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Author - Dr. Sanjay Dhawan
Last update -
03 March, 2005
Introduction
Dark-room procedures (DRP) constitute an essential part of the
examination of the eye beside being an important part of the undergraduate professional
examinations. The main DRPs are:
Preliminary examination at 1 m (including Retinoscopy).
Distant Direct Ophthalmoscopy at 22/25 cm.
Indirect Ophthalmoscopy.
Direct Ophthalmoscopy.
All DRP are performed in very low ambient illumination (not necessarily
pitch dark) and with the examiner preferably dark adapted.
Before describing DRP it is important to understand the principle of
coaxial illumination which is applicable to all DRP.
Coaxial Illumination
Why does the pupil appear black no matter how bright light we use to
examine the eye? And why does it appear red when seen through ophthalmoscope or
retinoscope even with very faint light? A very basic principle of optics is that in any
ideal optical system the light rays retrace their own path. Therefore the light
rays from the torch-light falling into the eye get reflected back in the same direction
and since the observer’s eye is not a source of light, no light comes in that
direction and therefore the pupil appears black (in eyes with high refractive errors the
pupil may appear red because the eye is no longer an ideal optical system). However the
ophthalmoscope or retinocsope have an ingenious optical system of coaxial illumination
i.e. the axis of illumination and the axis of observation are in same line making the eye
of the observer a virtual source of light, as depicted in the diagram below:
The light reflected from the patient’s eye gets transmitted
through the partially mirrored glass and falls into the observer’s eye, therefore the
pupil appears red (the fundal glow).
What is the color of the retina? Theoretically purple because of
the pigment visual purple (as deciphered from its absorption/emission spectrum) but in
practice when any amount of light falls on the retina the pigment gets bleached making it
almost transparent. Then why does the fundal glow appear red? Because of the light
reflected from the choroidal vasculature.
I. Preliminary Examination at 1m (including Retinoscopy)
Patient is seated in a dimly lit room with light source above and
behind patient’s left shoulder. Observer sits at 1m and using plane mirror of the
Priestley-Smith Retinoscope (described later) light is reflected onto patient’s eye
while looking through its hole. If the ocular media are clear the pupil appears red due to
fundal glow.
Retinoscopy (or Skiascopy) is an objective method of determining
the refraction of the eye (not to be confused with ophthalmoscopy which means
visualization of the retina).
To understand the methodology of retinoscopy we will make following
assumptions:
The reflected light from plane mirror is moved across the
patient’s eye by turning the mirror from left to right while the movement of the glow
(light) in the pupil is observed. There are 3 possibilities:
1. Glow in the pupil moves in the same direction as the light
outside. This means the patient can be any of the following:
Hypermetropic
Emetropic
Myopic less than -1.0 D
2. Glow in the pupil moves in the opposite direction. This means
patient is myopic by more than -1.0 D.
3. Glow does not move (and the pupil is either uniformly lit or uniformly dark).
This means patient has myopia of 1.0D.
Above assertions can be easily remembered by studying the following
diagram:
It may be seen from the retinoscopy line that if the glow moves in the
+ (same) direction patient is anywhere on the + side and if the glow moves in the -
(opposite) direction the patient is anywhere in the - side; but in both these situations
we do not know where exactly the patient is. However, when the glow in the pupil
does not move that we know for certain that the patient has myopia of -1.0 D. So what do
we do if the glow moves in the same or the opposite direction? We neutralize the movement
i.e. bring the optical system to the point where the glow stops moving, by putting in
front of the eye increasing power of + or - lenses depending on + or - direction of
movement of glow, respectively. If the glow moves in the + (same) direction we neutralize
it by + lenses and if it moves in the - (opposite) direction then by - lenses. When the
glow stops moving then the ‘optical system’ viz. the eye of the patient and the
lens in front of it, has come to the point of neutralization and therefore has myopia of
-1.0 D. Now if we prescribe -1.0 D lens to this ‘optical system’ it will become
emetropic, which is same as adding -1.0 to the power of lens in front of the eye (the
retinoscopic value). Thus, we get the refractive error of the patient.
Examples:
If the movement in the same direction gets neutralized by +5.0D lens
then the refractive error is +4.0D (-1.0 added algebraically to +5.0).
If movement in the same direction is neutralized by +1.0D then the
error is 0.0 i.e. the patient is emetropic[-1+(+1)].
If movement in same direction is neutralized by +0.25D then the error
is -0.75D[-1+(+0.25)].
If the movement in the opposite direction is neutralized by
-1.0D then the error is -2.0D[-1+(-1)].
NB:
The higher the refractive error the fainter the glow in the pupil and
slower does it move. But as one approaches the point of neutralization the glow gets
brighter and moves faster. And at the point of neutralization the glow is the brightest
and completely fills the pupil.
In some cases direction of movement of glow is not clearly defined,
instead there is scissoring of the glow; here the point of neutralization is reached when
two limbs of the scissors start from the center of the pupil and move equally in opposite
directions.
If two light reflexes are seen, one central and the other peripheral,
then one should neutralize the central glow because the central parts of cornea and lens
are more important in forming image on the retina.
Theoretically the ideal distance for doing retinoscopy is infinity (¥ ) because the retinoscopy directly gives the refractive error. The
neutralization point correspond to myopia of -1¸ distance in
meter which is also the amount to be added to the retinoscopy value.
At neutralization point the patient’s and the observer’s eyes
become conjugate foci of the optical system (as the image of the illuminated points
on the patient’s retina are formed at the observer’s pupil).
The method described above gives refractive error only in the
horizontal meridian, whereas the error may not be the same in all meridians as is seen in
astigmatism. However, as most patients have regular astigmatism in which two principal
meridians disposed at right angle to each other can be defined. Also, these meridians are
most commonly aligned vertically and horizontally. Therefore, it is customary to do
retinoscopy both vertically and horizontally, and note the values separately as
follows:
where x denotes retinoscopy value along horizontal meridian and y
denotes the value along the vertical meridian (obtained by moving the mirror vertically).
If these two values are equal then there is no astigmatism and a spherical lens alone will
correct the error. But if these two values are not equal then it denotes presence of
astigmatism which needs a cylindrical lens (alone or in combination with a spherical lens)
for its correction, as explained next.
A cylinder is a lens which has refractive power only in one meridian
(i.e. at right angle to its axis) and no power at right angle to it (i.e. along the axis).
Now if the retinoscopy values are e.g.:
Now a +3.0 D spherical lens would completely correct the vertical
meridian and would partly correct the horizontal meridian leaving a residual error of +1.0
D [+4.0-(+3.0) = +1.0]. This is corrected by +1.0 D cylinder whose axis is placed at 90° (vertically) because the power is required to act at 180° (horizontally). Thus the prescription would be:
+3.0 D sphere / +1.0 D cylinder at 90°
Transposition means an equivalent prescription with the cylinder of
opposite sign. While transposing a prescription the spherical-equivalent (and not the
sphere) of the lens is kept constant. Following are the steps to transpose a prescription:
Algebraic sum of the sphere and the cylinder gives the new sphere.
Same cylinder with opposite sign.
Axis is placed at right angle to the previous axis.
Spherical Equivalent of a spherocylindrical lens (combination of a
sphere and cylinder) is a spherical lens with same average refractive power
obtained by algebraically adding half the value of the cylinder to the sphere. Note the focal
point of the spherical equivalent coincides with the circle of minimal blur of
the spherocylindrical lens.
The use of Cycloplegia in retinoscopy
In retinoscopy a common source of error is accommodation which
is most active in young patients. When patient accommodates the refractive power of the
eye increases resulting in a variable shift towards myopia. A simple solution would be
to relax the accommodation by the use of a cycloplegic but cycloplegia leads to abolition
of basal tone of the ciliary body muscles resulting in manifestation of latent
hypermetropia. So if the patient accommodates there is a shift towards myopia, and if
we use cycloplegia there is a shift towards hypermetropia; however, the latter situation
is preferable as the amount of shift towards hypermetropia caused by a given
cycloplegic is known and the same amount can be reduced from the retinoscopy value to get
the refractive error.
Cycloplegic-Mydriatic Agents
Preparation |
Dosage |
Age
Group of patient |
Amount
to be reduced |
Duration
of action |
Complication |
1. Atropine sulfate 1 % oint.
|
TDS ´
3 days |
0 to 5 years |
1.0 to 1.5 D |
2 to 3 weeks |
Allergy, Fever, Flushing
of face, etc. |
2. Homatropine hydrobromide 2 % drops
|
every 15 min. ´ 4 applications |
5 to 7 years |
0.5 to 1.0 D |
24 to 48 hr. |
Dry mouth, Urine retention
in old |
3. Cyclopentolate HCl 1% drops
|
every 15 min. ´ 4 applications |
7 to 20 years |
0.5 to 1.0 D |
12 to 24 hr. |
Transient psychotic
reaction |
4. Tropicamide 0.5-1 % drops
|
every 15 min. ´ 4 applications |
not specified |
0.25 to 0.5 D |
2 to 4 hr. |
Dry mouth |
5. Phenylepherine HCl 5-10 % drops
|
every 15 min. ´ 4 applications |
where only mydriasis is
required |
Nil (it has no cycloplegic
action) |
6 to 8 hr. |
Increase in BP in
hypertensives |
NB- For patients of 20 to 30 years of age cycloplegia is used if needed
in a particular case, and after the age of 30 years cycloplegia is not required.
When retinoscopy is done under cycloplegia, patient is not prescribed
glasses at the same time but patient is examined again after the effect of the cycloplegic
has worn off i.e. usually after 1 week, for subjective verification (post cycloplegic
test). However, patient’s acceptance of the refractive correction is determined the
same day. Therefore, the sequence of steps of refraction are:
- Retinoscopy under cycloplegia.
- Get the acceptance by adding -1¸ distance (m). If the
distance is 1 m then add -1¸ 1 i.e. -1.0
- Prescibe after cycloplegia is worn off by reducing from above value the amount for given
cycloplegic used.
Note that the amount for cycloplegia is reduced only form the
spherical component of the refractive correction.
II. Distant Direct Ophthalmoscopy (DDO) at 22 cm using plane mirror
After preliminary examination at 1 m examiner moves closer to the
patient to a comfortable near vision distance of 22 cm (or 25 cm and concave mirror
according to some authors). In a normal eye with clear media one can see red fundal glow
in the pupil. The abnormalities that can be observed by this method are as follows:
Opacity in the media (cornea, aqueous, lens and vitreous) will
appear black against the background of red fundal glow. The opacity appears black
because no light goes in the eye or comes out of it from the area of opacity.
Depth of opacity (Parallactic Displacement) can be estimated
qualitatively by the method of parallactic displacement. While observing the pupil patient
is asked to move the eye up, down, right and left. An opacity located in the pupillary
plane does not seem to move relative to the pupil whereas any opacity anterior to the
pupil seems to move in the same direction and an opacity posterior to the pupil seems to
move in opposite direction. The further away an opacity is from the pupillary plane
the more does it move in relation to the pupil. This can be easily understood by studying
the diagram above. If we take a rod which is hinged at point 3, and turn it up. The points
1 and 2 which are in front of the hinge move in the same direction and points 4 and 5
behind the hinge move in the opposite direction, whereas the point 3 which is in the plane
of the hinge does not seem to move. When looked at end-on it appears like the figures on
the right with the circles representing the pupil. The pupil acts a frame to which all the
movements are referred. Contrary to expectation the reference plane in the eye is not the
center of rotation because the movement of any opacity can be appreciated only in relation
to the pupil.
Opacity in fluid or solid part of the media can be
differentiated by observing the after-movement of the opacity i.e. movement of the
opacity after the eye has halted. Presence of after-movement denotes opacity in the fluid
part of the media (aqueous or vitreous).
Keratoconus gives rise to a ring-shadow corresponding
to the base of the cone of the keratoconus. It occurs despite the fact that the cornea is
transparent, because of total internal reflection of light occurring at the base of
the cone. This reflects the light back into the eye and, thus, the base of the cone
appears as a dark ring. Similar ring shadow may also be seen in lenticonus and the two can
be differentiated by parallactic displacement.
Iris nevus and coloboma, both of which appear as black patch
on the iris, can be differentiated by DDO. Coloboma being a defect in the iris permits
light to pass through it making fundal glow visible. However, glow cannot be seen across
the nevus.
Cataract can be easily differentiated from nuclear sclerosis
which also appears gray with torch light examination. Cataract appears as dark opacity
against fundal glow but in nuclear sclerosis the media are clear. Moreover, as long as
some clear cortex is present in cataract some fundal glow is visible, thus differentiating
immature from mature cataract. In mature cataract no glow is seen, rather pupil appears
gray even with DDO due to light reflected from the cataract.
Subluxation of clear lens is not obvious on examination with
torch light as the pupil appears dark. With the DDO the edge of the lens crossing
the pupil stand out as a dark crescent against fundal glow. Though the lens is
clear yet the edge appears dark because of total internal reflection of light at
lens edge.
Lens Coloboma, a notch like defect in lens edge can be readily
seen with DDO.
Vitreous hemorrhage in its milder form gives rise to darkening
of the red fundal glow which then appears deep red or maroon. In severe vitreous
hemorrhage no glow can be seen and pupil appears dark as whatever light gets reflected
from fundus is absorbed by the blood pigments.
Retinal Detachment (RD) is separation of the neural layer of
retina from the retinal pigment layer leading to loss of nutrition of the former from the
choroidal vasculature. As a result of this the retina becomes grayish opaque and lies much
anteriorly (closer to the lens). This gives rise to grayish glow in the area of RD
and makes that part of the retina visible by DDO (detached retina becomes highly
hypermetropic). Retinal folds may be seen moving with the movement of the eye and retinal
vessels running over the folds as dark bands.
Retinal Tumors are also visible on DDO as gray glow and
sometimes the mass itself can be seen with a bunch of abnormal vessels on it. Unlike RD no
folds are seen and there is no after-movement in retinal tumor.
Gray glow on DDO; other causes are:
III. Indirect Ophthalmoscopy
Ophthalmoscopy means visualization of the fundus oculi. In principle,
indirect ophthalmoscopy involves making the eye highly myopic by placing a high
power convex lens (+13, +20 or +28 D) in front of the eye so that a real
inverted image is formed in front of the lens.
Classically indirect ophthalmoscopy was done using concave mirror of
the Priestly-Smith retinoscope but now it is done using a head mounted binocular
indirect ophthalmoscope. Patient lies on a couch with the pupils fully dilated. The
examiner stands at the head end of the bed and directs the light of the ophthalmoscope
onto the patient’s eye and while looking at the pupil interposes the convex lens in
front of the eye, then moving the lens away form the eye till the retina is seen clearly.
A real inverted image which is 3 to 5 times magnified is formed in between the lens
and the observer. The magnification depends on the power of the lens used and the
refraction of the eye and is given by the refraction of the eye divided by the power of
the lens used e.g. 60 D ¸ 13 D = 5 approx.
Advantages |
Disadvantages |
1. Stereopsis: the greatest advantage is the 3
dimensional view possible with this method which lets the depth or the solid nature of a
lesion to be appreciated.
2. Large field of view enables a wide area of the retina to be seen at a given time
(about 30° ). Thus a large lesion e.g. retinal-detachment,
tumors, etc. can be observed in its entirety.
3. Periphery of the retina can be seen, even up to ora serrata (combined with
indentation of the sclera) by this method. So the peripheral retinal lesions e.g. retinal
degenerations, breaks or holes etc. may be visualized.
4. Vitreous can be easily examined and various vitreous abnormalities diagnosed.
5. In hazy media this method is useful because the illumination is very bright and the
method does not make use of the refractive system of the eye, thus being of immense
benefit in corneal haze, cataract, vitreous hemorrhage, etc.
6. This can be used intra-operatively as there is reasonable distance between the
patient and the examiner so various maneuvers can be done on the eye, and also the lens
used for the purpose can be sterilized.
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1. The technique is difficult to learn as the image
is inverted; the orientation being achieved with lot of practice. Although by examining
the patient from the head end of the bed the retina is inverted thus resulting in an
‘erect’ image yet coordinating the indentation of the sclera with the observed
image requires learning.
2. It is very difficult to use this method in an upright patient.
3. The instrument is bulky and therefore not easily portable.
4. Magnification is less, therefore the small lesions e.g. macular
pathology, cannot be examined in all their details.
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IV. Direct Ophthalmoscopy
First given by Herman von Helmholtz, ophthalmoscopy is classically done
by just a plane mirror making use of the optical system of the patient’s
eye. The light of the hand held self illuminated ophthalmoscope is directed to
patient’s pupil while observing it through the fenestration in the ophthalmoscope.
The examiner approaches the patient’s eye to a distance within anterior principal
focus i.e. about 15 mm, where a virtual erect image of the fundus is seen formed
behind patient’s eye. The optical system of the eye acts as a simple microscope
which magnifies the image about 15 times. The magnification can be derived by
dividing the diopteric power of the eye by 4, thus 60 ¸ 4 =
15. The magnification is more in myopia and less in hypermetropia.
Advantages |
Disadvantages |
1. Easy to use and portable.
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1. No stereopsis. |
2. Greater magnification enables fine details to be
examined.
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2. The illumination is low
therefore if media are hazy the visibility is poor. |
3. Patient can be examined in any position.
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3. Cannot be used for
operative procedures because of close proximity with the patient and inability to
sterilize the ophthalmoscope. |
4. Erect image does not cause any difficulty in
orientation.
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4. Field of view is small
and the retinal periphery cannot be seen. |
APPLIANCES
I. PIN-HOLE
A black disc with a 0.5 to 1.5 mm diameter fenestration in the center.
Uses:
Diagnosis of refractive error - to differentiate the diminution of
vision caused by refractive errors from any other cause. Pin-hole is placed in front of
the patient’s eye if the vision improves to (nearly) normal then it denotes that the
diminution is caused by refractive error otherwise an organic cause may be present. The
pin-hole allows only a very narrow pencil beam of parallel rays to pass through the
optical center of the eye, which does not need any refraction to form a sharp image on the
retina. In cases with central media opacity and macular pathology the vision may
deteriorate with the pin-hole because pin-hole cuts out light from the peripheral media
and forms clear image on the macula.
Confirmation of the refractive correction is done by placing a
pin-hole in front of the corrective lenses, if the vision improves further it means
the correction applied is imperfect and needs improvement.
Uniocular diplopia or polyopia is differentiated from binocular
diplopia by placing the pin-hole in front of the eye with diplopia. Elimination of
diplopia denotes that the diplopia is uniocular.
Two point discrimination test is done using a disc with 2
pin-holes 2 mm apart placed close to the eye and a point source positioned 2 feet behind
the disc. If the patient sees 2 images of the point source of light then the macular
function is inferred to be normal.
As a low vision aid - patients with irregular cornea whose
refractive error cannot be corrected by glasses, can be benefited by using opaque discs
with multiple pin-holes which provide refractive correction whichever direction the
patient gazes.
As an aperture pin-hole is used in various optical instruments
to control the amount of light passing, increase the depth of focus and cut off the
internal glare generating within the tubes of the instrument.
Rest to the eye can be ensured by making the patient wear opaque
glasses with only one pin-hole in front of each eye. This forces the patient to move his
head to view his surroundings rather than moving his eyes. This method was used in past in
patients with traumatic hyphema and fresh retinal detachment.
II. STAENOPIC SLIT
This is an opaque black disc with a 1 mm thick slit running across the
center.
Uses:
Astigmatism can be diagnosed with a staenopic slit. It is placed in
front of the patient’s eye and slowly rotated by 180° .
If the vision of the patient is significantly better in a particular position than others
then astigmatism is present. The axis of the slit represents the axis of one of the two
principal meridia. The staenopic slit has the same effect as the pin-hole but only in
one meridian which is at right angle to the slit itself.
Confirmation of the power and axis of the cylinder can be made
by placing the staenopic slit in front of the cylinder along its axis. If the
vision of the patient improves further then the power of the cylinder is imperfect. Then
the axis of the slit is varied about the axis of the cylinder and if the vision improves
then the axis of the cylinder is required to be changed accordingly.
Fincham’s test is done using the staenopic slit to
differentiate the colored haloes caused by cataract (lenticular) from that caused by
glaucoma (corneal). Staenopic slit is moved across patient’s eye while the patient
looks at a bright point source of light which gives rise to halo. The halo caused by
cataract breaks into a fan the blades of which seem to move whereas the halo caused
by glaucoma remains unchanged or just becomes a little faint. This is caused by
peculiar disposition of the lens fibers which cause diffraction of light parallel to them.
Determination of meridian of optical iridectomy is done with the
help of staenopic slit. A patient who has a small central corneal or lens opacity may
benefit from optical iridectomy. Vision is first recorded with the normal pupil then the pupil
is dilated and staenopic slit is placed in front of the eye and rotated; the axis where
vision improves markedly, is chosen for optical iridectomy. Although empirically the
optical iridectomy is done inferonasally in patients whose occupation involves near
work, and temporally for out-door workers.
III. MADDOX ROD
Maddox rod or groove is a set of high power micro cylinders placed
close and parallel to each other. It converts a point source of light in to a line
image at right angle to its axis. Therefore, a point source of light seen through
Maddox rod with the axis placed horizontally, appears to be a vertical line.
Uses:
Latent squint can be diagnosed by placing Maddox rod in front of
one eye and asking the patient to look at a bulb in the center of Maddox cross
mounted on a wall. Patient sees bulb with the bare eye and a vertical line with the other
eye. If the eyes are aligned the bulb is seen in the center of the line. Whereas if there
is a latent squint the bulb is seen away from the line or eccentric on the line. In
esophoria the line is seen on the same side as the eye with Maddox rod (uncrossed
diplopia) and in exophoria on the opposite (crossed diplopia). The amount of deviation can
be measured by neutralizing the misalignment by putting prism in front of the Maddox rod
or by reading the scale mark corresponding to the line as seen on the Maddox cross.
Macular function test is performed by placing Maddox rod in
front of the eye with opaque media and shining a bright torch on it. With normal macula
the patient sees a smooth, continuos and unbroken straight line. If the macular function
is deranged patient sees an irregular or broken line.
Double Maddox rod test is done for the diagnosis of cyclotropia
or torsional squint. Two Maddox rods, one red and the other white, are placed in front of
the two eyes with the axes vertically. A 4D base-down prism
is placed in front of one of the rods to displace one of the lines upwards. A patient
without any cyclotropia will see two parallel lines one above the other. Patient with
cyclotropia will see one horizontal line and one tilted line. He is then instructed
to rotate the Maddox rod corresponding to the tilted line such that the two lines become parallel
to each-other. The axis of this rod gives the degree of cyclo-deviation.
IV. CONVEX LENS
Convex or plus lens is the one that converges the rays of light
(or decreases divergence).
Identification is done by holding the lens close to eye and
moving it side ways while looking at a distant target. The target seems to move in the opposite
direction. Then the lens is rotated while still viewing a distant target, there is no
distortion of the image in a spherical lens (as opposed to a cylindrical lens).
Uses: The convex lenses are used for following:
Hypermetropia
Presbyopia
Aphakia
Accommodative esotropia
Indirect ophthalmoscopy
Corneal loupe
As part of (almost all) optical instruments
V. CONCAVE LENS
Concave or minus lens diverges the rays of light (or decreases
convergence).
Identification is done by the method described above for convex
lens, however, the target seems to move in the same direction and there is no
distortion on rotating the lens.
Uses: The concave lenses are used for following:
Myopia
Hruby lens (-57.8 D) , which is a high power concave lens, is used for
visualizing the fundus on the slit-lamp examination.
As part of various optical equipment.
VI. CYLINDER
A cylinder is a lens which has refractive power only in one meridian
but not at right angle to it. It should be noted that the power of cylinder acts at
right angle to it. So if power is required vertically the cylinder axis is placed
horizontally and vice versa.
Identification is made by placing the lens close to eye and looking
at a distant target. On moving it horizontally or vertically target will move only with
one movement, and on rotation the image shows distortion. In convex cylinder the image
seems to move in opposite direction and in concave cylinder it moves in the same
direction.
Use: Cylinders are used to correct astigmatism.
VII. TORIC LENS
A toric or sherocylindrical lens is a combination of a sphere and a
cylinder.
Identification: On moving the lens the image seems to move in one
or the other direction (depending on convex or concave), regardless of which direction the
lens is moved. And on rotation there is distortion of the image.
Uses: Toric lenses are used to correct the following:
Compound astigmatism
Mixed astigmatism
VIII. PRIESTLEY-SMITH RETINOSCOPE
It is a device which has a plane and a concave mirror with following
properties:
Plane mirror
it is actually a very slightly concave mirror with a focal length of
1.5 m.
a fenestration (hole) in the center.
central hole is 2.5 mm on the mirrored side and tapers out to 4 mm on
the polished side.
the wall of the hole is painted dull black
Uses:
Concave mirror
Focal length 25 cm.
Central hole with same features as above.
In past the central hole used to contain a +2 D convex lens to relax
the accommodation of the observer. Although the convex lens is no longer placed in the
hole yet the manufacturers continue to print +2 at the back of concave mirror.
Looking into it the viewer’s own eye appears magnified (method
of identification).
Uses:
Retinoscopy in patients with hazy media, high refractive error where
the glow is faint and for confirmation of the point of neutralization.
Indirect ophthalmoscopy (as described classically).
Although not described conventionally yet DDO may be done by the
mirror.
IX. PRISM
A prism is an optical device which deviates the path of light without
converging or diverging the rays light. The light rays deviate towards the base whereas
the image is shifted towards the apex. The angle of deviation is half the angle of
the prism (a ) i.e. a ¸ 2. The power of the prism is measured in prism diopters (D ); one prism diopter denotes displacement by 1 cm of the image of
an object placed at 1 m from the prism. The power of the prism can be measured by any of
the following methods:
Identification is made using the same method described for lenses.
On moving the prism sideways the is no movement of the image. On rotating the prism
there is no distortion of the image, however, the image seems to swirl around
the base of prism. The base is marked + on the rim and the apex is marked -.
Uses:
Diagnostic
Objective measurement of the angle of squint.
Subjective measurement of the angle of squint.
Measurement of fusion range.
Preoperative assessment of possibility of postoperative diplopia.
4D base-out prism test for microtropia.
Test for malingering
As part of ( following) instruments
Indirect ophthalmoscope
Slit-lamp biomicroscope
Operating microscope
Gonioscope
Applanation tonometer
Therapeutic
To relieve diplopia in cases of paralytic squint while they are
waiting for surgery.
Severe convergence insufficiency which does not respond to
conservative treatment.
Suggestions and criticism may be addressed to:
Dr. Sanjay Dhawan
MBBS, MS (Gold Medalist), DO
Senior
Consultant Eye Surgeon
Max Medcenter
| G-28, Jangpura Ext. New
Delhi 110 014
Ph. 24321766, 2 4316094
E-mail sdhawan@sdhawan.com
Website: sdhawan.com |
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