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Progress tracking

You may hear your doctor talk about Visual Acuity (VA) and Central Retinal Thickness (CRT) at your appointments. Here's what they mean for your progress.

1

What is Visual Acuity (VA)?

Visual acuity (VA) refers to the sharpness of your vision and is measured by your ability to identify letters or numbers on a standardized eye chart from a specific viewing distance, usually as part of a visual acuity test. It is typically measured as one of the several tests performed to diagnose DME as well as to help determine whether you need treatment. VA measurements can use a number of different systems (e.g., Snellen, ETDRS, LogMar) and notations (e.g., fractions, decimals). Your doctor will choose an appropriate system and notation for recording these scores for you. If you have any questions about the meaning of your VA measurements, you can always ask your doctor.

2

What is a good Visual Acuity (VA) score?

Visual acuity (VA) scores vary from person to person for a number of reasons – patients start treatment from different VA starting positions, "normal" VA tends to vary from person to person, and some fluctuation in VA is normal. Typically, if your VA scores are improving from appointment to appointment, this is an indication you are making progress and your eyesight is improving.

Interpreting your VA scores can be difficult, as there are several different systems and notations used to represent them. Here is a brief guide to the main ones you may encounter.

Snellen:

This is one of the most commonly used systems. Snellen measurements are often expressed as fractions, e.g., 20/20 (in feet) or 6/6 (in metres). The easiest way to think about this is that the number at the top of the fraction (the numerator) represents the distance at which you (the subject) can just read the letter in question, and the number at the bottom of the fraction (the denominator) represents the distance at which a "normal" person can just read the same letter. A score of 20/20 is typically considered “normal”. Essentially, a person with a 20/20 score can see better than someone with a score of 20/100, as there is no difference between what they see and what a "normal" person sees. Snellen is also sometimes expressed as a decimal e.g., 1.0 or 0.2. The higher the decimal score, the better the VA.

ETDRS:

This is another commonly used system, and is named after a well-known eye study (The Early Treatment Diabetic Retinopathy Study). Unlike Snellen, ETDRS uses the number of letters that can be read on a standardized eye chart to measure how well a patient can see. You start at the top of the chart, working your way down until you reach a row where a minimum of three letters cannot be read. You are then scored on the basis of how many letters could be correctly identified. So, someone with a score of 85 can see better than someone with a score of 50. The higher the score, the better the VA.

LogMar:

This tends to be used less commonly than Snellen and ETDRS in the clinic, but still relatively commonly. LogMar, like Snellen, uses the idea of angular measurement, which involves measuring the distance at which letters on a chart can be seen, to measure how well a patient can see. It is expressed as a decimal number e.g., 0.0 or 0.7. So, someone with a score of 0.0 can see better than someone with a score of 0.7.

If you have any questions about the meaning of your VA scores, you can always ask your doctor.

3

What is Central Retinal Thickness (CRT)?

Central Retinal Thickness (CRT) is a measure of the swelling on the macula, the central part of the retina responsible for sharp vision. This swelling is caused by leakage from blood vessels weakened by diabetes and its complications, including DME.

It is typically measured using an Optical Coherence Tomography (OCT) scanner that scans the retina and produces visualizations of different kinds that are used by a doctor or healthcare professional to measure the thickness of the macula and evaluate disease progression, particularly after treatment.

If you have any questions about the meaning of your CRT scores, you can always ask your doctor.

4

What is a good Central Retinal Thickness (CRT) score?

Central Retinal Thickness (CRT) is measured in micrometres (one millionth of a metre). Since it is a measure of the amount of swelling on the macula, and the goal of DME treatment is to reduce this swelling, the lower the CRT measurement, the better for your DME. So, someone with a CRT score of 250 micrometers typically has DME that is less advanced than someone with a score of 450 micro meters.

As with Visual Acuity (VA), it is worth remembering that "normal" CRT scores can differ widely from person to person. This means that a good CRT score for one patient will not necessarily be a good score for another.

If you have any questions about the meaning of your CRT scores, you can always ask your doctor.

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An easy-to-understand guide to the medical terms