[Info] Logarithm in Ophthalmology

Logarithm in Opthalmology

This guide serves as more of a comparison between the different methods used in Ophthalmology, the main focus being logarithm.

Introduction
Logarithms have many hidden uses in society, and amongst these uses are its use in ophthalmology, where it is used to calculate the visual acuity of my patients. Everyday, I use the Bailey-Lovie logMAR chart. LogMAR is now the preferred visual acuity test in many eye clinics; it is effectively replacing the Snellen chart. We have come a long way since 1623 when visual acuity assessment required people to resolve two mustard seeds at a fixed distance!

Description of application: the logMAR score
LogMAR is the logarithm (to base 10) of the width (in minutes of arc) of the limbs of a letter. The LogMAR test use a special font, in which all the lines are of equal thickness. Unlike, for example, Times New Roman where the vertical lines are thicker than horizontal lines.



The conversion chart for the logMAR value




Why is logMAR (Logarithmic scale) better than Snellen (Ordinal Scale)?

Our eyes see objects in a logarithmic (geometrical) manner. Snellen chart is non-geometrical and hence, it does not give an accurate reflection of how our eyes actually work. Moreover, logMAR is superior with the following:

1. There is same number of letters on each line – the task is equivalent for reading each line.
2. It provides more letters (5 per line) for children with poorer visual acuities – with Snellen they get only one stab at the largest or top letter.
3. The letter size increases in equal steps of 0.1 logMAR per line i.e there is a geometric progression of letter size.
4. Each letter can be scored individually. This means that a child who can read all the letters on one line and half the letters on the next line, he can get a score between the two letter sizes. This is helpful to track the progression of eyesight.
5. The assessment can be carried out at any distance.
6. The scores are useful for research, as a logMAR score is a “real number” that can be used in calculations and statistics.

I hope this guide helps to stir up some interested in Ophthalmology!

 

http://www.cnetfrance.fr/telecharger/visual-acuity-39543163s.htm