RNFL analysis by OCT... onto the next level for glaucoma diagnosis - Ophthalmology Times Europe

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RNFL analysis by OCT... onto the next level for glaucoma diagnosis
Meeting the need for more sensitive diagnostic tests


Ophthalmology Times Europe



The early diagnosis of glaucoma and the early detection of glaucomatous progression are twin central challenges facing ophthalmologists. Since glaucomatous damage is irreversible, prevention of this injury before it occurs is the essential strategy available to those treating this disease.

Standard Automated Perimetry, (SAP) unarguably the gold standard to evaluate glaucomatous neuropathy and to monitor disease progression has poor sensitivity for detecting glaucoma. Quigley and co workers1 showed that significant axonal loss may precede the development of visual field defect and identifiable cupping. Since the test requires subjective input, it is therefore prone to short and long term fluctuation. Subsequently, there is clearly a compelling need for more sensitive glaucoma diagnostic tests.

Available technologies


Figure 1: After seven months.
Work during the past two decades has resulted in the development and implementation of several imaging technologies designed to detect glaucomatous neuropathy at early stages of the disease. The Optical Coherence Tomography (OCT) is a powerful tool for assessing the retinal nerve fibre layer thickness in glaucomatous eyes. It has the ability to visualize tissue cross-sections both qualitatively and quantitatively, to discriminate between healthy and diseased tissue and to evaluate disease progression.


Figure 2: HVF was within normal limits.
A healthy retina is only a quarter of a millimetre thick, but it contains multiple layers of specialized transparent cells. One layer converts light into nerve signals, another layer processes the nerve impulses, while another layer transmits these organised impulses to the brain where they are interpreted. The Stratus OCT (Carl Zeiss Meditec) provides diagnostic visualization, measurement, documentation and comparison of the thicknesses of these layers.


Figure 3: After nine months.
It could be useful in detecting pre-perimetric glaucoma, thereby allowing earlier commencement of treatment and preventing visual loss. In addition, monitoring of glaucoma progression with OCT over an extended period of time could arrest continued injury to the optic nerve.


Figure 4: After 27 months.
Circular OCT tomograms are acquired around the Optic disc at 2.3 mm and 3.4 mm diameters. Cylindrical sections are displayed unwrapped and correspond to a clockwise scan around the disc. In our two studies we used the 3.4 mm Fast RNFL scan.

•The first was a two year study, conducted where RNFL assessment in glaucoma suspects was correlated to Short Wave Automated Perimetry changes.

•The second was over five years and was conducted with 532 eyes where RNFL assessment was correlated to Standard Automated Perimetry changes.

The two-year study, which started with 132 eyes of glaucoma suspects converted to SWAP changes (p<0.01) in an average lead time of 7.6 months with a positive predictive value of 72%.

The five-year study began with 232 eyes of glaucoma suspects which went on to 532 eyes in five years and a statistically significant number (p<0.01) converted to Standard Automated Perimetry changes in an average lead time of 36.2 months with a positive predictive value of 76%

In addition, the locations of the RNFL changes corresponded closely with the regions of the visual field where the defects occurred.

Patient selection

We identified the patients in the study as glaucoma suspects on the basis of having an IOP of 22.0 mmHg or above, after correction for corneal thickness with ultrasound pachymetry, and having asymmetrical cupping of optic disc, or a cup/disc ratio of more than 0.6 in one eye. In addition, standard automated perimetry (SAP) with the Humphrey Visual Field analyser using SITA (Swedish interactive threshold algorithm) analysis showed all eyes as having visual fields within normal limits at the beginning of the study.

Patient examinations

Throughout a study period of 54 months, all patients underwent three-monthly examinations with SAP and peripapillary 3.4 mm circular scans by OCT. Abnormal SAP was defined as four points depressed with p value of <0.05 or a cluster of three points depressed with p value of < 0.01. RNFL thickness change was defined as one or more quadrant becoming abnormal or more than one clock hour becoming abnormal. The peripapillary OCT thickness data clearly reflected a statistically significant correlation between RNFL loss and abnormal SAP.

There was generally a good correlation with topographical changes in the RNFL and glaucomatous visual field defects measured by SAP. The OCT sectors 6, 7 and 8 o'clock correlated with SAP pattern deviations in the corresponding visual field zones, 13, 14 and 18.

Ironically, while changes in the inferior quadrant of the RNFL were the most sensitive in predicting visual field loss, changes in the corresponding superior hemifield detected with SAP were the least common indicator of glaucoma onset.

Findings support previous research

Our study's findings support those of several previous studies. For example, in a study by Cesar A Sanchez-Galeana et al,2 RNFL loss detected by OCT in the inferior and inferior temporal sectors corresponded with defects in superior hemifield, central and arcuate areas detected by short wave automated perimetry.

In another study,3 linear regression analysis showed that deviation from normal RNFL thickness detected by OCT in the inferior and inferior temporal quadrants of the optic nerve correlated most strongly with SAP pattern deviations in the corresponding superior arcuate and nasal step zones of the visual field.

In K. Nouri-Mahdavi et al.,4 OCT had a positive predictive value for glaucoma onset similar to our study, that is, 71% compared to 76%.

We therefore concluded that RNFL analysis by the Stratus OCT 3 gives the earliest structural evidence of Primary Open Angle Glaucoma.

References

1. H. A. Quigley et al., Archives of Ophthalmology 2006;124(6):853-9.

2. C.A. Sanchez-Galeana et al., Ophthalmology, 2004; 110 (10):1866-1872.

3. T.A. El Beltagi et al, Ophthalmology, 2003 110(11): 2185-91.

4. K. Nouri-Mahdavi et al., Am J Ophthalmol, 2004;137(2):228-35.

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