Instruments
for investigation and correction of the human eye disorders
HUMAN EYE
The human eye is an organ that
reacts to light and has several purposes. As a conscious sense organ, the
mammalian eye allows vision. Rod and cone cells in the retina allow conscious
light perception and vision including color differentiation and the perception
of depth. The human eye can distinguish about 10 million colors.
Similar to the eyes of other mammals,
the human eye's non-image-forming photosensitive ganglion cells in the retina
receive light signals which affect adjustment of the size of the pupil.
The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit. The smaller
frontal unit, more curved, called the cornea is linked to the larger unit
called the sclera. The corneal segment is typically about 8 mm
(0.3 in) in radius. The sclerotic chamber constitutes the remaining
five-sixths; its radius is typically about 12 mm. The cornea and sclera are
connected by a ring called the limbus.
The iris the color of the eye and
its black center, the pupil, are seen instead of the cornea due to the cornea's
transparency. To see inside the eye, an ophthalmoscope is needed, since
light is not reflected out. The fundus (area opposite
the pupil) shows the characteristic pale optic disk (papilla), where
vessels entering the eye pass across and optic nerve fibers depart the globe.
The dimensions differ among adults by
only one or two millimeters. The vertical measure, generally less than the
horizontal distance, is about 24 mm among adults, at birth about 1617
millimeters (about 0.65 inch). The eyeball grows rapidly, increasing to
22.523 mm (approx. 0.89 in) by three years of age. By age 13, the eye
attains its full size. The typical adult eye has an anterior to posterior
diameter of 24 millimeters, a volume of six cubic centimeters and a mass of 7.5
grams
The eye is made up of three coats,
enclosing three transparent structures. The outermost layer, known as the
fibrous tunic, is composed of the cornea and sclera. The
middle layer, known as the vascular tunic, consists of the choroid, ciliary
body, and iris. The innermost is the retina, which gets its
circulation from the vessels of the choroid as well
as the retinal vessels, which can be seen in an ophthalmoscope.
Blood vessels can be seen
within the sclera, as well as a strong limbal ring
around the iris.
Within these coats are the aqueous
humour, the vitreous body, and the
flexible lens. The aqueous humour is a
clear fluid that is contained in two areas: the anterior chamber between
the cornea and the iris, and the posterior chamber between the iris and
the lens. The lens is suspended to the ciliary
body. The vitreous body is a clear jelly that is much larger than
the aqueous humour present behind the lens. There is
an optic disc on the retina. The optic disc or optic
nerve head, or a blind spot, is the location where ganglion cell axons exit
the eye to form the optic nerve. There are no light sensitive rods or cones to
respond to a light stimulus at this point. This causes a break in the visual
field called "the blind spot" or the "physiological blind
spot". A macula is a yellow spot with highest concentration
of photosensitive cells
A
photoreceptor cell is a specialized type of neuron found in the
retina that is capable of phototransduction.
The great biological importance of photoreceptors is that they convert light (visible
electromagnetic radiation) into signals that can stimulate biological
processes. To be more specific, photoreceptor proteins in the cell
absorb photons, triggering a change in the cell's membrane potential.
The
two classic photoreceptor cells are rods and cones, each
contributing information used by the visual system to form a representation of
the visual world, sight. The rods are narrower than the cones and
distributed differently across the retina, but the chemical process in each
that supports phototransduction is similar. A
third class of photoreceptor cells was discovered during the 1990s: the
photosensitive ganglion cells. These cells do not contribute to sight
directly, but are thought to support circadian rhythms and pupillary reflex.
There are major functional
differences between the rods and cones. Rods are extremely sensitive, and can
be triggered by as few as 6 photons. At very low light levels, visual
experience is based solely on the rod signal. This explains why colors cannot
be seen at low light levels: only one type of photoreceptor cell is active.
Cones require significantly brighter
light (i.e., a larger numbers of photons) in order to produce a signal. In
humans, there are three different types of cone cell, distinguished by their
pattern of response to different wavelengths of light. Color experience is
calculated from these three distinct signals, perhaps via an opponent process.
The three types of cone cell respond (roughly) to light of short, medium, and
long wavelengths. Note that, due to the principle of univariance,
the firing of the cell depends upon only the number of photons absorbed. The
different responses of the three types of cone cells are determined by the
likelihoods that their respective photoreceptor proteins will absorb photons of
different wavelengths. So, for example, an L cone cell contains a photoreceptor
protein that more readily absorbs long wavelengths of light (i.e., more
"red"). Light of a shorter wavelength can also produce the same
response, but it must be much brighter to do so.
The human retina contains about 120
million rod cells and 6 million cone cells. The number and ratio of rods to
cones varies among species, dependent on whether an animal is primarily diurnal
or nocturnal. Certain owls, such as the tawny owl,have a tremendous number of rods in their retinae. In addition, there are about 1.5 million ganglion
cells in the human visual system, 1 to 2% of them photosensitive.
The approximate field of view of an
individual human eye is 95° away from the nose, 75° downward, 60° toward the
nose, and 60° upward, allowing humans to have an almost 180-degree
forward-facing horizontal field of view. With eyeball rotation of about 90°
(head rotation excluded, peripheral vision included), horizontal field of view
is as high as 270°. About 1215° temporal and 1.5° below the horizontal is the
optic nerve or blind spot which is roughly 7.5° high and 5.5° wide
Accommodation
Accommodation
(Acc) is the process by which the vertebrate eye changes optical power to
maintain a clear image or focus on an object as its distance varies.
Figure 2. Accomodation of
an eye.
Accommodation acts like a reflex, but
can also be consciously controlled. Mammals, birds and reptiles vary the
optical power by changing the form of the elastic lens using the ciliary body (in humans up to 15 diopters).
Fish and amphibians vary the power by changing the distance between a rigid
lens and the retina with muscles.
The young human eye can change focus
from distance (infinity) to 7 cm from the eye in 350 milliseconds. This
dramatic change in focal power of the eye of approximately 13 diopters (diopter is the
reciprocal of focal length in metres) occurs as a
consequence of a reduction in zonular tension induced
by ciliary muscle contraction. The amplitude of
accommodation declines with age. By the fifth decade of life the accommodative
amplitude has declined so that the near point of the eye is more remote than
the reading distance. The age-related decline in accommodation occurs almost
universally to less than 2 dioptres by the time a
person reaches 45 to 50 years, by which time most of the population will have
noticed a decrease in their ability to focus on close objects and hence require
glasses for reading or bifocal lenses. Accommodation decreases to essentially 0
dioptres at the age of 70 years.
Myopia
Myopia
(Ancient Greek: μυωπία,
muōpia, from myein
"to shut" ops (gen. opos)
"eye"), commonly known as being nearsighted (American English)
and shortsighted (British English), is a condition of the eye where the
light that comes in does not directly focus on the retina but in front of it,
causing the image that one sees when looking at a distant object to be out of
focus, but in focus when looking at a close object.
Eye care professionals most commonly
correct myopia through the use of corrective lenses, such as glasses or contact
lenses. It may also be corrected by refractive surgery, though there are cases
of associated side effects. The corrective lenses have a negative optical power
and are divergent (dispersive) (i.e. have a net concave effect) which
compensates for the excessive positive diopters of
the myopic eye.
Figure 3.
Myopia
Figure 4. Compensating for myopia using a corrective lens.
The opposite of Myopia is Hyperopia, (longsighted).
·
Curvature myopia is attributed to excessive, or
increased, curvature of one or more of the refractive surfaces of the eye,
especially the cornea.In those with Cohen syndrome,
myopia appears to result from high corneal and lenticular
power.
·
Index myopia is attributed to variation in the
index of refraction of one or more of the ocular media.
Elevation of blood-glucose levels can
also cause edema (swelling) of the crystalline lens as a result of sorbitol (sugar alcohol) accumulating in the lens. This
edema often causes temporary myopia (nearsightedness).
Myopia, which is measured in diopters by the strength or optical power of a corrective
lens that focuses distant images on the retina, has also been classified by
degree or severity:
Hyperopia
Hyperopia,
commonly known as being farsighted (American English), being longsighted
(British English), or hypermetropia, is a
defect of vision caused by an imperfection in the eye (often when the eyeball
is too short or the lens cannot become round enough), causing difficulty
focusing on near objects, and in extreme cases causing a sufferer to be unable
to focus on objects at any distance. As an object moves toward the eye, the eye
must increase its optical power to keep the image in focus on the retina. If
the power of the cornea and lens is insufficient, as in hyperopia,
the image will appear blurred.
People with hyperopia
can experience blurred vision, asthenopia,
accommodative dysfunction, binocular dysfunction, amblyopia,
and strabismus, another condition that frequently causes blurry near vision. Presbyopes who report good far
vision typically experience blurry near vision because of a reduced
accommodative amplitude brought about by natural aging changes with the
crystalline lens. It is also sometimes referred to as farsightedness, since in
otherwise normally-sighted persons it makes it more difficult to focus on near
objects than on far objects.
The causes of hyperopia
are typically genetic and involve an eye that is too short or a cornea that is
too flat, so that images focus at a point behind the retina.
The opposite of Hyperopia
is Myopia, (shortsighted).
Various eye care professionals,
including ophthalmologists, optometrists, orthoptists,
and opticians, are involved in the treatment and management of hyperopia. At the conclusion of an eye examination, an eye
doctor (ophthalmologist or optometrist) may provide the patient with an
eyeglass prescription for corrective lenses. Minor amounts of hyperopia are sometimes left uncorrected. However, larger
amounts may be corrected with convex lenses in eyeglasses or contact lenses.
Convex lenses have a positive dioptric value, which
causes the light to focus closer than its normal range.
Hyperopia
is correctable with various refractive surgery procedures, such as LASIK, Radial Keratocoagulation
or Thermokeratoplasty.
Figure 5. Hyperopia,
and restoring of vision with convex lens.
An eye examination is a series
of tests performed by an ophthalmologist (medical doctor), assessing
vision and ability to focus on and discern objects, as well as other tests and
examinations pertaining to the eyes. Health care professionals often recommend
that all people should have periodic and thorough eye examinations as part of
routine primary care, especially since many eye diseases are asymptomatic.
Ideally, the eye examination consists
of an external examination, followed by specific tests for visual
acuity, pupil function, extraocular muscle motility,
visual fields, intraocular pressure and ophthalmoscopy
through a dilated pupil.
A minimal eye examination consists of
tests for visual acuity, pupil function, and extraocular
muscle motility, as well as direct ophthalmoscopy
through an undilated pupil.
External examination of eyes consists
of inspection of the eyelids, surrounding tissues and palpebral fissure. Palpation of the orbital
rim may also be desirable, depending on the presenting signs and symptoms. The
conjunctiva and sclera can be inspected by having the individual look up, and
shining a light while retracting the upper or lower eyelid. The position of the eyelids are checked for abnormalities such
as ptosis which is an asymmetry between eyelid
positions.
Close inspection of the anterior eye
structures and ocular adnexa are often done with a slit
lamp which is a table mounted microscope with a special adjustable
illumination source attached. A small beam of light that can be varied in
width, height, incident angle, orientation and colour,
is passed over the eye. Often, this light beam is narrowed into a vertical
"slit", during slit-lamp examination. The examiner views the
illuminated ocular structures, through an optical system that magnifies the
image of the eye and the patient is seated while being examined, and the head
stabilized by an adjustable chin rest.
Figure 6.
Slit lamp examination
Slit lamp examination of the eyes in
an ophthalmology clinic
This allows inspection of all the
ocular media, from cornea to vitreous, plus magnified view of eyelids, and
other external ocular related structures. Fluorescein
staining before slit lamp examination may reveal corneal abrasions
or herpes simplex infection.
The binocular slit-lamp examination
provides stereoscopic magnified view of the eye structures in striking detail,
enabling exact anatomical diagnoses to be made for a variety of eye conditions.
Also ophthalmoscopy
and gonioscopy examinations can
also be performed through the slit lamp when combined with special lenses.
These lenses include the Goldmann 3-mirror lens, gonioscopy single-mirror/ Zeiss
4-mirror lens for (ocular) anterior chamber angle structures and +90D lens,
+78D lens, +66D lens & Hruby (-56D) lens, the
examination of retinal structures is accomplished.
Intraocular pressure (IOP) can be measured by tonometry
devices. The eye can be thought of as an enclosed compartment through which
there is a constant circulation of fluid that maintains its shape and internal
pressure. Tonometry is a method of measuring this
pressure using various instruments. The normal range is 10-21 mmHg.
Examination of retina (fundus examination) is an important part of the general eye
examination. Dilating the pupil using special eye drops greatly enhances
the view and permits an extensive examination of peripheral retina. A limited
view can be obtained through an undilated pupil, in
which case best results are obtained with the room darkened and the patient
looking towards the far corner. The appearance of the optic disc and
retinal vasculature are also recorded during fundus
examination.
INSTRUMENTS AND DEVICES FOR EYE
EXAMINATION
LENSMETER
A lensmeter
or lensometer, also known as a focimeter, is an ophthalmic instrument. It is mainly
used by optometrists and opticians to verify the correct prescription in a
pair of eyeglasses, to properly orient and mark uncut lenses, and to
confirm the correct mounting of lenses in spectacle frames. Lensmeters
can also verify the power of contact lenses, if a special lens support
is used.
The parameters appraised by a lensmeter are the values specified by an ophthalmologist or
optometrist on the patient's prescription: sphere, cylinder, axis, add, and in
some cases, prism. The lensmeter is also used to
check the accuracy of progressive lenses, and is often capable of marking the
lens center and various other measurements critical to proper performance of
the lens. It may also be used prior to an eye examination to obtain the last
prescription the patient was given, in order to expedite the subsequent
examination.
Figure 7. A simple lensmeter cross sectional view.
1 Adjustable
eyepiece 2 Reticle
3 Objective lens 4 Keplerian
telescope
5 Lens holder 6 Unknown lens
7 Standard lens 8 Illuminated target
9 Light source 10 Collimator
11 Angle adjustment lever
12 Power drum (+20 and -20 Diopters)
13 Prism scale knob
PHOROPTER
A
phoropter is an instrument commonly used by
eye care professionals during an eye examination, containing different lenses
used for refraction of the eye during sight testing, to measure an individual's
refractive error and determine his or her eyeglass prescription
Figure 9. Ukrainian simplified
version of phoropter, used to measure refractive
power correction of an human eye
Typically, the patient sits behind
the phoropter, and looks through it at an eye chart
placed at optical infinity (20 feet or 6 metres),
then at near (16 inches or 40 centimetres) for
individuals needing reading glasses. The eye care professional then changes
lenses and other settings, while asking the patient for subjective feedback on
which settings gave the best vision.
Phoropters
can also measure phorias (natural resting position of
the eyes), accommodative amplitudes, accommodative leads/lags, accommodative
posture, horizontal and vertical vergences, and more.
The major components of the phoropter are the JCC (Jackson
Cross-Cylinder) used for astigmatism correction, Risley
prisms to measure phorias and vergences,
and the (+), (−), and cylinder lenses.
From the measurements taken, the
specialist will write an eyeglass prescription that contains at least 6
numerical specifications (3 for each eye): sphere, cylinder, and axis and
possibly pupillary distance.
The lenses within a phoropter refract light in order to focus images on the
patient's retina. The optical power of these lenses is measured in 0.25 diopter increments. By changing these lenses, the examiner
is able to determine the spherical power, cylindrical power, and cylindrical
axis necessary to correct a person's refractive error. The presence of
cylindrical power indicates the presence of astigmatism which has an axis
measured from 0 to 180 degrees away from being aligned horizontally.
Similar to medical prescriptions,
eyeglass prescriptions are written on paper pads that frequently contain a
number of different abbreviations and terms:
Figure 10.
An example of eyeglasses prescription
SLIT LAMP
The
slit lamp is an instrument consisting of a high-intensity light source
that can be focused to shine a thin sheet of light into the eye. It is used in
conjunction with a biomicroscope. The lamp
facilitates an examination of the anterior segment, or frontal structures and
posterior segment, of the human eye, which includes the eyelid, sclera,
conjunctiva, iris, natural crystalline lens, and cornea. The binocular
slit-lamp examination provides a stereoscopic magnified view of the eye
structures in detail, enabling anatomical diagnoses to be made for a variety of
eye conditions. A second, hand-held lens is used to examine the retina. In
ophthalmology and optometry, the instrument is called a slit lamp, although
it is more correctly called a slit lamp instrument. Todays instrument is a
combination of two separate developments, the corneal microscope and the slit
lamp itself.
While a patient is seated in the
examination chair, they rest their chin and forehead on a support to steady the
head. Using the biomicroscope, the ophthalmologist
then proceeds to examine the patient's eye. A fine strip of paper, stained with
fluorescein, a fluorescent dye, may be touched to the
side of the eye; this stains the tear film on the surface of the eye to aid
examination. The dye is naturally rinsed out of the eye by tears.
A subsequent test may involve placing
drops in the eye in order to dilate the pupils. The drops take about 15 to 20
minutes to work, after which the examination is repeated, allowing the back of
the eye to be examined. Patients will experience some light sensitivity for a
few hours after this exam, and the dilating drops may also cause increased
pressure in the eye, leading to nausea and pain. Patients who experience
serious symptoms are advised to seek medical attention immediately.
Adults need no special preparation
for the test; however children may need some preparation, depending on age,
previous experiences, and level of trust.
Figure 11.
Side view of a slit lamp machine
Slit lamp
result interpretation
The
slit lamp exam may detect many diseases of the eye, including:
One sign that may be seen in slit
lamp examination is a "flare", which is when the slit-lamp beam is
seen in the anterior chamber. This occurs when there is breakdown of the
blood-aqueous barrier with resultant exudation of protein
Slit lamp structure, maintanance, and operational checks
The slit lamp is an essential and often used diagnostic instrument in
ophthalmology. It provides illumination and magnification for the examination
of many structures of the anterior segment. With complementary lenses, it is also
used to examine the chamber angle and a significant part of the retina. Its
name derives from the fact that a narrow slit of light is used to illuminate
the various structures being examined.
By following these simple
suggestions, you can ensure that a slit lamp performs optimally and remains
functional for longer.
Location
Replacing the bulb
Figure 12.
Slit lamp structure
Cleaning
The
following functions should be checked weekly. The hospitals maintenance team
or the service agent should be called if any problems are noticed during these
checks.
Other
Tips
Fundus
(retina of an eye) observation is known by the ophthalmic and the use of fundus cameras. With the slit lamp, however, direct
observation of the fundus is impossible due to the
refractive power of the ocular media. In other words: the far point of the eye
(punctum remotum) is so
distant in front of (myopia) or behind (hyperopia)
that the microscope cannot be focused. The use of auxiliary optics - generally
as a lens makes it possible however to bring the far point within the
focusing range of the microscope. For this various auxiliary lenses are in use
that range in optical properties and practical application
Figure 13.
Cataract in Human Eye- Magnified view seen on examination with the slit lamp
OPHTALMOSCOPE
Ophthalmoscopy (funduscopy or fundoscopy)
is a test that allows a health professional to see inside the fundus of the eye (includes the retina, optic
disc, macula) and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination
and may be done as part of a routine physical examination. It is crucial in
determining the health of the retina and the vitreous humor.
An alternative or complement to ophthalmoscopy is to perform a fundus photography, where the image can be analyzed
later by a professional.
Figure 14. Ophthalmoscopic exam: the medical provider would next move
in and observe with the ophthalmoscope from a distance of one to several cm.
Figure 15.
Basic explanation of ophthalmoscope operation
Figure 16.
Basic principle of light beam travel during ophthalmoscopic
operation
Figure 17.
Modern ophthalmoscope, which is equipped with camera to take photos of the fundus
Features |
Direct ophthalmoscopy |
Indirect ophthalmoscopy |
Condensing lens |
Not Required |
Required |
Examination distance |
As close to patient's eye as possible |
At an arm's length |
Image |
Virtual, erect |
Real, inverted |
Illumination |
Not so bright; so not useful in hazy media |
Bright; useful for hazy media |
Area of field in focus |
About 2 disc diameters |
About 8 disc diameters |
Stereopsis |
Absent |
Present |
Accessible fundus view |
Slightly beyond equator |
Up to Ora serrata i.e. peripheral retina |
Examination through hazy media |
Not possible |
Possible |
Each type of ophthalmoscopy
has a special type of ophthalmoscope:
Figure 18.
Ophthalmoscope (left) and otoscope combination by
Welch Allyn
Figure 19.
Ophthalmoscope external components
Figure 20. Fundus photographs of the right eye (left
image) and left eye (right image), seen from front so that left in each image
is to the person's right, demonstrating the structures that can be seen in ophthalmoscopy. Each fundus has
no sign of disease or pathology. The gaze is into the camera, so in each
picture the macula is in the center of the image, and the optic disk
is located towards the nose. Both optic disks have some pigmentation at the
perimeter of the lateral side, which is considered non-pathological. The left
image (right eye) shows lighter areas close to larger vessels, which has been
regarded as a normal finding in younger people.
Ophthalmoscopy is done as part of a
routine physical or complete eye examination. It is used to detect and evaluate
symptoms of retinal detachment or eye diseases such as glaucoma.
In patients with headaches, the finding of swollen optic discs on ophthalmoscopy is a key sign, as this indicates raised
intracranial pressure (ICP) which could be due to
brain tumor, amongst other conditions. Cupped optic discs are seen in glaucoma.
To allow for better inspection through the pupil, which constricts because of
light from the ophthalmoscope, it is often desirable to dilate the pupil by
application of a mydriatic agent, for instance tropicamide. It is primarily considered ophthalmologist
equipment. Recent developments like Scanning Laser Ophthalmoscope can make good
quality images though pupils as small as 2 millimeters, so dilating pupils is
no longer needed with these devices.
A red reflex can be seen when looking at a patient's pupil through a
direct ophthalmoscope. This part of the examination is done from a distance of
about 50 cm and is usually symmetrical between the two eyes. An opacity may indicate a cataract. The red reflex
refers to the reddish-orange reflection of light from the eye's retina that is
observed when using an ophthalmoscope or retinoscope
from approximately 30 cm / 1 foot. This examination is usually performed in a
dimly lit or dark room. According to Bate's
Guide to Physical Exams, retinal detachment would result in the absence
of red reflex in the affected eye.
Gonioscopy
describes the use of a goniolens (also known
as a gonioscope) in conjunction with a slit
lamp or operating microscope to gain a view of the iridocorneal
angle, or the anatomical angle formed between the eye's cornea and iris.
The importance of this process is in diagnosing and monitoring various eye
conditions associated with glaucoma.
Glaucoma is a term
describing a group of ocular disorders with multi-factorial etiology united by
a clinically characteristic intraocular pressure-associated optic neuropathy.
This can permanently damage vision in the affected eye(s) and lead to
blindness if left untreated. It is normally associated with increased fluid
pressure in the eye (aqueous humour).
|
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Figure 21.
Normal eye and Glaucoma eye
Mainly Glaucoma is
divided into two groups
1.
Congenital Glaucoma: This is seen
in newborn infants. It is due to developmental defect of draining channels and
the front portion of the eye ball. It is a serious sight threatening problem. If diagnosed in early stage, can be treated by surgery.
2.
Acquired Glaucoma: Again it can
be classified as
a.
Primary Glaucoma :
Which occurs without any underlying cause. It is due to the age and changes in
anatomical configuration. It can be again classified as
:
Open angle Glaucoma : This usually
occurs in people above 40 years. Though the drainage area is open, the draining
channels are blocked due to age related changes.
Narrow angle Glaucoma : The
irido corneal angle is narrow which causes
obstruction to the outflow of fluid in the eye ball. This increases the
intraocular pressure which in turn hampers the optic nerve functioning.
b. Secondary
Glaucoma is due to the underlying causes like injury, inflammation,
diabetes etc.
The goniolens
allows the ophthalmologist to view the irideocorneal
angle through a mirror or prism, without which the angle is masked by total
internal reflection from the ocular tissue.
The mechanism for this process varies
with each type of goniolens. Three examples of goniolenses are the:
There are many other goniolenses available for use, including modified versions
the aforementioned, which prove valuable for surgical use.
Although the details vary based on
the type of goniolens used, in general the gonioscopy process involves:
CORNEAL PACHYMETERS
Corneal pachymetry is the process of measuring
the thickness of the cornea. A pachymeter
is a medical device used to measure the thickness of the eye's cornea. It is
used to perform corneal pachymetry prior to LASIK surgery, and is useful in screening for patients
suspected of developing glaucoma among other uses.
Conventional pachymeters are devices that display the thickness of the
cornea, usually in micrometres, when the ultrasonic
transducer touches the cornea. Newer generations of ultrasonic pachymeters work by way of Corneal Waveform (CWF).Using this technology the user can capture an ultra-high
definition echogram of the cornea, somewhat like a corneal A-scan. Pachymetry using the corneal waveform process allows the
user to more accurately measure the corneal thickness, verify the reliability
of the measurements that were obtained, superimpose corneal waveforms to
monitor changes in a patient's cornea over time, and measure structures within
the cornea such as micro bubbles created during femto-second
laser flap cuts.
Figure 24. A
typical ultrasound pachymeter
Figure 25. Corneal thickness examination with a pachymeter.
TONOMETERS
Tonometry
is the procedure eye care professionals perform to determine the intraocular
pressure (IOP), the fluid pressure inside
the eye. It is an important test in the evaluation of patients at risk from
glaucoma. Most tonometers are calibrated to measure
pressure in millimeters of mercury (mmHg).
Palpation
Palpation (also known as digital tonometry) is the method of estimating intraocular pressure
by gently pressing the index finger against the cornea of a closed eye. This
method is notoriously unreliable
Goldmann tonometry
Goldmann tonometry is considered to be the gold standard test and is
the most widely accepted method. A special disinfected prism is mounted on the tonometer head and then placed against the cornea. The
examiner then uses a cobalt blue filter to view two green semi circles. The
force applied to the tonometer head is then adjusted
using a dial connected to a variable tension spring until the inner edges of
the green semicircles in the viewfinder meet. When an area of 3.06mm has been
flattened, the opposing forces of corneal rigidity and the tear film are
roughly approximate and cancel each other out allowing the pressure in the eye
to be determined from the force applied. Like all non-invasive methods, it is
inherently imprecise
Non-contact tonometry
Non-contact tonometry
(or air-puff tonometry) is different from pneumatonometry and was invented by Bernard Grolman of Reichert, Inc (formerly American Optical). It
uses a rapid air pulse to applanate (flatten) the
cornea. Corneal applanation is detected via an
electro-optical system. Intraocular pressure is estimated by detecting the
force of the air jet at the instance of applanation.
Historically, non-contact tonometers were not
considered to be an accurate way to measure IOP but
instead a fast and simple way to screen for high IOP.
However, modern non-contact tonometers have been
shown to correlate well with Goldmann tonometry measurements and are particularly useful for
measuring IOP in children and other non-compliant
patient groups.
KERATOMETER
FOR ASTIGMATISM ASSESSMENT
A keratometer,
also known as a ophthalmometer,
is a diagnostic instrument for measuring the curvature of the anterior
surface of the cornea, particularly for assessing the extent and axis of astigmatism.
It was invented by the German physiologist Hermann von Helmholtz
in 1880, although an earlier model was developed in 1796 by Jesse Ramsden and Everard Home.
A keratometer
uses the relationship between object size (O), image size
(I), the distance between the reflective surface and the object (d), and the
radius of the reflective surface (R). If three of these variables are known (or
fixed), the fourth can be calculated using the formula
There are two distinct variants of
determining R; Javal-Schiotz type keratometers
have a fixed image size and are typically 'two position', whereas Bausch and
Lomb type keratometers have a fixed object size and
are usually 'one position'.
To summarize, keratometer is an optical instrument for measuring the
radius of curvature of the cornea in any meridian. By measuring along the two
principal meridians, corneal astigmatism can be deduced. The principle is based
on the reflection by the anterior surface of a luminous pattern of mires
in the centre of the cornea in an area of about 3.6 mm in diameter. Knowing the
size of the pattern h and measuring that of the reflected image h′
and the distance d between the two, the radius of curvature R of
the cornea can be determined using the approximate formula.
R =
2d (h′/h)
In addition, a doubling system (e.g.
a bi-prism) is also integrated into the instrument in order to mitigate the
effect of eye movements, as well as a microscope in order to magnify the small
image reflected by the cornea. This instrument is used in the fitting of
contact lenses and the monitoring of corneal changes occurring as a result of
contact lens wear. The range of the instrument can be extended approximately 9
D by placing a +1.25 D lens in front of the objective to measure steeper
corneas. The range in the other direction can be extended by approximately 6 D
using a −1.00 D lens to measure flatter corneas.
Figure 26.
An eye doctor examining a patient with a keratometer
Figure 27. Keratometer structure
SCANNING LASER POLARIMETER
Scanning laser polarimetry
is the use of polarised light to measure the
thickness of the retinal nerve fiber layer as part of a glaucoma workup. The GDx-VCC is one example. The GDx
nerve fiber analyzers measure the retinal nerve fiber layer (RNFL) thickness with a scanning laser polarimeter
based on the birefringent properties of the RNFL. It projects a polarized beam of a light into the eye.
As this light passes through the NFL tissue, it changes and slow. The detectors
measure the change and convert it into thickness units that are graphically
displayed.
Figure 28.
Scanning laser polarimeter
For healthy eye, the image will show
yellow and red colour in superior and inferior at NFL
regions. But, in glaucoma, the image is absence of red and yellow colours. Superiorly and inferiorly more uniform blue
appearance. Picture indicates that the eye is at the advance stage of the
disease.
The deviation map reveals the
location and magnitude of RNFL thinning relative to a
normal value. This normal value was generated as an average of people from
various cultutres. Defects are colour-coded
based on probability of normality (e.g. yellow means that the probability is
below 5% of that RNFL at that location is normal). A
healthy eye has a clear deviation map.
A further representation is the TSNIT graph. TSNIT is stand for
Temporal
|
|
Figure 29. GDx - Deviation map |
Figure 30. TSNIT graph |
ELECTRORETINOGRAPH
Electroretinography
measures the electrical responses of various cell types in the retina,
including the photoreceptors (rods and cones), inner retinal cells (bipolar and
amacrine cells), and the ganglion cells. Electrodes
are usually placed on the cornea and the skin near the eye,
although it is possible to record the ERG from skin electrodes. During a
recording, the patient's eyes are exposed to standardized stimuli and
the resulting signal is displayed showing the time course of the signal's
amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG
is composed of electrical potentials contributed by different cell types within
the retina, and the stimulus conditions (flash or pattern stimulus, whether a
background light is present, and the colors of the stimulus and background) can
elicit stronger response from certain components.
If a flash ERG is performed on a
dark-adapted eye, the response is primarily from the rod system. Flash ERGs performed on a light adapted eye will reflect the
activity of the cone system. Sufficiently bright flashes will elicit ERGs containing an a-wave (initial
negative deflection) followed by a b-wave (positive deflection). The leading
edge of the a-wave is produced by the photoreceptors, while the remainder of
the wave is produced by a mixture of cells including photoreceptors, bipolar, amacrine, and Muller cells or Muller glia.
The pattern ERG, evoked by an alternating checkerboard stimulus, primarily reflects
activity of retinal ganglion cells.
Figure 31. Maximal response ERG waveform from a dark adapted eye.
Figure 32. A
patient undergoing an electroretinogram
Inherited retinal degenerations in
which the ERG can be useful include:
Other ocular disorders in which the
standard ERG provides useful information include:
The ERG is also used extensively in
eye research, as it provides information about the function of the retina that
is not otherwise available.
Other ERG tests, such as the Photopic Negative Response (PhNR)
and pattern ERG (PERG) may be useful in assessing
retinal ganglion cell function in diseases like glaucoma.
The multifocal
ERG is used to record separate responses for different retinal locations.
ELECTROOCULOGRAPH
Electrooculography (EOG/E.O.G.) is a technique for
measuring the resting potential of the retina in the human eye.
The resulting signal is called the electrooculogram.
Primary applications are in ophthalmological
diagnosis and in recording eye movements. Unlike the electroretinogram,
the EOG does not measure response to
individual visual stimuli.
To measure eye movement, pairs of
electrodes are typically placed either above and below the eye or to the left
and right of the eye. If the eye moves from center position toward one of the
two electrodes, this electrode "sees" the positive side of the retina
and the opposite electrode "sees" the negative side of the retina.
Consequently, a potential difference occurs between the electrodes. Assuming
that the resting potential is constant, the recorded potential is a measure of
the eye's position.
Principle:
The eye acts as a dipole in which the
anterior pole is positive and the posterior pole is negative. 1. Left gaze: the
cornea approaches the electrode near the outer canthus
of the left eye, resulting in a negative-trending change in the recorded
potential difference. 2. Right gaze: the cornea approaches the electrode near
the inner canthus of the left eye, resulting in a
positive-trending change in the recorded potential difference.
Figure 33. Electrooculograms for the left eye (LEOG)
and the right eye (REOG) for the period of REM sleep.
ULTRASOUND BIOMICROSCOPE
Ultrasound biomicroscopy
is a type of ultrasound eye exam that makes a more detailed image than regular
ultrasound. High-energy sound waves are bounced off the inside of the eye and
the echo patterns are shown on the screen of an ultrasound machine. This makes
a picture called a sonogram. It is useful in glaucoma, cysts
and neoplasms of the eye, as well as the
evaluation of trauma and foreign bodies of the eye.
VIDEOKERATOGRAPH
Corneal topography,
also known as photokeratoscopy or videokeratography, is a non-invasive medical imaging
technique for mapping the surface curvature of the cornea, the outer structure
of the eye. Since the cornea is normally responsible for some 70%
of the eye's refractive power, its topography is of critical importance in
determining the quality of vision and corneal health.
The three-dimensional map is
therefore a valuable aid to the examining ophthalmologist or optometrist and
can assist in the diagnosis and treatment of a number of conditions; in
planning cataract surgery and intraocular lens (IOL)
implantation (plano or toric
IOLs); in planning refractive surgery such as LASIK, and evaluating its results; or in assessing the fit
of contact lenses. A development of keratoscopy,
corneal topography extends the measurement range from the four points a few
millimeters apart that is offered by keratometry to a
grid of thousands of points covering the entire cornea. The procedure is
carried out in seconds and is completely painless.
Figure 34. Corneal videotopography or videokeratography results.
The patient is seated facing a bowl
containing an illuminated pattern, most commonly a series of concentric rings.
The pattern is focused on the anterior surface of the patient's cornea and
reflected back to a digital camera at the centre of the bowl. The topology of
the cornea is revealed by the shape taken by the reflected pattern. A computer
provides the necessary analysis, typically determining the position and height
of several thousand points across the cornea. The topographical map can be
represented in a number of graphical formats, such as a sagittal
map, which color-codes the steepness of curvature according to its dioptric value.
The corneal topograph
owes its heritage to 1880, when the Portuguese ophthalmologist Antonio Placido viewed a painted disk (Placido's
disk) of alternating black and white rings reflected in the cornea. The
rings showed as contour lines projected on the corneal tear film. Javal L., an pioneer in the field
in the 1880s incorporated the rings in his opthalmometer
and mounted an eyepice which magnified the image of
the eye. He proposed that the image should be photographed or diagrammatically
represented to allow analysis of the image.
Computerized corneal topography could
be employed for diagnostics. It is, in fact, one of the exams the patients have
to undergo prior to the Cross-linking and the Mini Asymmetric Radial Keratotomy
(M.A.R.K.). For example, the KISA%
index (keratometry, I-S, skew percentage,
astigmatism) is used to arrive at a diagnosis of keratoconus,
to screen the suspect keratoconic patients and analyse the degree of corneal steepness changes in healthy
relatives.
Nevertheless, topography in itself is
a measurement of the first reflective surface of the eye (tearfilm)
and is not giving any additional information beside the shape of this layer
expressed in curvature. keratoconus
in itself is a pattern of the entire cornea, therefore every measurement just
focusing on one layer, might not be enough for a state of the art diagnosis.
Especially early cases of keratoconus might be missed
by a plain topographic measurement, which is critical if refractive surgery is
being considered. The measurement is also sensitive to unstable tearfilms. Also, the alignment of the measurement can be
difficult, especially with eyes that have Keratoconus,
a significant astigmatism, or sometimes after refractive surgery.