Clinical method of calculating optic nerve head vertical disc height

Abstract

There is large variability between individuals on many measurable parameters of the optic disc. The retinal fundus is most commonly measured indirectly via slit-lamp biomicroscopic examination with a fundus spherical lens. Three optical methods of measuring the vertical disc height were compared: – Optical Coherence Tomographic (OCT) method – Calculation using a modified Littman’s equation: t = pqs – Measurement using five different double aspheric Volk fundus lenses on two different slit lamps (Haag-Streit and CSO) The aims of the study were: – To calculate a conversion equation of optic disc height measurement by clinical assessment using any one of five double aspheric fundus lenses at the slit-lamp – To compare the control mechanism of optic disc height measurement on OCT versus calculation using a modified Littman’s equation – To compare measurements between the Haag-Streit BM900 and CSO SL990 slit lamps – To compare accuracy and agreement of Volk conversion factors based on manufacturer magnification values to the control mechanism DESIGN & METHOD: A cross-sectional study was performed on patients presenting to the St John Eye Hospital from August ’11 to July ‘12. 100 eyes were measured for: – The optic nerve head vertical disc height on OCT – Vertical disc height (VDH) on Haag-Streit and CSO slit lamps with Volk super 66D, 78D, Superfield, Digital Wide Field, Digital High Magnification lenses – Axial length – Refraction Subsequently, 44 new eyes were measured and used to test agreement of the formulated equations. RESULTS: – Axial length was not found to have a correlation with VDH (p=0.64). – The modified Littman’s equation: t=pqs where p varies for each lens and q = 0.01306(axial length – 1.82), had very low correlation and poor agreement – Comparing formulated equations to the OCT VDH, the formulated Digital High Magnification and super 66D lenses were found to have the most accuracy – Measurements with the Volk Digital Wide Field lens were highly inaccurate and are therefore not recommended for vertical disc height measurement in future – Comparison of the fundus lens measurements using the two slit lamps showed alarmingly different results. There was no agreement between slit lamp measurements using four of the lenses CONCLUSIONS: – The use of axial length and other quantitative factors attained by measures of a schematic eye cannot be used accurately in clinical practice to determine fundus object sizes. – The slit lamp technique of measuring optic disc size with application of a simple equation yields useful results in close agreement to those yielded by expensive technology. It cannot replace accurate analysis using an OCT but is sufficient in aiding clinical decisions – Two important factors need to be taken into account when applying slit lamp fundus measurements: the type of lens used, and the slit lamp being used – as measurement standards are inconsistent between slip lamps