Ординатура / Офтальмология / Английские материалы / Comprehensive Ophthalmology_Khurana_2007

Fig. 21.13. Technique of Schiotz tonometry.

intraocular pressure in mm of mercury (mmHg) from the scale reading and the plunger weight.

The main advantages of Schiotz tonometer are that it is cheap, handy and easy to use. Its main disadvantage is that it gives a false reading when used in eyes with abnormal scleral rigidity. False low levels of IOP are obtained in eyes with low scleral rigidity seen in high myopes and following ocular surgery.

Applanation tonometry

The concept of applanation tonometry was introduced by Goldmann is 1954. It is based on ImbertFick law which states that the pressure inside a sphere

(P) is equal to the force (W) required to flatten its surface divided by the area of flattening (A); i.e., P = W/A.

The commonly used applanation tonometers are:

1. Goldmann tonometer. Currently, it is the most popular and accurate tonometer. It consists of a double prism mounted on a standard slit-lamp. The

Fig. 21.14. Technique of applanation tonometry.

2.Perkin’s applanation tonometer (Fig. 21.16). This is a hand-held tonometer utilizing the same biprism as in the Goldmann applanation tonometer. It is small, easy to carry and does not require slit lamp. However, it requires considerable practice before, reliable readings can be obtained.

3.Pneumatic tonometer. In this, the cornea is applanated by touching its apex by a silastic diaphragm covering the sensing nozzle (which is connected to a central chamber containing pressurised air). In this tonometer, there is a pneumatic-to-electronic transducer, which converts the air pressure to a recording on a paper-strip, from where IOP is read.

Fig. 21.16. Perkin’s hand-held applanation tonometer.

4.Pulse air tonometer is a hand-held, non-contact tonometer that can be used with the patient in any position.

5.Tono-Pen is a computerised pocket tonometer. It employs a microscopic transducer which applanates the cornea and converts IOP into electric waves.

Tonography

Tonography is a non-invasive technique for determining the facility of aqueous outflow (C-value). The C-value is expressed as aqueous outflow in microlitres per minute per millimetre of mercury. It is estimated by placing Schiotz tonometer on the eye for 4 minutes. For a graphic record the electronic Schiotz tonometer is used. C-value is calculated from special tonographic tables taking into consideration the initial IOP (P0) and the change in scale reading over the 4 minutes.

Clinically, C-value does not play much role in the management of a glaucoma patient. Although, in general, C-values more than 0.20 are considered normal, between 0.2 and 0.11 border line, and those below 0.11 abnormal.

TECHNIQUES OF FUNDUS EXAMINATION

A.Ophthalmoscopy, and

B.Slit-lamp biomicroscopic examination of the fundus by:

Indirect slit-lamp biomiscroscopy,

Hruby lens biomicroscopy,

Contact lens biomicroscopy

For details see page 564-568

PERIMETRY

The visual field is a three-dimensional area of a subject’s surroundings that can be seen at any one time around an object of fixation. The extent of normal visual field with a 5 mm white colour object is superiorly 50o, inferiorly 70o, nasally 60o and temporally 90o (Fig. 21.17). The field for blue and yellow is roughly 10o less and that for red and green colour is about 20o less than that for white. Perimetry with a red colour object is particularly useful in the diagnosis of bitemporal hemianopia due to chiasmal compression and in the central scotoma of retrobulbar neuritis.

The visual field can be divided into central, and peripheral field (Fig. 21.17):

Central field includes an area from the fixation point to a circle 30° away. The central zone contains physiologic blind spot on the temporal side.

Peripheral field of vision refers to the rest of the area beyond 30° to outer extent of the field of vision.

Methods of estimating the visual fields

Perimetry. It is the procedure for estimating extent of the visual fields. It can be classified as below:

Fig. 21.17. Extent of normal visual field.

Kinetic versus static perimetry

Kinetic perimetry. In this the stimulus of known luminance is moved from periphery towards the centre to establish isopters. Various methods of kinetic perimetry are: confrontation method, Lister’s perimetery, tangent screen scotometry and Goldmann’s perimetry.

Static perimetry. This involves presenting a stimulus at a predetermined position for a preset duration with varying luminance. Various methods of static perimetry adopted are Goldmann perimetry, Friedmann perimetry, automated perimetry.

Peripheral versus central field charting

Peripheral field charting

Central field charting

-Confrontation method

-Perimetery:Lister’s,Goldmann’sandautomated.

-Campimetry or scotometry

-Goldmann’s perimetry

-Automated field analysis

Manual versus automated perimetry

Manual perimetry

Automated perimetry

A. MANUAL PERIMETRY

Most of the kinetic methods of field testing are done manually as described below:

1.Confrontation method. This is a rough but rapid and extremely simple method of estimating the peripheral visual field.Assuming the examiner’s fields to be within the normal range, they are compared with patient’s visual fields.

The patient is seated facing the examiner at a distance of 1 metre. While testing the left eye, the patient covers his right eye and looks into the examiner’s right eye. The examiner occludes his left eye and moves his hands in from the periphery keeping it midway between the patient and himself. The patient and the examiner ought to see the hand simultaneously, for the patient’s field to be considered normal. The hand is moved similarly from above, below and from right and left.

2.Lister’s perimeter (Fig. 21.18). It has a metallic semicircular arc, graded in degrees, with a white dot for fixation in the centre. The arc can be rotated in different meridians.

The patient is seated facing the arc with his chin firmly in the chin-rest. With one eye occluded, he fixates the white dot in the centre. A test object (usually white and of size 3 to 5 mm) is moved along the arc from extreme periphery towards the centre, and the point at which the patient first sees the object is registered on a chart. The arc is moved through 30o each time and 12 such readings are taken. The details of the object regarding its colour and size are noted.

With the help of this perimeter extent of peripheral field is charted.

3.Campimetry (scotometry) is done to evaluate the central and paracentral area (30o) of the visual field. The Bjerrum’s screen is used and can be of size 1 metre or 2 metres square (Fig. 21.19). Accordingly, the patient is seated at a distance of 1 metre or 2 metres, respectively. The screen has a white object

for fixation in its centre, around which are marked concentric circles from 5o to 30o. The patient fixates at the central dot with one eye, the other being occluded.Awhite target (1-10 mm diameter) is brought in from the periphery towards the centre in various meridians. Initially the physiologic blind spot is charted, which corresponds to the optic nerve head and is normally located about 15o temporal to the fixation point. Dimensions of blind sports are horizontally 7-8o and vertically 10-11o.

Other important advantages of automated perimeters are data storage capability, ease of operation, well controlled fixation, menu driven software and on line assistance making them easy to learn and use.

Automated perimetry also provides facility to compare results statistically with normal individuals of the same age group and with previous tests of the same individual.

Interpertation of automated perimetry print out field charts

Before embarking on the interpertation of automated perimetry printout field charts, it will be worth while to have a knowledge about:

Automated perimeter variables and

Testing strategies and programmes

The following description is mainly based on

Humphrey’s field Analyser (HFA).

Automated perimeter variables

1.Background illumination. HFA uses 31.5 apostilb (asb) background illumination. Apostilb (asb) is a unit of brightness per unit area (and is defined as 35–1 candela / m2).

2.Stimulus intensity. HFA uses projected stimuli which can be varied in intensity over a range of more than 5% log units (51 decibels) between 0.08 and 10,000 asb. In decibel notation (db), the value refers to retinal sensitivity rather than to stimulus intensity. Therefore, 0 db corresponds to 10,000 asb and 51 db to 0.08 asb (Fig. 21.22). In contrast to kinetic perimetry, the higher numbers indicate a logarithmic reduction in test object brightness, and hence greater sensitivity of vision (Fig. 21.22).

3.Stimulus size. HFA usually offers five sizes of stimuli corresponding to the Goldmann perimeter stimuli I through V. Unless otherwise instructed, the standard target size for automated perimetry is equivalent to Goldmann size III (4 mm2) white target.

4.Stimulus duration. Stimulus duration should be shorter than the latency time for voluntary eye movements (about 0.25 seconds). HFA uses a stimulus duration of 0.2 sec. while octopus has 0.1 sec.

Testing strategies and programes

The visual threshold is the physiologic ability to detect a stimulus under defined testing conditions. The normal threshold is defined as the mean threshold in normal people in a given age group at a given location in the visual field. It is against these values that the machine compares the patient’s sensitivity. Thresholds are reported is decibels in a range of 0-50. Fifty decibels (db) is the dimmest target the perimeter can project. 0 db is the brightest illumination the perimeter can project. The lower the decibel value the lower the sensitivity; the higher the decibel value, the higher is the sensitivity.

Two basic testing strategies are used in automated static perimetry:

A.Suprathreshold testing. It uses targets that are well above the brightness that the patient should be able to see (suprathreshold). It is simply a screening procedure to detect gross defects.

B.Threshold testing. Threshold testing provides more precise results than suprathreshold testing and is thus preferred by most clinicians, although it takes more time and the equipment often costs more. Strategies used for threshold testing are:

1.Full threshold testing. A full threshold test determines the threshold value at each point by the bracketing technique (4-2 on the Humphrey and 4-2-

1on the Octopus perimeter). In it, a stimulus is presented at a test point for 0.2 seconds and the machine waits for Yes/No response. If the stimulus is not seen, the intensity of the stimulus is increased in

4db steps till it is seen. Once the threshold is crossed, the stimulus intensity is decreased in 2db steps till the stimulus is not seen. A full threshold test is appropriate for a patient’s first test, because it crosses the threshold twice (first with a 4 dB increment). Accurately determinied threshold values make subsequent tests easier because it allows the perimeter to begin with the previous threshold values for determining future data points. Full threshold test is, however, a time consuming process.

2.Fast Pac. It is a more rapid testing strategy where the threshold is only crossed once (in 3dB increments), but this strategy is often not appropriate.

3.SITA (Swedish Interactive Threshold Alogarithm). It is a strategy of threshold testing which dramatically reduces test time. It is available as SITA - standard and SITA-fast.

Test programmes

The standard test programmes used with static threshold strategy on the Humphrey’s Field Analyser (HFA) can be grouped as below:

A.Central field tests

Central 30 - 2 test,

Central 24 - 2 test,

Central 10 - 2 test,and

Macular test

B.Peripheral field tests

Peripheral 30/60-1,

Peripheral 30/60-2,

Nasal step, and

Temporal crescent

C.Speciality tests

Neurological-20,

Neurological -50,

Central 10-12, and

Macular test

D.Custom tests

Central field tests are more commonly required. These include:

1.Central 30-2 test. It offers the most comprehensive form of visual field assessment of the central 30 degrees. It consists of 76 points 6 degrees apart on either side of the vertical and horizontal axes, such that the inner most points are three degrees from the fixation point.

2.Central 24-2 test. In it, 54 points are examined. It is near similar to the 30-2 test except that the two peripheral nasal points at 30 degrees on either side of the horizontal axis are not included while testing the central 24 degrees.

3.Central 10-2 test. When most points in the arcuate region between 10 and 30 degrees show marked depression then this test helps to assess and followup 68 points 2 degrees apart in the central 10 degree are examined.

4.Macular grid test is used when the field is limited to central 5 degrees. This test examines 10 points spaced on a 29 degree square grid centred on the point of fixation.

Evaluation of Humphrey single-field print-out

The standard HFA single field printout is obtained using a software called Statpac printout. For the purpose of evaluation, the Humphry single-field printout (Statpac printout) with central 30-2 test can be studied in eight parts or zones I to VIII as described below (Fig. 21.23):

I. Patient data and test parameters. At the top of printout page (part I or zone I) are printed:

Patients data (name, date of birth, eye (right/ left) pupil size visual acuity).

Test parameters (test name, strategy, stimulus used,background)

II. Reliability indices. The part II or zone II of the printout shows the reliability indices and test duration (Fig. 21.23). The visual field examination is considered unreliable if three are more of the following reliability indices have below mentioned values:

Fixation losses ≥ 20%,

False positive error ≥ 33%,

False negative error ≥ 33%,

Short-term fluctuations ≥ 4.0 dB,

Total questions ≥ 400.

III. Gray scale simulation of the test data is depicted in zone III or part III of the printout (Fig. 21.23). The darker the printout, the worse is the field. The gray scale provides the field defects at a glance. However,

Fig. 21.23. Arbitraty division of humphry single field print out (statpac printout) with central 30-2 test in sparts

(zones) for the purpose of discription and understanding.

in general we do not make a diagnosis based on the grey scale. The main empahasis on statistical help shows in zone IV to VIII of the printout (threshold values).

IV. Total deviation plots provide the deviation of patient’s threshold values from that of age corrected normal data. The two total deviation plots are numeric value plot and the probability plot (grey scale symbol plot).

Numeric value plot represents the differences in decibels. A zero value means that the patient has the expected threshold for that age. Positive numbers reflect points that are more sensitive than average for that age; whereas negative numbers reflect points that are depressed compared with the average.

Probability plot (grey scale symbol plot). In the lower part of zone IV of the printout, the total deviation plot is represented graphically. The darker the graphic representation. the more significant it is.

Note: In general, the total deviation plot is an indicator of the general depression and is not

capable of revealing the hidden scotomas that may be present in the overall depressed field.

V. Pattern deviation plots. The two pattern deviation plots (numeric pattern deviation plot and probability pattern deviation plot) shown in zone V of the printout are similar to the total deviation plots except that here Statpac software has corrected the results for the changes caused by cataract, small pupil, etc.

VI. Global indices are depicted in the zone VI of the printout. Global indices refer to some calculations made by Statpac to provide overall guide lines to help the practitioner assess the field results as a whole rather than on point-to-point basis as shown in the total deviation and pattern deviation plots.

Below mentioned four global indices are provided with the full threshold program which summerize the status of the visual field at a glance. Principally, the global indices are used to monitor progression of glaucomatous damage rather than for initial diagnosis.

1.Mean deviation (MD). This is the mean difference (in decibel value) between the normative data for that age compared with that of collected data. It is more an indicator of the general depression of the field. Worse than normal value is indicated by a negative value.

2.Pattern standard deviation (PSD). It is a measure of variability within the field i.e, it measures the difference between a given point and adjacent points. It actually points out towards localized field loss and is most useful in identifying early defects. It loses its advantage in marked depression.

3.Short-term fluctuation (SF). It is a measure of the variability between two different evaluations of the same 10 points in the field. It is not available with SITA strategy. A high SF means either decreased reliability or an early finding indicative of glaucoma.

4.Corrected pattern standard deviation (CPSD). It is the PSD corrected for SF. It indicates the variability between adjacent points that may be due to disease rather than due to intra-test variability.

VII. Glaucoma hemifield test (GHT) comapares the five clusters of points in the upper field (above the horizontal midline) with the five mirror images in the lower field. These clusters of points have been developed based on the anatomical distribution of

the nerve fibres and are specific to the detection of gluacoma. Depending upon the differences between the upper and lower clusters of points the following five messages may be displayed:

Outside normal limits. The GHT outside normal limits denotes that either the values between upper and lower clusters differ to an extent found in less than 1% of the population or any one pair of clusters is depressed to the extent that would be expected in less than 0.5% of the population.

Border line. The GHT is considered border line when the difference between any one of the upper and lower mirror clusters is what might be expected in less than 3% of population.

General reduction in sensitivity. The GHT is considered to have general reduction in sensitivity if the best part of visual field is depressed to an extent expected in less than 0.5% of the population.

Abnormally high sensitivity is labelled when the best part of the visual field is such as would be found in less than 0.5% of the population.

Within normal limits. GHT is considered within normal limits when none of the above criteria is met.

VIII. Actual threshold values shown in part VIII of the printout (Fig. 21.23) may be inspected for any pattern or scotoma when clinical features are suspeciant and even if all the seven other parts of the printout are normal. A scotoma by definition is the depressed part of the field as compared to the surrounding and not as compared to normals. When the actual test threshold values are below 15dB, the sensitivity of the test is lost.

Diagnosis of glaucoma field defects on HFA single-field printout

(See page 220).

FUNDUS FLUORESCEIN ANGIOGRAPHY

Fundus flourescein angiography (FFA) is a valuable tool in the diagnosis and management of a large number of fundus disorders.

Basically, FFA gives information by allowing the examiner to study the changes, produced by various fundus disorders, in the flow of fluorescein dye along the vasculature of the retina and choroid.

Indications. It is indicated in many disorders of ocular fundus, viz., (1) Diabetic retinopathy (2) Vascular occlusions; (3) Eales’ disease. (4) Central serous retinopathy, (5) Cystoid macular oedema.

Technique. The technique of FFA comprises rapidly injecting 5 ml of 10 per cent solution of sterile sodium fluorescein dye in the antecubital vein and taking serial photographs (with fundus camera) of the fundus of the patient who is seated with pupils fully dilated. The fundus camera has a mechanism to use blue light (420-490 nm wavelength) for exciting the fluorescein present in blood vessels and to use yellow-green filter for receiving the fluorescent light (510-530 nm wavelength) back for photography.

The first photograph is taken after 5 seconds, then every second for next 20 seconds and every 3-5 seconds for next one minute. The last pictures are taken after 10 minutes.

Complications. FFA is comparatively a safe procedure. Minor side effects include: discoloration of skin and urine, mild nausea and rarely vomiting. Anaphylaxis or cardiorespiratory problems are extremely rare. However, a syringe filled with dexamethasone and antihistaminic drug along with other measures should be kept ready to deal with such catastrophy.

Phase of angiogram. Normal angiogram consists of following overlapping phases:

1.Pre-arterial phase. Since the dye reaches the choroidal circulation 1 second earlier than the retinal arteries, therefore in this stage choroidal circulation is filling, without any dye in retinal arteries.

2.Arterial phase. It starts 1 second after prearterial phase and lasts until the retinal arterioles are completely filled.

3.Arteriovenous phase. This is a transit phase and involves the complete filling of retinal arterioles and capillaries with a laminar flow along the retinal veins (Fig. 21.24).

4.Venous phase. In this phase, veins are filling and arterioles are emptying. This phase can be subdivided into early, mid, and late venous phase.

Abnormalities detected by FFA. In the blood fluorescein is readily bound to the albumin. Normally the dye remains confined to the intravascular space due to the barriers formed by the tight junctions between the endothelial cells of retinal capillaries

Fig. 21.24. Normal fluorescein angiogram

(arteriovenous phase).

(inner blood-retinal barrier) and that between the pigment epithelial cells (outer blood-retinal barrier).

In diseased states abnormalities in the form of hyperfluorescence and hypofluorescence may be detected on FFA.

1. Hyperfluorescence. The causes are:

A window defect in RPE due to atrophy shows background choroidal fluorescence.

Pooling of dye under detached RPE.

Pooling of dye under sensory retina after breakdown of the outer blood-retinal barrier as occurs in central serous retinopathy (CSR).

Leakage of dye into the neurosensory retina due to a breakdown in inner blood-retinal barrier e.g., as seen in cystoid macular edema (CME).

Leakage of dye from the choroidal or retinal neovascularization e.g., as seen in cases of proliferative diabetic retinopathy, and subretinal neovascular membrane in age-related macular degeneration.

Staining i.e., long retention of dye by some tissues e.g., as seen in the presence of drusen.

Leakage of dye from optic nerve head as seen in papilloedema.

2. Hypofluorescence. The causes are:

Blockage of background fluorescence due to abnormal deposits on retina e.g., as seen due to the presence of retinal haemorrhage, hard exudates and pigmented clumps.

Occlusion of retinal or choroidal vasculature, e.g., as seen in central retinal artery occlusion and occlusion of capillaries in diabetic retinopathy.

Loss of vasculature as occurs in patients with choroideremia and myopic degeneration.

ELECTRORETINOGRAPHY AND ELECTRO-OCULOGRAPHY

The electrophysiological tests allow objective evaluation of the retinal functions. These include: electroretinography (ERG), electro-oculography (EOG), and visually-evoked response (VER).

Electroretinography (ERG)

Electroretinography (ERG) is the record of changes in the resting potential of the eye induced by a flash of light. It is measured in dark adapted eye with the active electrode (fitted on contact lens) placed on the cornea and the reference electrode attached on the forehead (Fig. 21.25).

Fig. 21.25. Technique of electroretinogram (ERG) recording.

Normal record of ERG consists of the following waves (Fig. 21.26):

a-wave. It is a negative wave possibly arising from the rods and cones.

b-wave. It is a large positive wave which is generated by Muller cells, but represents the acitivity of the bipolar cells.

c-wave. It is also a positive wave representing metabolic activity of pigment epithelium.

Both scotopic and photopic responses can be

elicited in ERG. Foveal ERG can provide information about the macula.

Fig. 21.26. Components of normal electroretinogram (ERG).

Uses. ERG is very useful in detecting functional abnormalities of the outer retina (up to bipolar cell layer), much before the ophthalmoscopic signs appear. However, ERG is normal in diseases involving ganglion cells and the higher visual pathway, such as optic atrophy.

Clinical applications of ERG

1.Diagnosis and prognosis of retinal disorders such as retinitis pigmentosa, Leber’s congenital amaurosis, retinal ischaemia and other chorioretinal degenerations.

2.To assess retinal function when fundus examination is not possible, e.g., in the presence of dense cataract and corneal opacity.

3.To assess the retinal function of the babies where possibilities of impaired vision is considered.

Abnormal ERG response. It is graded as follows:

1.Subnormal response. b-wave response is subnormal in early cases of retinitis pigmentosa even before the appearance of ophthalmoscopic signs. A subnormal ERG indicates that a large area of retina is not functioning.

2.Extinguished response is seen when there is complete failure of rods and cones function e.g., advanced retinitis pigmentosa, complete retinal detachment, central retinal artery occlusion and advanced siderosis.

3.A negative response indicates gross disturbances of the retinal circulation.

Electro-oculography (EOG)

Electro-oculography is based on the measurement of resting potential of the eye which exists between the cornea (+ve) and back of the eye (–ve).

Technique (Fig. 21.27). Electrodes are placed over the orbital margin near the medial and lateral canthi. The patient is asked to move the eye sideways (medially and laterally) and keep there for few seconds, during which recording is done. In this procedure, the electrode near the cornea (e.g., electrode placed near lateral canthus, when the eye is rotated laterally) becomes positive. The recording is done every minute for 12 minutes. This procedure is performed first in the dark adapted stage and then repeated for light adapted stage.

Normally, the resting potential of the eye decreases during dark adaptation and reaches its peak in light adaptation.

Fig. 21.27. Technique of electro-oculography (EOG) and record of normal electro-oculogram.