RxPG thanks Dr Sanjay Dhawan, Maulana Azad Medical College and GNEC, for providing these high yield notes.
DARKROOM PROCEDURES : ENLIGHTENED
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
Distant Direct Ophthalmoscopy at 22/25 cm.
All DRP are performed in very low ambient
illumination (not necessarily pitch dark) and with the examiner preferably dark
Before describing DRP it is important to
understand the principle of coaxial illumination which is applicable to all DRP.
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
I. Preliminary Examination at 1m (including
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
1. Glow in the pupil moves in the same
direction as the light outside. This means the patient can be any of the
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.
If the movement in the same direction gets
neutralized by +5.0D lens then the refractive error is +4.0D (-1.0 added
algebrically 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)].
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
Theoretically the ideal distance for doing
retinoscopy is infinity (Ą ) 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
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
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
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.
This article is part 1 of the three article series. For other parts see the ophthalmology section of this site.