Instruments for investigation and correction of the human eye disorders
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 16–17 millimeters (about 0.65 inch). The eyeball grows rapidly, increasing to 22.5–23 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 12–15° 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 (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 (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, 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
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
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
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”. Today’s 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.
Replacing the bulb
Figure 12. Slit lamp structure
The following functions should be checked weekly. The hospital’s maintenance team or the service agent should be called if any problems are noticed during these checks.
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
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
As close to patient's eye as possible
At an arm's length
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
Accessible fundus view
Slightly beyond equator
Up to Ora serrata i.e. peripheral retina
Examination through hazy media
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).
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 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.
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 (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 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 (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
Figure 29. GDx - Deviation map
Figure 30. TSNIT graph
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.
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.
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 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.
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.