Objective Methods Of Measuring The Rnfl Biology Essay

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Automated perimetry inc perimetry stimuli, psychophysical methods of detection, probability summation and riccos law and related findings

The dimensions of the optic nerve head are diagnostic in glaucoma due to the excavation of the cup due to loss of axons and rearrangement of neural tissue. The signs and characteristics of glaucoma at the ONH are summarized by Fingeret et al (2005). One third of the neural retinal rim is made from support cells such as glial cells and blood vessels and the remaining tissue is RGC'S and their axons. This affects the measurements taken in OCT where the thickness of the nerve fibre layer is estimated (Hood et al. 2006, 2009)

There are several (new) of taking measurements of the retinal nerve fibre layer and these include OCT and SCLP scanning confocal laser polarimetry.

There are several ways to take objective measurements of the retinal nerve fibre layer and to assess the visual field loss as a result of glaucoma. Optical Coherence Tomography (OCT) is commonly used to assess changes in the retinal layers including the RNFL and the macula, for example cystoid macula oedema or AMD. Infrared light is reflected back from the retina and its speed compared to a reference signal; each layer can be determined by the time it takes for the reflecting light to return, the accuracy can be up (x micrometres). Structures such as the RNFL and the choriocapillaris are highly reflective (Hee et al. 1995) and this allows the RNFL to be visualised and used in glaucoma diagnosis and follow-up. Newer models of the OCT use spatial domain methods which are more accurate than the original time domain technology due to faster image acquisition and resolution (Sakata et al. 2009). OCT divides the optic nerve head into (4 or 6) sectors and can calculate measurements such as cup-disc ratio, area, neuroretinal thickness and make comparisons OCT measurements of the RNFL are comparable to those by CSLO and SLP (Greany et al. 2002, Hoh et al.2000) but they are all strongest when used in conjunction. OCT c

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Scanning laser polarimetry is a technique which measures the thickness of the retinal nerve fibre layer by utilising its bifringence property whereby a split polarized light beam is reflected back at a different speed from the back and front of the RNFL (De Pozzo et al 2009). It is often used as a tool in cases of suspected glaucoma, for example ocular hypertension or suspicious optic nerve appearances rather than after SAP defects where a reduction in nerve fibres has already occurred by definition. SLP allows a visual representation of the NFL at each of the quadrants around the optic nerve to (extent - source) and compares to age related norms. It correlates with analyzing red-free photographs for nerve fibre defects (Woolstein et al. 2000) and calculates/stores measurements for future comparison, however it is not always diagnostically correct (Meideiros et al. 2004) therefore it is used in conjunction with other tests not alone.

(((((((((((Limits of PPA , macula, refractive error and tilted discs. An corneal compensation)))))))))))))))))))

Previously, before new technology allowed the visualisation and measurement of the retinal layers, the optic nerve head photograph taken with a fundus camera or by observation using a slit lamp and (condensing?) lens, was used for glaucoma diagnosis. This is still a valuable method however studies by ( Varma et al 1989) and ( ) showed differences in observer estimates. Varma et al. showed greater observer variability in the extremes of small and large cup to disc ratios. (need to read two journals from Bute for two more views on reliability). Clearly, it is beneficial to have newer, objective ways to assess the optic nerve head, particularly for accuracy in research purposes. However, DeLeón-Ortega et al (2006) found that subjective evaluation of the ONH was more accurate than each of the objective methods, so a combination of both will probably elicit the best results, though, the latter did admit that the training and experience of the observers in the study may have had an effect on the outcome. Greaney et al. (2002) also found that experienced observers of stereo photographs were just as accurate when identifying glaucomatous features as new methods such as SLP and OCT. Although the human skill will be redundant in the near future it is being replaced by astounding new technology which can measure every detail of the neural retinal rim and optic nerve head within micron resolution as well as being objective and repeatable.

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Objective methods of measuring the RNFL are one thing, but assessing the visual field loss always requires a subjective response from the patient. Static perimetry is a psychophysical method of testing whereby a small light of varying intensity is presented to one area of the visual field and the patient needs to respond whether they have seen it or not. If it is not seen then the intensity is increased in finite steps and the results will show if there are any relative or absolute defects in the visual field comparing to age related norms. The Goldman perimeter uses kinetic perimetry where the target is brought in from a'non-seeing' to 'seeing' area of the field but has been surpassed by automated perimeters due to their sensitivity to visual field loss, programming ability and objectivity (Trope and Britton 1987). The latter are also objective perimeters whereas Goldmann perimetry requires a skilled perimetrist to perform it. Despite being replaced by perimeters such as the Humphrey Field Analyzer and Frequency Doubling Technology perimeters on the high street, Goldmann Perimetry still has its uses in hospitals and is particularly good for plotting large visual field defects, neurological defects and for patients who would find SAP difficult due to disorders affecting fixation or co-ordianation (Szatmary et al 2002). Of course, it also allows for the whole of the visual field to be plotted as it extends to 90 degrees, which is incomparable as the closest is a 60 degree plot using the HFA but the time is roughly 30 minutes which would be intolerable (Szatmary et al.2002). However, for early, small (and medium) visual field defects seen in early stage glaucoma, standard automated perimetry is sensitive and is the most common diagnostic field test, particularly the HFA SITA 24-2 programme which analyzes 54 points in the central 30 degree field or the 30-2 programme which is slightly longer and requires more from the patient. Other programmes are also available which have variations in the decibel changes or test a different size field. SAP will find the patients threshold by using the staircase method of detection and provide a greyscale field plot, decibel values for each visual point tested compared to age related norms, as well as indices to assist the analysis of the results. Glaucoma is a progressive neuropathy so successive field plots are compared to follow either the development of visual field loss or effect of treatment. To help in this, the HFA can have software programs such as Linear Regression analysis or Glaucoma Change Probability Analysis which can measure the probability of concurrent visual field tests showing change or not, as field plots become invariably unreliable as defects increase in size, however the results from each algorithm do not always concur and may underestimate visual field loss (Nouri-Mahdavi et al. 2007)

There are two other types of perimeters used to assess glaucomatous damage, these are Frequency Doubling Technology (FDT), as previously mentioned, and Short Wavelength Automated Perimetry. Both are designed to assess the sensitivity of retinal cells which are less abundant than cells sensitive to white-on-white perimetry and therefore may be selective for glaucoma at an earlier stage. For example, FDT has a stimulus of low spatial frequency set at a high temporal frequency so that it appears doubled, and is a stimulus to the magnocellular pathway which is primarily composed of parasol ganglion cells.

SWAP is the only chromatic visual field test, it utilises the scarcity of the small bistratified ganglion cells by adapting the retina to a yellow background stimulus and using blue stimuli. Both FDT and SWAP have been shown to be more sensitive to early visual field loss by as much as two years ( ); the SWAP program is available on the HFA so it may be used more in clinical practice for early glaucoma detection.

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