Each eyeball is a
cystic structure kept distended by the pressure inside it. An eyeball acts as a
camera which perceives the images and relays the sensation to the brain
(occipital cortex) via visual pathway.
Anterior segment of an
eyeball includes from anterior to posterior are cornea, iris, aqueous humour,
ciliary body and lens.
Anterior segment can
be divided into two chambers –
Anterior chamber –
anterior chamber is bounded anteriorly by back of the cornea and posteriorly by
the iris and part of ciliary body. It contains about 0.25ml of aqueous humour.
Posterior chamber –
posterior chamber is a triangular space, contains 0.06ml of aqueous humour.
Posterior chamber is bounded anteriorly by the posterior surface of iris and
ciliary body and posteriorly by the crystalline lens and its zonules.
Examination of
Anterior Segment –
Anterior segment
examination can provide important information regarding any pathology of ocular
structures.
Anterior segment of an
eyeball can be examined with the help of some these instruments –
Torchlight
Keratometer
Slit lamp
Tonometer
Pachymeter
Gonioscope
Torch Light
An external torch
light examination helps to inspect –alignment, position of the eyes, eyelids,
ocular adnexa, conjunctiva, sclera, cornea, iris, pupil, extra ocular
movements, eyelid abnormalities, palpebral aperture, presence of any discharge.
Torch light has become
the most common used device to assess the pupil diameter.
If the torch is moved
from one pupil to other pupil, each pupil should become small (constrict), when
the light shown it.
If this does not
happen, this may indicates a relative afferent pupillary pathway(RAPD).
Figure- 1.1
Keratometer
Kerato- Cornea, Metry
– Measurement
Keratometer is an
instrument, used to measurement of curvature of anterior surface of cornea,
across a fixed chord length, usually 2-3mm, which is situated with in the
optical spherical zone of the cornea.
The process of these measurements is called keratometry.
HISTORY -
Hermann von Helmholtz invented the first keratometer. The earlier keratometer was
designed by Jesse Ramsden and Everard Home in 1796.
Figure- 2.1
Principal -
The keratometry is based on the principle that the anterior
corneal surface behaves like a convex mirror and the size of the image varies
with corneal curvature. Hence the curvature of the cornea can be calculated
based on image size from the anterior corneal surface.
The relationship between the object, image, radius of refractive surface
and denoted as
r = 2ul/ o,
where r is the radius of the reflective surface, o is the object size, l is the image size, and
u is the distance measured between the reflective surface and object.
Parts of Keratometer [figure 2.2]
Headrest
Chinrest
Eyepiece
Horizontal knob
Vertical knob
Focusing knob
Locking knob
Elevating knob
Chin height knob
Keratometer height knob
Occluding shield
Horizontal measuring drum
Vertical measuring drum
Axis scale
Figure- 2.2
Types of Keratometer -
The various type of keratometers can be Helmholtz
keratometer, Bausch and Lomb, and Javal Schiotz keratometer.
Javal-Schiotz Principles
The Javal-Schiotz keratometer is a two position
instrument which uses a fixed image and doubling size and adjustable object
size to determine the radius of curvature of the reflective surface. It uses
two self illuminated mires (the object), one a red square, the other a green
staircase design, which are held on a circumferential track in order to
maintain a fixed distance from the eye. In order to get repeatable, accurate
measurements, it is important that the instrument stays focused. It uses the
Scheiner principle, common in autofocus devices, in which the converging
reflected rays coming towards the eyepiece are viewed through (at least) two
separate symmetrical apertures.
Bausch and
Lomb keratometer [figure 2.1, 2.3]
This keratometer is based on the
principle of constant object size and image size variability.
There is a circular mire with two
plus and two minus signs. A bulb is used to illuminate the mires with the help
of mirrors placed diagonally. When light from the mire strikes the patient
cornea, it makes a diminished image behind it.
Figure- 2.3
Through objective lens, helps focus the light from the image of
the mire.
A diaphragm has four aperture is
placed near the objective lens. Beyond the diaphragm, Two doubling prisms are
placed, one base up and another a base out prism. The prisms can move
independently and parallel to the central axis of the instrument.
The upper and lower apertures
diaphragm act as Scheiner's disc, doubling the image size. The doubling
phenomenon of image is a unique phenomenon for Bausch and Lomb keratometer. This
helps in calculating the power of the cornea in two different meridians without
any rotation of the instrument. Hence, this is also known as one position
keratometer.
Eyepiece lens helps get a
magnified view of the doubled image.
Procedure [figure 2.4]
Firstly, the keratometer
is calibrated with steel balls. A whiteboard is placed in front of
the objective lens, and a black line is seen sharply focussed on it. When the
mires are focused, the instrument is said to be calibrated. Then, the head
should be against the headrest, and the chin should rest on the chin rest. Ask
the patient to occlude the non-tested eye and to look at the centre of the
instrument. After all adjustments, the mires are focused in the center of
cornea.
To assess the curvature in the
horizontal meridian, the plus sign of the central and left circle is
superimposed, by horizontal alignment knob and for vertical alignment, central
and upper circles is superimposed, by vertical measuring knob. If two-plus signs
will not be aligned, than oblique astigmatism may occur. Until, both plus signs
are aligned, the instrument has to be rotated.
Figure- 2.4
Slit Lamp
Biomicroscopy
A slit
lamp is an instrument used for eye examination. It consists of a high illumination light source that can be focused to interior
to the eye. It gives your ophthalmologist a clear picture at the
different structures at the front of the eye and interior to the eye. Slit lamp not only provides a magnified view of intraocular
structures but also help in qualitative and quantitative analysis of various
parameters such as corneal endothelial cell count, corneal thickness, anterior
chamber cells, and flare assessment, depth of anterior chamber, pupil size,
grading of cataract.
History
In 1823,
Purkinje was tried to make a hand-held slit lamp by using one lamp to magnify
the handheld lens with oblique illumination. The modern slit lamp was developed
in 1911 by the Swedish physicist and Nobel laureate Allvar Gullstrand.
Principle
Narrow beam of very bright light
produced by lamp. The beam focused on to the eye which is then viewed under
magnification with a microscope.
Parts of Slit Lamp [figure 3.1]
There are three main parts of a
slit lamp -
1.
Observation system
2.
Illumination system
3.
Mechanical support system
1)
Observation System/
Microscope -
The system is a compound
microscope system with two optical components:
The objective lens
The eyepiece lens
Objective Lenses:
It has two plano-convex lenses with the convexities close together, giving a
total power of +22D.
Eyepiece Lenses:
It has a power of +10 D. The tubes converge at 10 to 15 degrees and
to provide good stereopsis.
Prisms:
There are a pair of prisms, to reinvert the image produced by the objective and
the eyepiece.
2)
Illumination system [figure
3.2]
It is composed of the following components.
Light Source: The light source is
a Nernst lamp, followed by Nitra lamps, arc lamps, mercury vapor lamps, and
halogen lamps. It gives illumination of 2x10 to 4x10 lux.
Condenser Lenses: Two plano-convex
lenses with the convex surface in opposition.
Slit and Diaphragms: The height
and width can be changed by two knobs, respectively.
Filters: The cobalt blue filter
and red-free filters are provided in modern models of bio-microscope.
Projection Lenses: These
lenses helps in forming the image at the slit of the eye.
Reflecting Mirrors or Prism: These form the last part of the reflecting mirror.
3)
Mechanical Support System–
The mechanical support system can
be -
Joystick:
for movements side to side and up-down are usually achieved.
Up-and-down movement:
can be obtained through a screw device.
Patient Support System:
for chin rest and head rest.
Fixation Target:
facilitates the examination under some conditions.
Mechanical Coupling System:This
system supports and helps in coupling the microscopic system and the
illumination assembly along the central axis of rotation that overlaps with
their focal planes.
Magnification control:
two pair of readily changeable objective lens and two sets of eyepiece.
Methods of Illumination [Figure
3.3]
1. Diffuse Illumination
In it, the angle between the
illumination beam and microscope is at 30 to 45 degrees. The width of slit
is maintained as fully wide, diffuse illumination, magnification should be medium,
and illumination should be from medium to high.
It is used to Gross examination
of the anterior segment of the eye [lids, lashes, conjunctiva, cornea, sclera,
pupil] and movement of contact lens.
2. Direct Illumination
In it, the slit beam is focused
until it coincides with the correct focus of the microscope. The technique has
three subcategories of examination -
Conical Beam
- The slit beam is kept as a small circular pattern. The microscope is kept in
front of the eye, and magnification is high. The aqueous flare and aqueous
cells are seen as white dots.
Parallelopiped
- It is observed using a wide focussed slit.
Pathologies of corneal epithelium
and stroma, corneal scar or infiltrate and cells and flare in the anterior
chamber.
Optical Section
This section is produced by a
very narrow obliquely focused slit lamp beam. The optical section produced appears
like a knife on the deeper part of eye. A slit beam is used progressively to
focus on the deeper layers of the cornea, lens and anterior one-third of the
vitreous face.
3. Indirect Illumination
In this, the slit beam is focussed
attached to the area to be examined. The angle between the slit-lamp light
source and the viewing arm should be between 30 to 45 degrees. A width beam is moderately
used, and illumination low to high based on the need. The indirect illumination
examines corneal infiltrates, corneal microcysts, vacuoles, or epithelial
cells.
4.
Retro-illumination
In it, the microscope is focused
on the cornea, while the light is reflected from the iris or the retina. The
retro-illumination examines neovascularization, oedema, microcysts, vacuoles
and infiltrates. It has been further classified as direct or indirect -
Direct Retro-illumination
In this, the examiner is in the
direct path of the light reflected from the ocular structures. The pathology is
focused on an illuminated background.
Indirect Retro-illumination
In this method, the examiner is at
a right angle to the pathology or ocular structure, easily observed. In
non-illuminated background, the pathology of structures is seen against a dark
background.
5.
Specular reflection
When the angle
between the illumination system and microscope is 60 degree, then the angle of
incidence is equal to the angle of reflection. With this technique, the
endothelial cells, tear film cells can be measured.
6.
Sclerotic scatter illumination
This technique
is used to detect the smallest and finest corneal irregularities. The light ray
is directed towards the limbus. The light rays are passed through the cornea
and illuminate the opposite side of the limbus, because of total internal
reflection . A magnification of 6 to 10 X is used.
7. Oscillating illumination
In this
technique, the slit lamp is given an oscillatory movement by which it is easy
to detect filaments or minute objects in the aqueous humour.
Technique of Biomicroscopy
Patient adjustment:
The patient should be made to sit comfortably on an adjustable chin rest and
head rest.
Instrument Adjustment:
The illumination system and the microscope should be adjusted with the
patient's eye for examination.
1. Bio-microscope examination should
be carried into a semi dark room.
2. Both the patient and examiner
should be positioned comfortably.
3. Diffuse illumination should be
used for as a short a time as necessary.
4. Regulate the illumination or brightness
through the desired filters.
5. Adjust the width and height of
the beam.
6. Low to high magnification should
be used to detect any pathology.
Figure- 3.1
Figure- 3.2
Figure- 3.3
Tonometer
Tonometer is an
instrument to measure the intra ocular pressure (IOP) of the eye and procedure is
known as tonometry. It is an important test to diagnose and treatment of
glaucoma.
History
William Bowman
introduced digital tonometry as routine eye examination, in 1826. In 1885,
Malklov made first applanation tonometer. Hjalmar Schiotz, in 1905, introduced
indentation tonometer. Goldmann designed a new prototype applanation tonometer
constant area in 1954. Grolmann in 1972, made non contact tonometer (NCT).
Types of Tonometer
Tonometer is of two
types –
A] Direct (Manometry) – In this
method, needle is inserted into AC, in which needle is connected to a fluid
filling tube than hight of the fluid in tube corresponds to IOP. It is not
practical methods for human beings but used in experiment as a research work on
animal eyes.
B] Indirect tonometry– It is an
indirect method of measuring the IOP. It includes –
1. Digital tonometer
2.
Indentation tonometer
3.
Applanation
tonometer
4.
Non-contact tonometer
1. Digital tonometer/Palpation Method [figure
4.1]
In
this method, it is done with the use of index finger of both hands. The patient
is aske d to look down and one finger is kept stationary which feels the
fluctuation produced by the indentation of globe by the other finger. When IOP
is raised – fluctuation is absent and the eyeball feels firm to hard and when IOP is very low – eye
feels soft like a partially filled balloon.
Figure- 4.1
Advantages –
Easier to perform
No anaesthesia
required
No equipment required
No staining required
Estimation of IOP with
irregular cornea, where applanation tonometry is not possible.
Disadvantages –
Reading no proper
Over-estimation or
underestimation
Only depends on
examiner
2] Indentation
tonometer/ Schiotz tonometer
In
indentation tonometry, a weight (which
is 5.5gm/7.5 gm/10.5 gm) is placed on
the cornea, and the IOP is estimated by measuring the indentation of the globe.
The extent of tonometer which placed on cornea is indented by plunger is
measured as the distance from the foot plate curve to the plunger base and a
lever system moves a needle on calibrated scale. The scale reading and the
plunger weight are converted to an IOP measurement. More the plunger indents
the cornea, higher the scale reading and lower the IOP. Each scale unit
represents 0.05mm protrusion of plunger.
Figure- 4.2
Procedure [figure 4.3]
Patient
should be anaesthetised with 0.5% proparacaine or lignocaine. Patient is asked
to lie on a couch on supine position, and look up to a fixation target while
the examiner separates the lids. Now, lowers the tonometer plate to rest on the
cornea so that the plunger is free to mover vertically. Scale reading is
measured. The 5.5 gm weight is initially used. If scale reading is 4 or less,
then additional weight is added to plunger. IOP measurement is repeated until 3
consecutive readings. Conversion table is used to derive IOP in mm Hg from
scale reading and plunger weight.
Advantages
Easy to use
Easy to carry
Less expensive
Easy to clean and
maintain
Does not require any
other device
Disadvantages
Due to ocular
rigidity, false or high IOP readings
Used only in supine
position
Can not be used in
traumatic case or corneal disease.
3.
Applanation tonometer
Applanation tonometer
was introduced by Goldmann in 1954.
Figure- 4.3
Principle [figure 4.4]
It is based on the
Imbert Fick Law in which pressure inside the sphere P is equal to force F,
necessary to flatten its surface divided by the area of flattening A.
P=F/A
Parts of applanation Tonometer
[figure 4.5A, B, C]
Biprism
Weight controller
Adjusting knob
Feeder arm
Knob
that tonometer connects to slit lamp
Figure- 4.4
Preparation –
Figure- 4.5 A
Figure- 4.5 B
Figure- 4.5 C
Before starting the
procedure, the Goldmann tonometer should
be calibrated. If the tonometer is not within 0.1 gm (1mmHg) of the correct
calibration, the instrument should be repaired.
Tonometer tip is
cleaned with sterilizing solution and tonometer tip and prism are set in
correct position on the slit lamp. Knob is set at 1 gm. If the knob is set at
zero, the prism head may vibrate when it touches the eye and damage the corneal
epithelium.
Cobalt blue filter is
used with the slit lamp beam. The angle between illumination system and
microscope should be 60°. The room illumination is reduced. Height of the slit
lamp, chair and chin rest are adjusted until the patient is comfortable and in
correct position for the measurement.
Procedure
Firstly, the examiner
is explained the purpose f the test and reassured that the measurement is not
painful. The patient is instructed to relax, maintain position and hold the
eyes open widely. One drop of topical anaesthetic (0.5% proparacaine), is
instilled in each eye and tip of a moist fluorescein strip is touched to the
inner surface of lower lid, two semicircles of fluorescein rings are seen in
the center of the field.
[figure 4.6 A, B] If
the two semicircles are not in equal size, IOP is overestimated/underestimated.
Than turn the tension knob in both directions to ensure that the instrument is
in good position. Fluorescein rings should be 0.25 – 0.3 mm in diameter. If the
rings are too narrow the tension knob is rotated until the inner borders of the
fluorescein rings touch each other at the mid-point. IOP is measured in the
right eye with 3 successive readings are within 1mmHg than the left eye is
measured. Reading obtained in grams is multiplied by 10 to give the intra
ocular pressure (IOP) in mmHg.
These are the source
of error of measurement of falsely low IOP
or falsely high IOP
Figure- 4.6A
Figure- 4.6B
Too
little fluorescein or too much fluorescein, thin cornea, thick cornea, corneal
oedema, steep cornea and astigmatism
Advantages
More accurate result
Can be done on injury
cases
Not effected
corneo-scleral rigidity
No need to indentation
– so not much force is applied on cornea.
Readings are directly
from knob
Disadvantages
Requires dark room
Need to staining and
cobalt blue filter
Need to slit lamp
More expensive
Types
of applanation tonometer
1. Perkins tonometer
[figure 4.7] – It developed by E S Perkins in 1956. It is similar to Goldmann
applanation tonometer. It uses same prism tips as GAT. The prism is illuminated
by battery powered bulb. It is hand held tonometer and both eyes must always be
anaesthetised to reduce the movements. Patient to look straight or slightly
upward and if necessary use of fixation target. Rest the instrument on the
patient’s forehead so that the probe can make contact with the centre of the
cornea. Adjust the force by turning the thumb wheel until the inner edges of
the semi circles coincide. The reading is multiplied by 10 to give the IOP in
mmHg. Being portable, it is practical when measuring IOP in infants, children
and lying patient.
Advantages –
Portable
Can
be used home visits
Being hand held, may
be used with the patient either sitting up or lying down.
Does not require slit
lamp
Does no require
electricity
Disadvantages –
Less stable, low
magnification, costly
Pneumatic tonometer
The pneumatic
tonometer pencil is resting on the table and is connected to the base unit by
hollow tubing that carries air to the tip of the pen. It has a sensory device
that consists of a gas chamber covered by a polymeric silicon diaphragm. A
transducer converts the gas pressure in the chamber into a electrical signal
that is recoded on a paper strip and IOP is read and noted down.
Mackey Marg tonometer
This type of tonometer
has 1.5 mm diameter plunger and rubber sleeve. Movement of plunger is electronically
monitored by a transducer and recorded on a moving paper strip. Useful for
measuring IOP in eyes with scarring, irregular or oedematous cornea because the
end point does not depend on the evaluation of light reflex sensitive to
optical irregularity.
Tono-pen [figure 4.8
A, B]
Tonopen is a computerised
pocket tonometer. It employs a microscopic transducer which applanates the
cornea and converts IOP into electronic waves.
Figure- 4.8A
Figure- 4.8B
Non-Contact Tonometer
[figure 4.9]
There are two types of
non-contact tonometer
Air puff tonometer
This tonometer also a
non-contact applanation type tonometer, using a standard puff of air. An air
puff is directed towards the cornea which is gradually flattens the corneal
surface and IOP is recorded. This method has advantage of no anaesthetic or no
risk of infection.
Pulse air tonometer
It is also a non
contact tonometer which can be used at any positions.
sss
Figure- 4.9
Pachymeter/Pachometer
Pachos – thickness,
metry – measurement
Pachymetry is a simple
procedure to measure the thickness of the cornea. The instrument is known as pachymeter. Pachymeter is an
important indicator of health status of cornea also helps to assess the
function of the corneal endothelium.
Normal corneal
thickness 500-575 micron. Abnormally thick or thin measurement of cornea may
indicate - corneal thinning, corneal oedema.
Factors affecting central corneal thickness
Higher in younger male
persons .
Increased corneal thickness
measurements are found in patients with diabetes.
Increased/ decreased
central corneal thickness is found in
patients with ocular hypertension or low tension glaucoma, respectively.
Techniques
of Pachymetry
Ultrasonic
techniques – Conventional ultrasonic pachymetry, Ultrasound biomicroscopy
Optical techniques – Optical
pachymetry(ORBSCAN), optical coherence tomography, scanning slit technology,
confocal microscopy, specular microscopy, laser doppler interferometry
Alternative
techniques – Pentacam, pachycam, ocular response analyser
Ultrasonic
Pachymetry [figure 5.1]
This is most commonly
used method.
Principle
– instruments
functions by measuring the amount of time needed for ultrasound pulse pass from
one end of transducer to descemet membrane
and back to the transducer.
Corneal thickness =
(amount of time x propogation velocity) /2
Components
Handle
of probe – it
has piezoelectric crystal that emits an ultrasonic beam of 20 mHz.
Transducer – it sends
ultrasound waves through the probe to the cornea and receives echoes from the
cornea.
Tip – the
diameter of the tip should not be more than 2 mm.
Figure- 5.1
Advantages
Portable
Faster , Easy to use
It can be used intra
operatively.
Disadvantages
Not accurate
oedematous cornea.
Accuracy is depend on
perpendicularity of the probe.
Figure- 5.2
Figure- 5.3
Gonioscope/Gonioscopy
Gonioscopy is an examination
to see the area where fluid drains
out of your eye. It uses a special lens that is gonio lens with slit lamp to
evaluate angle of anterior chamber. The angle is formed between peripheral
cornea and iris. If the drainage angle is opened/closed, you may have suspect glaucoma.
Gonioscopy helps to see angle of anterior chamber structures from posterior to
anterior; the iris, ciliary body band, scleral spur, trabecular meshwork,
schwalbe’s line.
History
Trantas
in 1907, firstly examine angle of anterior chamber in keratoglobus eye and
introduced the term gonioscopy. Maximilian Salsmann in 1914, introduced
goniolens to observe the angle. Gonioprism was introduced by Goldmann in 1938.
Otto Barkan introduced use of gonioscopy in the management of glaucoma.
Principal
[figure 6.1]
Angle of anterior
chamber cannot be visualized
directly through an intact cornea because the light emitted from angle
structures undergoes total internal reflection (TIR). Goniolens eliminates the
total internal reflection by placing the cornea.
Goniolens/Gonioscope
Figure- 6.1
There
are two types of goniolens
Direct
goniolens [figure 6.2]
Koeppe
lens – used as a diagnostic goniolens
Swan Jacob – used in
children
Barkan – surgical
goniolens
Richardson Shaffer –
used in infants
Figure- 6.2
Layden
– used in premature infants
Indirect goniolens
[figure 6.3]
Goldmann 3 mirror –
surface is slightly large than cornea, requires gonioscopic gel.
Zeiss 4 mirror –
mirrors are inclined at 64°, surface is slightly smaller than cornea, does not
require fluid.
Posner 4 mirror –
modified zeiss goniolens , with attached handle.
Sussmann 4 mirror –
handheld zeiss type gonioprism.
Thorpe 4 mirror –
inclined at 62°, requires fluid.
Figure- 6.3
Procedure
[figure 6.4]
Firstly,
explain the procedure to the patient in brief.
Cleaning and
sterilising the front surface of goniolens.
Applying lubricating
fluid to the front surface of lens, if needed.
Anaesthetising the
patient’s cornea with topical anaesthetic.
Prepare the slit lamp
for viewing structures through the goniolens.
Figure- 6.4
Slowly
applying the goniolens to the ocular surface and interpreting the gonioscopic
images.
To view each section of the iridocorneal angle
by moving the goniolens.
Direct
Gonioscopy
Advantages
–
Greater flexibility
Panoramic view is
obtained.
Minimal or no
distortion of the angle of anterior chamber.
Can be performed on
both eyes simultaneously.
Disadvantages
–
Time cosuming
Instrumentation
expensive and difficult to obtain.
Less magnification so
less detail visible.
Indirect
Gonioscopy
Advantages
–
All four quadrants can
be seen .
Coupling agent is not
used , so visualization of fundus and photography is possible.
Easy in learning
technique
Greater visibility of
detail with higher magnification.
Disadvantages
–
Tendency to
overestimation, narrowness of the angle, if the rim of the lens indents the
limbus.
Tendency to
underestimation, narrowness of the angle, if goniolens indents the cornea
anterior to the limbus.
Clinical Uses
Gonioscopy can be done
for both diagnostic and therapeutic purpose
Diagnostic
purpose[figure 6.5]
To differentiate
between POAG and PACG.
To diagnose secondary
glaucoma.
To evaluate tumours of
anterior segment.
Evaluation of
congenital anomalies in anterior segment.
To assess angle
recession and angle pigmentation, foreign body.
Figure- 6.5
Therapeutic
purpose
To perform argon laser
trabeculolasty, laser iridoplasty, laser sclerostomy, goniotomy and
trabecolutomy.
Limitations
Patient’s with
recurrent corneal erosions, corneal abrasions.
Patient with mydriatic
eyes.
Perforated globe,
bullous keratopathy and punctuate keratopathy.
1. Lam AK. A hand-held keratometer. Ophthalmic Physiol Opt. 1995
May;15(3):227-30. doi: 10.1016/0275-5408(95)90575-m. PMID: 7659423.
2. Oshika T, Yoshitomi F, Oki K. The pachymeter guide: a new
device to facilitate accurate corneal thickness measurement. Jpn J Ophthalmol.
1997 Nov-Dec;41(6):426-7. doi: 10.1016/s0021-5155(97)00074-9. PMID: 9509312.
3. Thomas R, Thomas S, Chandrashekar G. Gonioscopy. Indian J
Ophthalmol. 1998 Dec;46(4):255-61. PMID: 10218313.
4. Friedman DS, He M. Anterior chamber angle assessment
techniques. Surv Ophthalmol. 2008 May-Jun;53(3):250-73. doi:
10.1016/j.survophthal.2007.10.012. PMID: 18501270.
https://commons.wikimedia.org/wiki/Special:MyLanguage/Commons:Wiki_Loves_Folklore_2023
5. Mannan R, Pruthi A, Sud R, Khanduja S. Slit lamp examination
during COVID-19: Where should the protective barrier be? Indian J Ophthalmol.
2021 Feb;69(2):376-383. doi: 10.4103/ijo.IJO_2204_20. PMID: 33402656; PMCID:
PMC7933898.
6. Junk AK, Chang TC, Vanner E, Chen T. Current Trends in
Tonometry and Tonometer Tip Disinfection. J Glaucoma. 2020 Jul;29(7):507-512.
doi: 10.1097/IJG.0000000000001566. PMID: 32459693; PMCID: PMC7335349.
7. Dance S, Scholefield BR, Morris KP, Kanthimathinathan HK.
Characteristics of a Brisk or Sluggish Pupillary Light Reflex: A Nursing
Perspective. J Neurosci Nurs. 2020 Jun;52(3):128-131. doi: 10.1097/JNN.0000000000000501.
PMID: 32175991.
8. Gurnani B, Kaur K. Keratometer. 2022 Dec 6. In: StatPearls
[Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan–. PMID:
35593824.
9. Morlet N, Maloof A, Wingate N, Lindsay P.
Reliable keratometry with a new hand held surgical keratometer: calibration of
the keratoscopic astigmatic ruler. Br J Ophthalmol. 1998 Jan;82(1):35-8. doi:
10.1136/bjo.82.1.35. PMID: 9536877; PMCID: PMC1722336.
