patient fixates on a
spotlight which is seen as vertical red line with
right eye and vertical white line with left eye.
Diplopia charts of patients with paresis of
different extraocular muscles are shown in
Figs 12.13 and 12.14.
Disadvantages of diplopia plotting test
• This test is only qualitative, therefore, it is not
possible to comment on the minor changes
of the improvement or deterioration from
the records of different dates in the same
patient.
• The test requires intelligent patient, especially
to comment where the separation is maximum.
• It is not possible to perform the test in colour
blind patients.
• This test is not of use in congenital palsies and
those of long-standing onset, because due
to deep suppression diplopia cannot be
elicited.
III. QUANTITATIVE MEASUREMENT OF
ACTIONS OF EXTRAOCULAR MUSCLES
The quantitative measurement of actions of
extraocular muscles is essential to comment
about the paretic muscles and the pathological
sequelae of the paralysis, viz. overaction,
contracture and secondary inhibitional palsy.
The tests employed for quantitative measurements of ocular movements are based on
haploscopy. The haploscopic tests are based on
the principle described by Burian that in the
presence of normal retinal correspondence, the
two test objects presented to the two eyes will
be superimposed, if they stimulate the foveae
of the two eyes, irrespective of the position of
the two eyes (Fig. 6.20).
Commonly used haploscopic tests to have a
graphic record of the relative power of
extraocular muscles in all directions of gaze
include:
• Lancaster red-green test
• Hess screen test, and
• Lees screen test
• Videooculography (VOG)
• Magnetic search coil or scleral coil
• Three-dimensional eye tracking
1. Lancaster red-green test
The Lancaster red-green test is a haploscopic
test. It utilizes a Lancaster red-green screen
which is window-shade type of screen that can
be rolled up when not in use. The screen
contains horizontal and vertical lines forming
squares of 7 cm (Fig. 6.21). All the squares are
of the same size and the tangential error is not
taken into account. While performing the test,
the patient's eyes should be in level with the
129Evaluation of a Case of Strabismus and Orthoptic Instruments
Fig. 6.20 Burian's principle of haploscopic tests. Note, the
right eye is esotropic and two different objects (red and
green) presented to the two eyes (A) are stimulating the
foveas and are thus subjectively localized as superimposed over each other (B).
centre zero mark, and he/she can be seated at
either 1 or 2 metres. At 2 metres, each square
subtends an angle of 2o = 3.5; at 1 metre it
subtends an angle of 4o = 7. The patient is given
a red-green reversible goggles (e.g. red glass in
front of right eye and green glass in front of left
130 Theory and Practice of Squint and Orthoptics
2. Hess screen test
Principle
The Hess screen test is based on the haploscopic
principle. It utilizes the Hering's law of equal
innervation, which states that in all voluntary
movements of the eye, equal and simultaneous
innervation flows from the brain to the muscles
of both eyes concerned in the respective
direction of gaze (yoke muscles).
Prerequisites
Patient should have:
1. Full understanding about what he/she is
supposed to do, since the test is purely
subjective.
2. Good vision in both eyes.
3. Central fixation.
4. Normal retinal correspondence.
Discription of conventional Hess screen
Original Hess screen consisted of a single
tangent screen made up of a black cloth 3 ft wide
× 3½ ft long, marked by a series of horizontal
and vertical red lines (Fig. 6.22). The distance
between each line subtends a visual angle of 5°.
Fixation points are indicated at the centre of the
Fig. 6.21 Lancaster red-green test.
eye) and green flashlight that projects a linear
image. The examiner has a similar red flash light
and projects the red streak of light on the zero
mark on the screen. The patient is asked to
superimpose his/her green light on the
examiner's red light. This is then repeated in all
cardinal directions of gaze. The distance
between the streaks of light represents the
measurement of the objective deviation provided
retinal correspondence is normal. The results are
plotted on a chart that is an exact replica of the
screen. Since the projected image is a line, the
patient's response may indicate the presence of
cyclotropia, when his/her streak is tilted. This
test is most useful in patients with ocular
paralysis and least useful in patients with
heterophoria or intermittent heterotropias.
Fig. 6.22 The Hess screen.
131Evaluation of a Case of Strabismus and Orthoptic Instruments
screen and at the intersections of the 15° and
30° lines by red dots. Thus, the red dots form an
inner square of 8 dots along the 15° lines and an
outer square of 16 dots along the 30° lines. The
inner square represents the 8 cardinal directions
of gaze and the outer square the extreme
directions of gaze. In the original Hess screen,
indicator consists of a knot tying three green
cords together to form the letter Y. The end of
central vertical green cord is fastened to a
movable black rod 50 cm long. The ends of the
other two green cords, forming upper two limbs
of the letter Y, are kept taut by black threads
that pass through loops to small weights at
corresponding upper corners of the screen. This
arrangement enables the patient to move the
indicator freely and smoothly over the whole
surface of the screen in all directions.
Modified wooden Hess screen. One of the
modifications of the original Hess screen is a
wooden screen with small red lights forming the
fixation points (Fig. 6.23) and a green dot light
projecter as the indicator. Presently, it is more
commonly in use.
Procedure Hess screen test
The patient wears red-green goggles and sits
50 cm from the commonly used modified
wooden Hess screen. The patient now sees the
fixation points (red light) with one eye and the
indicator (green light of projector) with the other
eye. The patient is asked to superimpose the
indicator successively on each of the fixation
points, and the relative position of the eyes is
plotted for each of these directions of gaze on a
chart which is replica of the Hess screen.
Digital Hess screen
Digital or the PC Hess screen provides the
clinician, a new computer-based tool for
assessing patients suffering from paralytic
strabismus, using image manipulation
technology and software technology. It is
designed to run on any computer operating
under Windows, with a 19’’ (or larger) monitor.
When the program is run for the first time, the
user is required to calibrate the size of the screen
(by measuring the dimensions of a box
displayed on the screen) and to enter the
preferred viewing distance (usually 25–50 cm).
It provides integration function of diagnosis of
strabismus, data record and analyze.
Key features
• Rapid and accurate assessment of the size and
direction of phoria/tropia
• Results plotted in conventional Hess screen
format allow the clinician to establish whether
a deviation is concomitant or incomitant and
which muscle is affected.
• A variety of analytical tools to help the
clinician form a diagnosis
• Built in database allows results to be archived
for future reference
• Results can be printed or pasted into referral
letters and reports
• Runs on a standard PC
• Voice instructions, possible.
Fig. 6.23 Modified Hess screen.
Operating methods
Digital Hess screen is a computer program
which is designed to run on any computer.
Steps of use are as below:
• Patient wears red and green goggles and is
positioned in front of the computer screen at
the appropriate distance (Fig. 6.24A).
• Room lights are extinguished and a red and a blue
circle are displayed on the screen (the right eye
sees the red circle and the left the blue).
132 Theory and Practice of Squint and Orthoptics
Fig. 6.24A Patient is seated in front of a PC Hess screen
after wearing red and green goggles.
Initially the red circle is placed in the top left
of the screen and the patient is instructed to
move the blue circle using the mouse until it
appears to be centred on the red circle.
• As the eyes are dissociated, any deviation in this
direction of gaze will result in a misalignment
of the circles (Fig. 6.24B). This is repeated for
either 9 or 25 directions of gaze (depending
on the option selected). The colour of the
circles is then reversed and measurements
repeated with the left eye fixating. The nine
point test takes approximately 4 minutes to
complete.
• Results are then displayed in the conventional
format on the screen.
• Multiple plots can be superimposed to assess
longitudinal changes and the exact amplitude
of any deviation can be displayed at any point
on the chart (Fig. 6.25).
• A number of analytical tools can then be applied
to the data to help the clinician establish a
diagnosis. For example, the program will
automatically calculate the relative areas of the
plots for the left and right eyes, helping the
clinician to determine which eye has a palsied
muscle and providing an index for monitoring
the progression of an incomitant deviation.
3. Lees screen test
Lees screen, also known as the Hess-Lees screen,
is another modification of the original Hess
Fig. 6.24B Misalignment of the circles seen due to any
deviation in the direction of gaze.
Fig. 6.25 Hess screen plots superimposed to assess longitudinal changes and the exact amplitude of any deviation.
screen. The Hess-Lees screen (Fig. 6.26) consists
of two tangent screens made of white
Fig. 6.26 Lees screen.
translucent material placed at a right angle with
a plane mirror bisecting this right angle and
dissociating the fields of the two eyes. The
tangent pattern similar to the original Hess
screen printed in black dots (fixation points) on
a white background is placed just behind both
the translucent screens, and is seen only when
the translucent screens are illuminated for
performing the test.
Procedure
To perform the test with right eye fixing, the
patient sits facing the left tangent screen at
50 cm from it with his/her forehead leaning
against the central vertical rim of the mirror. The
line bisecting the mirror horizontally lies in the
same plane as the centre of the horizontals of
the tangent screens. Patient's pupils are levelled
with this horizontal line by an adjustable chin
rest. The patient is given a pointer with a ring at
its tip. The examiner has another pointer with a
small disc at its tip, half the diameter of the
patient's pointer. The right screen is illuminated
and left screen is kept non-illuminated. The
patient's right eye vision is intercepted by the
mirror, in which he/she sees the right screen
projected forward as a virtual image that
appears to be superimposed upon the left screen.
133Evaluation of a Case of Strabismus and Orthoptic Instruments
The patient's left eye sees the left screen directly
(he/she is facing that screen) but cannot see the
right screen, situated laterally. The examiner
now places his pointer on the zero (central) point
of the right screen. Its image that appears to be
superimposed on the left screen is seen by the
patient's right eye. Patient is asked to superimpose his/her pointer on the examiner's
pointer with his/her left hand on the left screen.
By means of a foot pedal, the left screen is
illuminated for 1–2 seconds so that the examiner
can plot on the diagnostic Hess chart the precise
location of the patient's pointer on the left screen.
The 8 dots of the inner square are then plotted
in sequence and, wherever necessary, this is
followed by the plotting of the 16 dots of the
outer square.
To perform the test with left eye fixing, patient
sits facing the right screen which is kept
unilluminated while looking with his/her left
eye into the mirror, where he/she sees the
virtual image of the left (illuminated) screen. The
procedure described above is then repeated.
Diagnostic interpretation of the Hess chart
The diagnostic interpretation of the Hess chart
is done by comparing the two fields, i.e. one, of
the left eye plotted while the right eye fixing and
other, of the right eye plotted while the left eye
is fixing. The interpretation should be done as
follows:
1. Compression of the space between the two
plotted fixation points indicates underaction
of a muscle acting in that direction.
2. Expansion of the space between the two
plotted fixation points indicates overaction of
the muscle acting in that direction.
3. Smaller field belongs to the eye with the
paretic muscle (Fig. 6.27A).
4. Non-affected eye shows the larger field
expressing the overaction of the contralateral
synergist (Fig. 6.27B).
5. Fields of similar shape and size are suggestive
of comitant deviation, while the fields of
dissimilar shape and size indicate incomitance.
6. In the smaller field, the greatest displacement
(compression) away from the normal cardinal
direction will indicate the paretic muscle
(underaction). In many cases, displacement in
134 Theory and Practice of Squint and Orthoptics
Fig. 6.27 Hess chart of a patient with right lateral rectus
palsy.
the direction of the field of the antagonist (due
to contracture) may also be seen.
7. In greater field, the greatest displacement
(expansion) away from the normal cardinal
direction will indicate the overacting muscle
(contralateral synergistic or yoke muscle of the
paretic muscle). In many cases (especially in
those of long duration), there may also be
displacement of the field away from the
direction of the antagonist of this muscle (due
to inhibitional palsy of the contralateral
antagonist).
4. Videooculography (VOG)
VOG systems use video cameras to record eye
movements with high spatial and temporal
resolution. They provide quantitative data on
eye movements, including saccades, smooth
pursuits, and vergence, and can be used to assess
EOM function in different gaze directions.
5. Magnetic Search Coil or Scleral Coil
Magnetic search coil or scleral coil techniques
involve attaching tiny coils to the surface of the
eye or contact lenses. Magnetic fields generated
around the eye's movement can be measured
with high precision, allowing for quantitative
assessment of eye movements in multiple
directions.
6. Three-dimensional eye tracking
Advanced techniques involve tracking eye
movements in three dimensions (horizontal,
vertical, and torsional) to provide a
comprehensive assessment of EOM function.
This can be accomplished using specialized
equipment and software.
Quantitative measurement of extraocular
muscle actions plays a significant role in
diagnosing and managing eye movement
disorders, ensuring accurate surgical planning,
and evaluating treatment outcomes. These
measurements are typically performed by
specialized eye care professionals or
neurologists with expertise in eye movement
disorders using a combination of clinical tests
and advanced instrumentation.
IV. FIELD OF BINOCULAR FIXATION
It should be tested in patients with incomitant
squint, where applicable, i.e. if patient has some
field of binocular single vision. The area of
binocular single vision is opposite to the
direction in which ocular motility is impaired.
In general, the field of binocular fixation
represents the extreme limits of conjugate
movement of the eyes in all directions in the
absence of any movement of the head.
135Evaluation of a Case of Strabismus and Orthoptic Instruments
Procedure
The test is performed on the perimeter using a
central chin rest. The patient fixates a small
(3.5 mm) movable white target in the primary
position, which is then moved along the arc until
diplopia results or it goes out of the fixation
limits. This point is recorded. The arc is then
moved on successively in 15° steps and the test
repeated for each position until the whole field
has been examined. Normal field of fixation is
shown in Fig. 6.28.
If the patient has diplopia in central position
fixation, the target is moved in the periphery
till the target becomes single or the target goes
out of fixation point. The record of binocular
field of fixation is completed as above.
The diplopia as well as binocular fixation is
better appreciated, when the test is performed
using red and green goggles in front of the eyes
and a movable spot of white light as target. In
the area of binocular single fixation, the target
spotlight will appear as mixture of red and
green, and when binocular fixation is lost it will
appear red or green.
It is standard to shade the area of binocular
single vision after plotting on the chart. In the
presence of suppression, the test can be
performed with filters, but this may diminish
Fig. 6.28 Field of fixation: (A) left eye; (B) right eye and (C) binocular.