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| Newer imaging devices |
Stereoscopic photography, the current gold standard for assessing optic nerve change allows the eye doctor to document longitudinal changes in the optic nerve head. Colour photography with a 15 degree field gives optimal magnification. Interpretation may be subjective .Reputed clinical trials and studies have used qualitative evalution of stereoscopic optic disc photographs as an outcome measure indicating an acceptance of stereoscopic optic disc photography as a valid tool for detection and monitoring of glaucoma .
The last few years have seen the emergence of a host of newer instruments - optic nerve imagers and retinal fiber layer analyzers, capable of providing objective and reproducible information about the optic disc and retinal nerve fibre. This information provided is usually beyond what's obtainable from a clinical examination and is useful for diagnosing glaucoma and also to assess deterioration over time
Scanning Laser Polarimetry( SLP ), GDX VCC provides for a quantitative assessment of the nerve fibre layer by measuring the rotation of polarized light reflected from the retina. It is assumed that the rotation is proportional to the thickness of the nerve fibre layer and the main birefringence tissue is the retina and its birefringence is homogeneous. Birefringence from the cornea and the lens are additional confounding factors and require compensation. Recently the fixed anterior segment compensator has been replaced by a custom anterior segment birefringence compensation ( ASBC ) .
Optical Coherence Tomograph (OCT ) provides for high resolution cross sectional imaging of the human retina and nerve fiber layer. The nerve fiber layer measurements are automated and displayed by quadrants, clock hour and an overall mean. OCT assessment of peripapillary retinal nerve fiber layer thickness has been reported to differentiate normal from glaucomatous eyes. With changing technology within the OCT , available longitudinal data at present, is insufficient to make conclusions about the ability of the OCT to detect change over time. There is also no evidence at present to suggest that the OCT can be used for the diagnosis of glaucoma .
Confocal Scanning Laser Ophthalmoscopy: The Heidelberg Retinal Tomograph (HRT), is a scanning laser ophthalmoscope utilizing confocal scanning diode technology to provide topographic measurements of the optic disc and peripapillary retina. In a confocal system a laser beam is scanned across the retina and reflected back to a detector through a system of two conjugate pinholes, one in front of the laser source and the other in front of the detector. Through the use of algorithms that account for eyes movements, each scanned image in the series can be aligned .The HRT II uses a fixed 150 field of view with 384 x 384 pixels per image plane and is more friendly clinically with automated fine focus and quality control checks to ensure image quality.
HRT II is most helpful in evaluating the optic nerve for change, for evaluating glaucomatous progression. Measurements of optic disc stereometric parameters by HRT are highly reproducible. Research suggests that there may be a subset of patients with ocular hypertension in whom sequential follow-up with HRT can reveal optic nerve head changes which predate development of glaucomatous field changes . Advantages of the HRT II include rapid image acquisition time, lack of need for pupillary dilation and images can be obtained through contact lenses or refractive errors can be compensated for prior to scanning.
Once the scan is obtained, a contour line must be drawn around the disc , which is most easily accomplished utilizing the black and white image or three-dimensional image of the nerve on the monitor. The recommended location to draw the contour line is on the inner edge of Elschnig's scleral ring .
The following are displayed on the initial HRT II printed report ( figure ).
A topography image, where the cup is displayed in red and the rim in blue and green, representing sloping and stable neuroretinal rim, respectively.
Reflection image of the optic nerve is divided into 6 sectors. The rim (green and blue) and the disc area (red, green and blue) for each sector are compared to a normal database and classified by Moorfield's Regression Analysis as within normal limits (signified by a green ? mark), borderline (yellow ! point), or outside normal limits (red x).
A graft with red and green vertical bars represents the results of Moorfields regression analysis. Each whole column represents the total optic nerve head area for a specific sector and is divided into the percentage of rim area (green) and cup area (red). Four black lines across the red / green graph reflect the percentage of optic nerve heads (ONH) in the normal database that have a rim area larger than the limit delineated by the line. The "predicted" line indicates that 50% of the ONH in the normal database have a larger rim area than this limit. From upper to lower, respectively, the lower lines indicate that 95.0/99.0/99.9% of the ONH in the database have a larger rim area than these limits. If the percentage of the patient's rim area is ? the 95% limit, the respective sector will be classified with a green ? (within normal limits), between 95% and 99% limits with a yellow ! (borderline), and with red x (outside normal limits) if lower than the 99% limit .
A table with the stereometric analysis of the ONH. This table provides absolute quantification of the patient's optic nerve head parameters and is not a comparison to a database. The most important parametrers here are the rim area, rim volume, cup shape measurement, height variation contour, and mean RNFL thickness, of which cup shape measure appears to be the more predictive characteristic - the more negative the better. The cup shape summarizes the distribution of depth within the cup and is independent of the reference plane. The topography standard deviation number serves as an image, quality control.
Mean height contour graph, where the height difference between the red reference line and the green height profile line corresponds to the thickness of the RNFL along the contour line drawn in the reflectance image. The profile line always starts temporal at 00 and is drawn clockwise for the right eye and counterclockwise for the left. The subadjacent red reference line indicates the location of the reference plane (separation between cup and neuroretinal rim); it is approximately at the base of the nerve fibre layer
Horizontal and vertical height profiles- provide information about the shape, slope, and the depth of the cup and its walls. Walls that tend to be steep or deep are suspicious.
A FOLLOW-UP REPORT ( figure ) can be generated beginning with the second follow-up examination (third actual scan; the first and second scans are used to determine a baseline). The viewer will diplay a row of topographic and reflectance images of the first and subsequent examinations. The quality of all the examinations being compared should be of similar quality (assessed via the standard deviation value). For objective comparability. All follow-up examinations are matched to the initial one, correcting for possible shift, tilt, rotation, or magnification differences. These displacements can be cause black edges to appear around the images of the follow-up examination. With the HRT II, the contour line drawn in the first examination will automatically be placed in the same location on subsequent tests to allow for proper serial analysis. On the printed follow-up report, the image is always in black and white, with significant changes in the ONH displayed in red (decreased height) or green (increased height) overlays. A stereometric analysis table will show any changes in the values .
The HRT is a patient and technician friendly technique with excellent reproducibility ( HRT II ) allowing it to better detect topographical changes over time . It is also the automated technique with the longest track record and largest number of publications .
Conclusion : Role of Newer Imaging Devices
1) Role in diagnosis of glaucoma : The parameters of IOP and visual field assessment can miss the diagnosis of glaucoma, especially in the early stages. In fact upto 40% of the ganglion cells must be lost for a detectable loss on automated perimetry. Because optic nerve damage is irreversible early detection is crucial. The objective of imaging of the optic nerve head and retinal nerve fibre layer is to precisely quantify structural measurements. A comparison with normals allows early detection of glaucoma.
A word of caution: Categorizing patients to have glaucoma or no glaucoma by measurements obtained from imaging devices is not the same as diagnosis. The diagnosis of glaucoma requires the integration of available information obtained by clinical examination of the optic nerve and retinal nerve fibre layer,visual field status , gonioscopy and possible risk factors .
At this point of time The Association of International Glaucoma Societies (AIGS) does not acknowledge the role of any newer imaging modalities in the diagnosis of glaucoma,.
2) Role in assessing deterioration in glaucoma : Comparison of these optic nerve and retinal nerve fibre layer changes over time helps to assess possible deterioration. Currently the confocal scanning laser ophthalmoscope Heidelberg Retinal Tomograph (HRT II ), is most sensitive to assess progressive changes from glaucoma in the optic nerve head. It has been shown in peer review literature that the HRT II can detect possible worsening in 75% of glaucomatous population a year and a half before conventional tests can pick up the same. This makes it a very useful technology in glaucoma.
Based on published peer review literature, The Association of International Glaucoma Societies (AIGS) has accepted the role of the HRT II to assess deterioration in glaucoma over all other newer imaging technologies.
Excerpts from the lecture "Role Of Newer Imaging Devices In Glaucoma"by Dr. Devindra Sood at the Annual Meeting of the Vidarbha Ophthalmological Society at Nagpur, Maharashtra, September 2008 |
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