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.
1. Easy to use and portable.
|1. No stereopsis.
2. Greater magnification enables fine details to be
|2. The illumination is low
therefore if media are hazy the visibility is poor.
3. Patient can be examined in any position.
|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
|4. Field of view is small and
the retinal periphery cannot be seen.
A black disc with a 0.5 to 1.5 mm diameter
fenestration in the center.
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
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
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
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.
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.
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
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
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.
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
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
VIII. PRIESTLEY-SMITH RETINOSCOPE
It is a device which has a plane and a concave
mirror with following properties:
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
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).
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
Although not described conventionally yet
DDO may be done by the mirror.
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 -.
Objective measurement of the angle of
Subjective measurement of the angle of
Measurement of fusion range.
Preoperative assessment of possibility of
4D base-out prism test for microtropia.
Test for malingering
As part of ( following) instruments
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, DO, MS (Gold Medalist)
This article is part 3 of the 3 article series. For rest of the article see the ophthalmology section of this site.