OCT in Glaucoma

1 June 2007 Rajiv Sukumaran et al. - Cyclosporine183 OCT in GlaucomaDr. Valsa Stephen MS DO DNB, Dr. Arup Chakrabarti MS DO, Dr. Sonia Rani John DNB,Dr. Meena Chakrabarti MS DO DNBG laucoma is the clinical manifestation of irreversibledamage to the retinal ganglion cells resulting in retinalnerve fibre layer loss. This becomes functionally evidentas visual field loss which is detected on perimetry. It isnow known that structural damage detected by RNFL defects precedes functional loss. 40 % axonal loss hasto occur before any detectable change occurs in visualfunction.

2 1 Another study 2 showed that RNFL loss wasdetected nearly six years before visual field defectsoccurred in ocular hypertensives. The recognition ofRNFL loss in patients with normal visual fields has ledto the concept of Pre-perimetric Glaucoma , signifyingearly glaucomatous damage not evident on standardautomated perimetry. RNFL loss also precedes optic discchanges which occur before visual field changes. It hasalso been shown that early treatment is important inlowering the patient s risk of Glaucoma progression. 3 However, clinical evaluation and photography of theRNFL is a difficult technique, is subjective, qualitativeand variably reproducible.

3 With the advent of OpticalCoherence Tomography (OCT) or scanning laserpolarimetry (GDx), highly reproducible RNFL assessment and quantification is possible, thus enablingimproved management of of OCT:OCT is a non-invasive imaging technology thatuses light to create high resolution (<10 ),cross-sectional images in an acquisition time of of OCT for Glaucoma :1. OCT measurement were found to be highlyreproducible 4 especially in a circle diameter mm and with internal Ability to distinguish normal from glaucomatouseyes is greater as established by several studies 5,6, Effect of age : OCT normative values have beendeveloped to adjust for the RNFL thinning with SCANS FOR DETECTING GLAUCOMAThe OCT enables scanning three regions for glaucomadetection-1.

4 The Peripapillary retinal nerve fiber layer (RNFL)2. The macular The optic nerve RNFL MeasurementRNFL measurements with OCT have sensitivity andspecificity in differentiating glaucomatous from normalsubjects. 8,9 Of these, RNFL thickness in the inferiorregion and mean RNFL thickness are the best to detectearly to moderate Macular ThicknessThe mean macular thickness of glaucomatous eyes hasbeen shown to be significantly lower than that of normalcontrol eyes. 10 A significant correlation between OCTmacular thickness and visual field mean defect inglaucomatous eyes has been demonstrated.

5 11 Howeverthis is inferior to RNFL Eye Care Centre, Kochulloor, Trivandrum 695 011 Address for correspondence: Dr. Valsa Stephen, Chakrabarti Eye Care Centre,Kochulloor, Trivandrum 695 011 E-mail: Journal of OphthalmologyVol. XIX, No. 23. Optic Nerve Head (ONH) AnalysisThe utility of this needs to be further evaluated asvariation may occur, for example disc marginevaluation may be influenced by changes in theRPE- choriocapillaris , a study has shown the rim area and horizontalintegrated rim width as the best ONH parameters, 8while another has shown the cup disc area ratio as thebest ONH discriminator.

6 12 IMAGE ACQUISTIONT here are 2 basic OCT scan patterns- lines and Scan placement for RNFL protocols : These are circlescans centred in the middle of the optic Optic Nerve head Protocols : These are line scansarranged like spokes of a wheel the center of whichshould be in the middle of the optic SCAN PROTOCOLSThe OCT scan protocols (Fig. 1) described for glaucomadetection and management include the following:c. Fast RNFL thickness ( ) : acquires mm diameter scans in seconds andcompresses them into one RNFL Map : consists of a set of six concentriccircle scans at increasing distances from thedisc ONH Protocols(a) Optic Disc (b) Fast optic disc : Compressessix optic disc scans into one scan in Macular Thickness Protocolsa.

7 Macular Thickness Mapb. Fast Macular Thickness Map : Compresses sixmacular thickness scans into one scan in 2. RNFL AnalysisFig 1. Glaucoma scan protocolsFig. 3. ONH AnalysisI. RNFL Protocolsa. RNFL thickness ( ): Consists of 3 circle scansof mm diameter around the optic discwhich are RNFL thickness ( x disc ) : a single circularscan around the optic disc that is timesthe radius of the aiming 2007 Valsa Stephen et al. - OCT in Glaucoma185 ANALYSIS PROTOCOLS FOR Glaucoma (Fig. 2 & 3)1. RNFL Thickness (Single Eye)The normal RNFL graphs appear as a double hump due to increased RNFL thickness at the superiorand inferior poles of the disc.

8 The output chart includescircle characteristics like quadrant and clock hour RNFL thickness averages. The RNFL is depicted in hot a red band superficial to the green retina (Fig. 4)Fig. 5. RNFL thickness average showing normal thickness ofRNFL in both eyesFig. 4. RNFL thickness report100%95 %5 %1 %0 %Of the normal population (Fig 6): 5 % fall within the white band: 5 to 95% fall within the green band: 1 to 5 % fall within the yellow band.: 1 % fall within the red band (Outside normal limits)2. RNFL Thickness Average (OU)Two maps, one showing RNFL thickness using a colourcode (A) and the other showing average RNFL thicknessin microns (B) are created (Fig 5).

9 The black graph represents the RNFL thickness of theeye being tested in the nasal, superior, temporal andinferior quadrants. The OCT has incorporated normativeage- matched RNFL thickness data. The softwaredisplays graphs which are colour coded according tothe probability of the RNFL thickness measured inthe particular patient being normal when comparedto age- matched output chart also provides RNFL thickness values inclock hours and quadrants using the same colour code. (B).Summary parameters which include average thicknessin each quadrant, maximum RNFL thickness in superiorand inferior quadrants and ratios of RNFL thickness invarious quadrants ( I max/S max, S max/I max; S max/T avg; I max/ T avg) are also provided (C).

10 Usually the(A)(B)(C)Fig. 6. Graph showing probability of distribution of RNFL thickness in the normal population186 Kerala Journal of OphthalmologyVol. XIX, No. 2inferior RNFL is the thickest and the I max/ S max ratiois greater than In Glaucoma , the ratio may be lessthan due to inferior RNFL RNFL Thickness Serial Analysis (OU)This allows comparison of RNFL thickness over timefor up to 4 visits which are superimposed on the samechart (A) and each visit is colour coded (B) (Fig. 7).RNFL thickness change analysis shows the differencein RNFL thickness in two visits (Fig.)