Ординатура / Офтальмология / Английские материалы / Clinical Ophthalmology A Systematic Approach 7th Edition_Kanski, Bowling_2011

kanski 7th

Fig. 10.19 Specific subtypes of glaucomatous damage. (A) Type 1 – focal ischaemic; (B) type 2 – myopic; (C) type 3 – senile sclerotic; (D) type 4 – concentrically enlarging

Non-specific signs of glaucomatous damage

Other disc signs of glaucomatous damage, though of variable specificity, include:

1Baring of circumlinear blood vessels is a sign of early thinning of the NRR. It is characterized by a space between a superficial blood vessel that runs from the superior or inferior aspects of the disc towards the macula, and the disc margin (Fig. 10.20A). ‘Overpass cupping’, in which there is loss of NRR underlying vessels, leaving space between the bridging vessels and the remaining nerve tissue, is similar.

2Bayoneting is characterized by double angulation of a blood vessel (Fig. 10.20B). With NRR loss, a vessel entering the disk from the retina may angle sharply backwards into the disk and then turn towards its original direction to run across the lamina cribrosa.

3Collaterals between two veins at the disc (Fig. 10.20C), similar to those following central retinal vein occlusion are rare.

4Loss of nasal NRR (Fig. 10.20D) is a sign of moderately advanced damage; a space may develop between the NRR and the central retinal vasculature.

5Lamina dot sign occurs in advancing glaucoma. The gray dot-like fenestrations in the lamina cribrosa become exposed as the NRR recedes (Fig. 10.20E). The fenestrations sometimes appear linear, and this itself may be a sign of advanced damage, indicating distortion of the lamina. The dot sign is not specific for glaucomatous atrophy, and may be seen in normal eyes.

6Disc haemorrhages often extend from the NRR onto the retina, most commonly inferotemporally (Fig. 10.20F). Their presence is a risk factor for glaucoma and they may also be a marker of inadequate control. They can also occur in healthy individuals as well as in patients with hypertension, diabetes and those taking antiplatelet agents.

7‘Sharpened edge’ or ‘sharpened rim’ is a sign of advancing damage. As NRR is lost adjacent to the edge of the disc, the disc margin contour assumes a sharper angle backwards. Bayoneting of vessels is often seen at a sharpened edge. This should not be confused with a ‘sharpened nasal polar edge’, which refers to the sharp angulation of the NRR at the nasal margin of a focal vertical polar notch.

419 / 1137

kanski 7th

Fig. 10.20 Non-specific signs of glaucomatous damage. (A) Inferior baring of circumlinear blood vessels; (B) inferior bayoneting; (C) collaterals; (D) loss of nasal neuroretinal rim; (E) lamellar dots; (F) disc haemorrhage

Peripapillary changes

Peripapillary atrophy surrounding the optic nerve head may be of significance in glaucoma (Fig. 10.21) and may be a sign of early damage in patients with ocular hypertension.

1Alpha (outer) zone is characterized by superficial retinal pigment epithelial changes. It tends to be larger and possibly more common in glaucomatous eyes.

2Beta (inner) zone is characterized by chorioretinal atrophy. It is larger and more common in glaucoma.

Fig. 10.21 Parapapillary changes. Zone beta (black arrow); zone alpha (white arrow)

420 / 1137

kanski 7th

It is important to note the distinction from the scleral lip or rim, the white band of exposed sclera central to the beta zone

Retinal nerve fibre layer

In glaucoma subtle retinal nerve fibre layer (RNFL) defects precede the development of detectable optic disc and visual field changes; their onset often follows disk haemorrhages. Two patterns occur: (a) localized wedge-shaped defects (Fig. 10.22A) and (b) diffuse defects that are larger and have indistinct borders. Red-free (green) light increases the contrast between normal retina and defects on slit-lamp biomicroscopy and typically makes identification easier (Fig. 10.22B). Defects may be easier to detect on (black-and-white) photographs than during clinical examination. Optical coherence tomography (OCT) and scanning laser polarimetry are highly effective means of quantifying the RNFL. It should be noted that RNFL defects are not specific to glaucoma, and can be seen in a range of neurological disease, as well as apparently normal individuals.

Fig. 10.22 Retinal nerve fibre layer defects. (A) Superotemporal wedge-shaped defect; (B) same eye seen with a green filter

(Courtesy of P Gili)

Copyright © 2011 Elsevier Inc.All rights reserved. Read our Terms and Conditions of Use and our PrivacyPolicy.

If you find this useful please saythanks in your way: dramroo

421 / 1137

kanski 7th

Imaging in glaucoma

Stereo disc photography

Stereo photography has historically been regarded as the reference standard in optic disc imaging, and remains a valuable option. The images are taken by repositioning the camera slightly between shots, either manually or using a stereo separator built into the camera.

Confocal scanning laser tomography

1Physics. The scanning laser ophthalmoscope (SLO) produces images of the optic nerve head and retina by scanning a diode laser beam across tissues at progressively varying depths, utilizing the confocal principle to capture information from only a thin layer at a time and thereby building a three-dimensional image. The Heidelberg Retinal Tomograph (HRT) has been widely adopted in the assessment of glaucoma patients. A HRT3 version has been introduced, though the HRTII is still the most commonly used.

2Indications

•To distinguish normal from glaucomatous eyes by comparison against a normative database (Moorfields regression analysis).

•To monitor disease progression in individual glaucoma patients.

•The macula can also be examined, although the OCT has more commonly been adopted for this.

3Technique. Keratometry values must be entered and significant (>1.0 dioptre) astigmatism corrected by means of a cylindrical lens. High quality images can usually be acquired without pupillary dilatation and through mild–moderate lens opacity. After image capture, the operator must manually mark the contour line defining the edge of the neuroretinal rim.

4Display. Images, data and analysis can be examined on a computer screen or printed. Sample monocular printouts from the HRTII are shown (Figs 10.23 and 10.24).

•Images of the disc and peripapillary retina are shown at the top of the display.

•In the topographic image (top left) the cup is represented in red, the neuroretinal rim in green and the connecting slope in blue.

•The reflectivity false colour image (top right) is divided into six sectors. Both the neuroretinal rim (green and blue on the topographic image) and the disc area (green, blue and red) are assessed using Moorfields regression analysis, taking into account age and overall disc size. A green tick within a sector indicates it is within normal limits, a yellow exclamation mark borderline and a red cross outside normal limits.

•The two cross-sectional images (top centre and middle left) show the amount of cupping in the vertical and horizontal planes. Two lines represent the edge of the optic disc and the single red line represents the arbitrary reference plane.

•The mean height contour graph (centre right) displays the variation of the retinal surface height along the contour line (green). The reference line (red) below this shows the position of the reference plane, designated as the plane of separation between the cup below and the neuroretinal rim above. This reference plane is parallel to the peripapillary retinal surface and is located 50 µm below the retinal surface at the location of the papillomacular bundle on the contour line. It is thus approximately located at the lower extent of the RNFL.

•The display of the retinal surface height variation along the contour line begins temporally at 0° (approximate centre of the papillomacular bundle). The height profile is plotted in a clockwise direction for a right eye and a counter-clockwise direction for a left eye. The graph largely corresponds to the course of the RNFL thickness along the disc margin.

•The Moorfields regression analysis is depicted as seven colour bar graphs, one bar for each segment and one global bar (bottom right). If the top of the green bar lies above the 95.0% prediction interval then the corresponding disc segment is classified as within normal limits, if it lies between the 95.0% and 99.9% it is borderline, and if it lies below 99.9% it is outside normal limits.

•Detailed stereometric data are presented in a table (bottom left). Readings outside normal are indicated with an asterisk.

422 / 1137

kanski 7th

Fig. 10.23 HRTof a normal eye

423 / 1137

kanski 7th

Fig. 10.24 HRTof a glaucomatous eye

Scanning laser polarimetry

1Physics. The GDx (Glaucoma Diagnosis) RNFL analyzer assesses the nerve fibre layer thickness by using its assumed ‘birefringent’ (resolving or splitting a light wave into two unequally reflected or transmitted waves) nature to change the polarization of incident polarized diode laser light; the amount of alteration is directly related to the thickness of the layer. The degree of polarization is assessed over an area of 1.75 disc diameters concentric to the disc and the profile of the density of the RNFL established; the thicker the RNFL the greater the polarization. The newer GDxVCC (Variable Corneal Compensation) version has eliminated many of the problems of the previous model which hindered its ready clinical acceptance.

2 Indications are similar to those of the SLO, although there is no macular facility.

3Display provides colour images of the optic nerve head and RNFL maps in the four quadrants (Fig. 10.25):

•The fundus image of the left and right eyes at the top is useful in identifying image quality.

•The thickness maps are presented in a colour-coded spectrum from blue to red. Red followed by yellow indicates a thick RNFL whereas blue followed by green shades are consistent with thin RNFL. The map has an hourglass appearance because the RNFL is thickest superiorly and inferiorly.

•The deviation maps show the location and magnitude of RNFL defects as tiny colour coded squares (pixels).

•The TSNIT (temporal-superior-nasal-inferior- temporal) graph is displayed at the bottom. It shows the actual values for that eye along with a shaded area that represents the 95% normal range for that age. The curve in a healthy eye should fall within the shaded area and has a double hump pattern because the superior and inferior fibres are thickest. The central printout shows the values for both eyes together.

•Parameters for each eye are displayed in a table (top centre). The nerve fibre indicator (NFI) at the bottom of the table indicates a global value based on the entire thickness map and is the optimal parameter for discriminating normal from glaucoma. Normal is 1–30, borderline is 31–50 and abnormal is 51–100.

424 / 1137

kanski 7th

Fig. 10.25 GDx VCCshows reduction in retinal nerve fibre density in the right eye and abnormal parameters

(Courtesy of J Salmon)

Optical coherence tomography

OCT has become a routine part of the management of macular and other retinal disease, but is also widely used for the assessment of glaucoma. The principles are discussed in detail in Chapter 14. The following imaging strategies are applicable to glaucoma:

1Peripapillary retinal nerve fibre layer. This involves the acquisition of a circular scan of diameter 3.4 mm of the retina around the optic nerve head. Retinal thickness is compared with normals. Sensitivity and specificity are around 90%.

2Optic nerve head. Radial cross-sectional scans permit an objective and repeatable assessment of disc morphology, with reasonable discriminatory value. This function has tended to be less commonly used than RNFL analysis in practice.

Anterior chamber depth measurement

Objective measurement of the depth of the anterior chamber is sometimes clinically useful in glaucoma management. Indications include monitoring of progression in conditions where the anterior chamber is shallowed such as post-trabeculectomy hypotony and cilio-lenticular block, and as a diagnostic tool, including the comparison of the two eyes. Older methods involved using a slit-lamp with or without a special attachment, but an accurate and repeatable measurement can be obtained using ultrasonographic or optical interferometric methods (e.g. ACD function on Zeiss IOLMaster). Utility and accuracy is limited in pseudophakic eyes.

Copyright © 2011 Elsevier Inc.All rights reserved. Read our Terms and Conditions of Use and our PrivacyPolicy.

If you find this useful please saythanks in your way: dramroo

425 / 1137

kanski 7th

Perimetry

Definitions

1The visual field can be represented as a three-dimensional structure akin to a hill of increasing sensitivity (Fig. 10.26A). The outer aspect extends approximately 50° superiorly, 60° nasally, 70° inferiorly and 90° temporally. Visual acuity is sharpest at the very top of the hill (i.e. the fovea) and then declines progressively towards the periphery, the nasal slope being steeper than the temporal. The ‘bottomless pit’ of the blind spot is located temporally between 10° and 20°, slightly below the horizontal.

2An isopter is a line connecting points of the same sensitivity, and on a two-dimensional isopter plot encloses an area within which a stimulus of a given strength is visible. An isopter plot of the right eye is shown in Fig. 10.26B. When the field is represented as a hill, isopters resemble the contour lines on a map.

3A scotoma is an area of reduced (‘relative’) or total (‘absolute’) loss of vision which is surrounded by a seeing area.

4Luminance is the intensity or ‘brightness’ of a light stimulus, measured in apostilbs (asb). A higher intensity stimulus has a higher asb value; this is the inverse of sensitivity.

5Logarithmic scale rather than a linear scale is used for stimulus intensity and sensitivity, so that for each log unit intensity changes by a factor of 10. With a log scale, greater significance is given to the lower end of the intensity range. The normal eye has a very large sensitivity range, and assessment of the lower end of the scale is of critical significance so that early damage can be detected. With a linear scale, the lower end would be reduced to a very small portion of a graphical chart axis. The visual system itself operates on close to a logarithmic scale, so using this method more closely matches the physiological situation.

6Decibels. Simple log units are not used in clinical perimetry, but rather ‘decibels’ (dB), where 10 dB = 1 log unit. Decibels are not true units of luminance but a representation, and vary between visual field machines. Perimetry usually concentrates on the eye's sensitivity rather than the stimulus intensity. Therefore, the decibel scale goes up as retinal sensitivity goes up, which obviously corresponds to reducing intensity of the perceived stimulus. This makes the assessment of visual fields more intuitive, as a higher number corresponds with higher retinal sensitivity. If the sensitivity of a test location is 20 dB (= 2 log units), a point with a sensitivity of 30 dB would be the more sensitive. The blind spot has a sensitivity of 0 dB. If, on a given machine, seeing a stimulus of 1000 asb gives a value of 10 dB, a stimulus of 100 asb will give 20 dB.

7Differential light sensitivity represents the degree by which the luminance of a target must exceed background luminance in order to be perceived. The visual field is therefore a three-dimensional representation of differential light sensitivity at different points.

8Threshold at a given location in the visual field is the brightness of a stimulus at which it can be detected by the subject. It is defined as ‘the luminance of a given fixed-location stimulus at which it is seen on 50% of the occasions it is presented’. In practice we usually talk about an eye's sensitivity at a given point in the field rather than the stimulus intensity. The threshold sensitivity is highest at the fovea and decreases progressively towards the periphery. After the age of 20 years the sensitivity decreases by about 1 dB per 10 years.

9Background luminance. The retinal sensitivity at any location varies depending on background luminance. Rod photoreceptors are more sensitive in dim light than cones and so due to their preponderance in the peripheral retina, at lower (‘scotopic’) light levels the peripheral retina becomes more sensitive in proportion to the central retina; the hill of vision flattens, with a central crater rather than a peak at the fovea due to the high concentration of cones, which have low sensitivity in scotopic conditions. It should be noted that it takes about 5 minutes to adapt from darkness to bright sunlight and 20–30 minutes from bright sunlight to darkness.

426 / 1137

kanski 7th

Fig. 10.26 (A) Hill of vision; (B) isopter plot

Types of perimetry

Kinetic

Kinetic perimetry is a two-dimensional assessment of the boundary of the hill of vision. It involves the presentation of a moving stimulus of known luminance or intensity from a non-seeing area to a seeing area until it is perceived (Fig. 10.27A). The stimulus is moved at a steady speed along various meridia (clock hours) and the point of perception is recorded on a chart. By joining these points along different meridians an isopter is plotted for that stimulus intensity. Using stimuli of different intensities a contour map of the visual field with several different isopters can be plotted. Kinetic perimetry can be performed by simple confrontation or by means of a perimeter such as the Goldmann.

427 / 1137

kanski 7th

Fig. 10.27 Principles of perimetry. (A) Kinetic; (B) static

Static

A method of assessing fields in which the location of a stimulus remains fixed at a certain location within the field, with the intensity increased until it is seen by the subject (or decreased until it is no longer seen). In other words, the target intensity is increased (or decreased) until threshold is reached (Fig. 10.27B). The most frequently used automated perimeter is the Humphrey Field Analyzer (HFA); others include the Henson, Dicon and Octopus. Automated static perimetry now constitutes the method used for the great majority of visual fields monitoring in the care of patients with glaucoma.

Suprathreshold

Suprathreshold perimetry involves testing with stimuli of luminance above the expected normal threshold levels for an age-matched population to assess whether these are detected; in other words, testing to check that a subject can see stimuli that would be seen by a normal person of the same age. It enables testing to be carried out rapidly to indicate whether function is grossly normal or not. However, it is not highly quantitative, and so is usually reserved for screening.

Threshold

Threshold perimetry is used for detailed assessment of the hill of vision by plotting the threshold luminance value in various locations in the visual field and comparing the results with age-matched ‘normal’ values. In Humphrey perimetry (see below), a stimulus of higher than expected intensity is presented; if seen, the intensity is decreased in 4 dB steps until it is no longer seen (‘staircasing’). The stimulus is then increased again in 2 dB steps until seen once more (Fig. 10.28). If the stimulus is not seen initially, its intensity is increased in 4 dB steps until seen, then decreased in 2 dB steps until no longer seen. Essentially, the threshold is crossed in one direction with large increments, then crossed again to ‘fine-tune’ the result with smaller increments. Threshold testing is quantitatively detailed and is therefore used for monitoring glaucomatous fields.

Fig. 10.28 Determination of threshold

Sources of error

The skill of the perimetrist in setting up the test, explaining the procedure to the patient, reassuring the patient and monitoring performance is fundamental to obtaining an accurate field. However, errors may still occur as a result of one or more of the following factors:

1Poor performance by the patient.

2Uncorrected refractive error can cause a significant decrease in central sensitivity. If a hypermetropic patient who usually wears contact lenses is tested wearing spectacles, this will have the effect of magnifying and enlarging any scotomas as compared with contact lenses. Most perimetry is performed with a stimulus at approximately reading distance, so a near correction should be used for presbyopic patients.

3Spectacle rim artefact. Spectacles can cause rim scotomas if small aperture lenses are used or if incorrectly dispensed. Some (narrow-aperture) trial frame lenses are unsuitable for perimetry.

4Miosis decreases sensitivity in the peripheral field and increases variability in the central field in both normal and glaucomatous eyes. Pupils less than 3 mm in diameter should therefore be dilated prior to perimetry; a consistent mydriatic should be used for serial tests.

5Media opacities (usually cataract) can have a profound effect, exaggerated by miosis.

6Ptosis, even if mild, can suppress the superior visual field. Similar effects result from dermatochalasis, prominent eyelashes, and deeply set eyes.

7Inadequate retinal adaptation may also lead to error if perimetry is performed soon after ophthalmoscopy.

Humphrey Field Analyzer

The Humphrey Field Analyzer (HFA) consists of a hemispherical bowl onto which a target can be projected at any location in the visual field (Fig. 10.29).

•A monitor on the side of the instrument presents a series of menus. Background luminance is set at 31.5 asb, considered to be at the lower end of the photopic illumination range.

•Variation in stimulus intensity can be achieved by altering either target size or luminance. Size is set prior to the test; 4 mm2 is used routinely, the same as the Goldmann perimeter stimulus size III.

•The other stimulus sizes available on the Humphrey correspond to the rest of the Goldmann stimulus size range (I, II, IV and V), but these are rarely utilized and usually only luminance is altered – this can be varied between 0.08 asb and 10 000 asb brighter than the background: between about 51 dB and 0 dB.

428 / 1137