y [mm]

[mm] y

In common with all imaging modalities, the quality of OCT images is dependent on the clarity of the ocular media. Current technology allows lateral scanning (with a trade-off between speed and resolution) through the retinal thickness, but clearly there are situations where a larger area of retina could

usefully be imaged. En-face OCT techniques are under development, which, combined with lateral scans, will allow three-dimensional reconstructions of large ‘blocks’ of retinal thickness. This development will allow more clinically interpretable representations of the retina in depth.

a

b

c

Fig. 17.14 (a) The appearance of macular oedema on an optical coherence tomography (OCT) scan.

(b) An OCT scan of a macular hole. (c) The OCT appearance of a pigment epithelial detachment.

Nerve fibre analyser

(GDx and GDx VCC, Carl Zeiss Meditec, Dublin, CA, USA)

The RNFL is thought to exhibit birefringence because of parallel cylindrical structures within the axons (Weinreb et al., 1990). This property forms the basis of the measurement of RNFL thickness by the GDx, and the recently modified version, the GDx VCC.

The essential imaging device is a scanning laser polarimeter using a near-infrared laser (780 nm). The scan raster covers an image field of 40º horizontally and 20º vertically. This field encompasses the ONH, peripapillary RNFL and the macula. The retina

Nerve Fibre Analyser

Box 17.1

Indications for optical coherence tomography (OCT) scanning

Retinal disease

■Preand post-macular hole surgery: OCT is a sensitive imaging technique for demonstrating the presence and grade of a macular hole, and to determine whether anatomical closure has occurred postoperatively

■Detection and monitoring of macular oedema during treatment

■Preand post-treatment features of subretinal neovascular membranes

Glaucoma

■Diagnostic OCT scan patterns for the optic nerve head and peripapillary retinal nerve fibre layer

■Glaucoma follow-up to detect evidence of progression

is illuminated with linearly polarised light that undergoes birefringence as it passes through the cornea, lens and, importantly, the nerve fibre layer. Reflected rays, polarised so that they are travelling perpendicular to the ONH fibres, travel at a different speed from those that are parallel to the fibres. The degree of polarisation in the reflected light is quantified using two analysers to measure light polarised parallel and perpendicular to the illuminating laser. The difference in velocity between these two differently polarised beams is termed retardation. It provides a measure of RNFL thickness: the greater the retardation, the thicker the nerve fibre layer.

Principle

The retardation image (Fig. 17.15), which allows estimation of the RNFL thickness, is produced by calculations involving the relative signal detected by the two analysers. A confounding factor is anteriorsegment birefringence, mainly produced by the cornea, which will obviously confuse the results. In the first machines this was compensated for in a uniform fashion but this did not work for all patients. The GDx VCC employs a variable corneal compensator to neutralise this birefringence. Essen-

Fig. 17.15 GDx image from a normal eye with a normal retinal nerve fibre layer (RNFL) thickness map. The image shows a healthy ‘double hump’ of thickest RNFL at the upper and lower peripapillary regions.

tially it measures the change in polarisation from the macular region, where there are no nerve fibres, to calculate the birefringence of the cornea. During imaging the compensator is adjusted to abolish the bright ‘bowtie’ macular reflectance, which is characteristic of anterior-segment artefact.

Interpretation

The GDx and the GDx VCC express the overall result of the retinal scan using the nerve fibre index (NFI). This is a single statistic, which is the output of the ‘support vector machine’ which uses data from hundreds of scans of normal and glaucomatous eyes. Values range from 0 (unequivocally normal) to 100 (advanced glaucoma). NFI has been shown

to be the best single parameter for distinguishing between normal and glaucomatous eyes. Research has shown that glaucoma-affected eyes rarely have an NFI of < 35, and normal eyes rarely have an NFI of > 44. Clearly, there will be a group of eyes with intermediate values between 35 and 44 which are not classified by the device. The likelihood of glaucoma in these eyes may be indicated by analysis of other output parameters, though undoubtedly, the original GDx failed to discriminate normals from glaucoma eyes in a significant proportion of cases. The more recently described GDx VCC is subject to investigation. Early indications suggest a more clinically useful imaging device (Reus and Lemij, 2004).

The original GDx device was sensitive to artefactual birefringence produced by anteriorsegment structures, especially the cornea. Severely affected GDx scans did not allow accurate discrimination between healthy and glaucomatous eyes. The GDx VCC may have overcome this difficulty, but further research is required. As with the HRT, software to look for progressive change in the RNFL is incorporated into the technology.

Indications

The original GDx is not recommended as a reliable discriminator in the early diagnosis of glaucoma. The GDx VCC may be more accurate, and may eventually have an important role in the early diagnosis of glaucoma eyes prior to the development of gross ONH damage and/or visual field defects (Fig. 17.16).

Nerve Fibre Analyser

Fig. 17.16 GDx image from a glaucomatous eye showing global reduction in peripapillary retinal nerve fibre layer (RNFL) thickness. There is relative loss of the characteristic RNFL thickness relationships at the upper and lower poles of the optic nerve head.

■Advantages of digital fundus photography include:

■Instant image review, ensuring good-quality images at the initial visit

■Better patient education, as patients can view the images as well

■Instant recall of images from databases in different locations

■The Heidelberg retinal tomograph scanner is not only useful for optic disc analysis but also has new applications in analysing macular pathology

■The optical coherence tomography scanner can be used to monitor:

■Postoperative cystoid macular oedema

■Macular holes preand postoperatively

■Optic disc anatomy

■Retinal nerve fibre layer changes in glaucoma

New Imaging Techniques

References

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Budenz D, Michael A, Chang R et al. Sensitivity and specificity of the STRATUSoct for perimetric glaucoma. Ophthalmology 2005; 112: 3–9.

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Coleman A, Sommer A, Enger C et al. Interobserver and intraobserver variability in the detection of glaucomatous progression of the optic disc.

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Index

Acanthamoeba, 130, 133 Adenovirus, 131–2 Alternate-cover test, 108–9 Altitudinal defect, 76 Amsler chart, 86–7

Angle structures, 40–3 gonioscopic findings, 41–3 iris contour, 43

Spaeth classification system, 43 Anomaloscopes, 9

Anterior-segment ultrasound biomicroscopy, 147 Applanation tonometry, 29–34

Arcuate scotomas, 76

Argyll Robertson pupil, 125, 126 A-scan ultrasound biometry, 153–4

contact, 155 immersion, 155 performing, 154 sources of error, 154–8

Astigmatism, tonometry in, 32–3 Automated perimeters, 77–84

artefacts, 83–4

assessment of progression, 84 glaucoma hemifield test, 82 grey-scale plot, 81 interpretation of results, 80–3 mean defect, 82–3

pattern deviation plot, 81–2 performing test, 79–80 short-wavelength perimetry, 84–5 test strategies, 78

threshold plot, 80 total deviation plot, 81

Axial length measurement, 153–8 performing A-scan, 154 sources of error, 154–8

Axis notation, 62

Bacterial infection, 129 examination, 130–1 Bagolini glasses, 116–17 Bailey-Lovie chart, 2–3

Bausch and Lomb wavefront analyser, 67 Bell’s phenomenon, 93

Bielschowsky head tilt test, 113–14 Binocular single vision, 118, 120 Binocular visual field, 72 Biometry, 66, 151–67

advice to patients, 151

axial length measurement, 153–8 calibration of equipment, 166 difficult, 164

IOL power and A-constant, 161–4 keratometry see Keratometry

partial coherence interferometry, 158–61 principles, 151

target refraction, 164–6 Bitemporal hemianopia, 188 Blepharoptosis (ptosis), 91–5, 96

classification, 91–2 examination, 92–4 Blind spot, missed, 83 Brown’s syndrome, 105

Cardiff cards, 4 Central island, 76 Central scotoma, 76

Central serous retinopathy, 218 Chiasmal glioma, 190 Children, visual assessment, 4–5 Chlamydia, 129, 132

Chlamydia trachomatis see Chlamydia Choroid

melanoma, 50

ultrasound examination, 144–5 City University test, 10

Cocaine, 127 Cogan’s twitch, 93

Colour Doppler imaging, 147–9 clinical applications, 149

Colour vision, 1, 5–9 anomaloscopes, 9

hue discrimination/arrangement tests, 8–9 pseudoisochromatic plates, 7–8

red desaturation, 9 Computed tomography

dacryocystography, 186 lacrimal system, 183–4 orbit, 171–7

visual pathways, 188 Confrontation field testing, 72–4

finger-counting, 72, 73 hand identification, 72, 73 light projection, 72 neurological pin, 72, 74 red desaturation, 74

Facial nerve palsy, 96 Fatigability, 93

Field defects, 75–6 altitudinal defect, 76 arcuate scotomas, 76 central island, 76 central scotoma, 76 contraction, 76

generalised depression, 76 hemifield defect, 76

nasal step, 76 split fixation, 76

Finger-counting, 72, 73 Fluorescein, 25

anaphylaxis, 220 extravasation, 218–19 intra-arterial injection, 220 properties of, 211 side-effects, 212, 220

Fluorescein angiography, 211–21 indications, 212

interpretation of angiogram, 214–16 iris angiography, 216

pitfalls and complications, 217–20 principles, 211

sequence of angiogram, 212–13 technique, 212

Fluorescein dye disappearance test, 97 Forced duction test, 111

Force generation test, 111 Four-mirror gonioscopy lenses, 38–9 Fourth-nerve palsy, 193 Frequency-doubling, 85–6 Frequency-of-seeing curve, 72

Full-field (Ganzfeld) electroretinography, 199–203 Functional acuity contrast test (FACT) chart, 10–11 Fundoscopy, 13

Fundus

digital fundus photography, 223–5 examination, 47–9, 120 retroillumination, 21

Fungal infection, 129, 130, 132–3

Ganzfeld electroretinography, 199–203 Glasgow acuity test, 4, 5

Glaucoma hemifield test, 82 Goldmann perimetry, 74–5

Goldmann single-mirror gonioscopy lens, 37–8

Index

Goldmann three-mirror lens, 37–8, 56 Goldmann tonometer, 13, 29–34

astigmatic patients, 32–3 calibration, 33–4

intraocular pressure measurement, 30–2 problems with, 32

Goldmann-Weekers dark adaptometer, 12 Gonioscopy, 37–43

angle structures, 40–3 direct, 39–40 epithelial oedema, 40 indirect, 37–9 principle of, 37

Haemangioma, 145 Hand-held keratometers, 63–4 Hand identification, 72, 73 Head posture, 104–5

Heat test, 93

Heidelberg retinal tomograph II, 226–32 Helmoholtz keratometer, 61–2 Hemianopia

bitemporal, 188 homonymous, 190–1

Hemifield defect, 76 Hering’s law, 93, 110 Herpes simplex, 132 Herpetic dendritic ulcer, 129 Hess chart, 120

Hill of vision, 71, 73

Hirschberg corneal light reflex test, 107 Holmes-Adie pupil, 124, 125

Horner’s syndrome, 91, 93, 123, 126 Hruby lens, 55

Hue discrimination/arrangement tests, 8–9 Humphrey automated perimeter, 78 Hydrocephalus, 189 Hydroxyamphetamine, 127

Hypopyon, 26

Ice test, 93, 94 Imbert-Fick law, 29 Indentation tonometry, 29 Indirect gonioscopy, 37–9

Indirect ophthalmoscopy, 51–3 hat (headband), 51–2 indentation, 52

pitfalls, 52–3

Index

Indocyanine green angiography, 216–17 Infection

acanthamoeba, 130 bacterial, 129, 130–1 chlamydia, 129 external eye, 129–30 fungal, 129, 130, 132–3 intraocular, 133–4 viral, 129, 131–2

International Commission on Illumination (CIE), 6 Internuclear ophthalmoplegia, 193–4

Intraocular infection, 133–4 Intraocular pressure measurement, 30–2 Intraocular tumours, 145–6 Intraoperative keratometry, 65–6 Iridocorneal angle, 42, 43

Irido-corneal-endothelial syndrome, 124–5 Iris

angiography, 216 coloboma, 124 contour, 43 retroillumination, 20–1

Ishihara plates, 7–8

Jarval-Schiotz keratometer, 60–1

Jerk nystagmus, 121

Jones test, 96–7

Kay pictures, 5

Keeler logMAR crowded test, 4, 5 Keratoconus, 24

Keratometry, 59–67, 152–3 applications, 66–7

axis notation, 62

computerised topography, 64–5 corneal disease, 152

hand-held keratometers, 63–4 instruments, 59–62 intraoperative, 65–6

previous refractive surgery, 152–3 recording results, 62–3

sources of error, 66 see also Biometry

Kinetic perimetry, 75 Koeppe lens, 40 Krimsky reflex tests, 115

Lacrimal system, radiology, 183–7 computed tomography, 183–4

computed tomography dacryocystography, 186 dacryocystography, 184–6

dacryoscintigraphy, 187

magnetic resonance dacryocystography, 186–7 magnetic resonance imaging, 184

plain radiographs, 183 Lambert-Eaton syndrome, 94 Landolt C test, 1

Latent nystagmus, 120–1 Levator function test, 92 Light projection, 72 Light reflex tests, 107 LogMAR charts, 2

Macroadenoma, 189 Maddox rod test, 115–16 Magna view goniolens, 37–8 Magnetic resonance imaging

dacryocystography, 186–7 lacrimal system, 184 orbit, 177–80

visual pathways, 188–95 Mainster macular contact lens, 56 Maloney keratoscope, 65 Manifest nystagmus, 120–1

Marcus Gunn jaw-winking phenomenon, 91 Marcus Gunn pupil, 125

Margin reflex distance, 92 Melanoma, 145

choroidal, 50 Meningioma, 174

Microbial examination, 129–36 bacterial infections, 130–1 external eye infections, 129–30 fungal infections, 132–3 intraocular infection, 133–4 viral infections, 131–2

Microreflux test, 96 Miosis, 126

Molluscum contagiosum, 129, 130, 133 Motion detection, 1

Multifocal electroretinography, 204 Multiplanar nystagmus, 121 Myasthenia gravis, 94

Nasal step, 76 Near response, 126

Nerve fibre analyser, 237–9 indications, 239