H WHAT ARE THE PRACTICAL ASPECTS OF SCREENING FOR DIABETIC RETINOPATHY?
1 Visual Acuity Measurement and Pupillary Dilation
VISUAL ACUITY
The measurement of visual acuity gives one measure of visual performance or the ability to see.
When an object is perceived on the retina it has to be focused by the cornea and the lens. The retina receives the picture formed by these light rays and sends the image to the brain through the optic nerve. If the light rays are not focused accurately onto the retina then the image will be blurred. There are four common refractive problems:
- Myopia or short-sighted
- Hypermetropia or long (hyperopia)
- Presbyopia
- Astigmatism
In myopia the image is either focused too far forward or the eye is too long. In hypermetropia the opposite problem occurs. In astigmatism the curve of the cornea is asymmetrical resulting in distortion of the light rays.
Normally when we look at objects close up such as when reading the muscles suspending the lens relax. In a young eye (under the age of 41 years) the lens shortens and widens inducing relative myopia.
As we get older the lens is no longer able to relax and presbyopia occurs resulting in things having to be held further away to be in focus.
Glasses and contact lenses correct refractive errors by adding or subtracting focusing power to your cornea and lens. The power needed to focus images directly on your retina is measured in diopters. This measurement is also known as your eyeglass prescription. When measuring visual acuity it is essential that the patient uses the appropriate distance or reading glasses. If the patient cannot read the bottom of the chart with their glasses a pinhole should be used. This can correct for about 4 diopters of refractive error by allowing only central rays of light to enter the eye.
The Snellen Chart
The commonest way of measuring visual acuity in clinical practice is the Snellen chart
The main problem with the Snellen chart is the fact that the progression of letter sizes does not represent equal steps in terms of difficulty. This is particularly true for the poorer visual acuities where the difference in performance between 6/60 and 6/36 is huge compared to the difference between 6/9 and 6/6. For this reason trials such as the ETDRS and the UKPDS have used the logMAR (” ETDRS”) chart.
If a patient can only can read half of the 6/9 line then the vision would be recorded either as:
6/9 – number of letters missed or
6/12 + number of letters seen on 6/9 line
In some countries, visual acuity is expressed using a different notation. Instead of 6/6, in the USA they use 20/20. In Japan and most of the countries of the world, visual acuity is expressed as a decimal that is equal to the corresponding Snellen acuity. For example, 6/6 (or 20/20) is expressed as 1.0 and 6/12 (or 20/40), which corresponds to 0.5.
The ETDRS Chart
This chart was developed by the National Institutes of Health (NIH) for use in the Early Treatment of Diabetic Retinopathy Study (EDTRS), and it has become a popular configuration for clinicians and scientists who require a well-designed standard for measuring visual acuity:
- Five letters on every line
- All letters have equal height and width
- Spaces between letters are equal to one letter width
- Letters are limited to the Sloan letter set
- The lines progress in 0.1 logMAR steps
- Every letter read counts as 0.02 of each line. This avoids the use of pluses or minuses appended to the Snellen fraction.
For example, if a person sees all of the logMAR 0.3 line (20/40) and two letters from the next line (20/40+2), it is scored as 0.3 minus 0.02 for each additional letter read from the next line. That is, 0.3 minus 0.04 or 0.26. If the person could read all of the next line, their logMAR acuity would have been 0.2. Similarly, if a person reads logMAR line 0.2 (20/32), but misses two letters, you would add 0.02 for each letter missed
- On a logMAR chart: the 20/20 letter has a value of logMAR value of 0.
- As acuity becomes worse, the value of the logMAR increases.
- As acuity improves better than 6/6 or 20/20, the logMAR score will have an increasing negative sign value.
You can convert Snellen acuity to logMAR using the following steps:
- Convert Snellen acuity to decimal acuity
- Compute the inverse of this number; it is the MAR
- Compute the log of this number; it is the logMAR, e.g. 6/6
- Decimal acuity = 1 (divide bottom into top)
- Inverse of 1 = 1/1 (1 over number) = 1
- Log of 1 = 0, therefore 6/6 Snellen = 0 logMAR
You can convert back between logMAR to Snellen acuity using the following steps:
- Compute the MAR, which is the anti-log of logMAR. This is the same as computing 10 raised to the logMAR power.
- Multiply this by 6 to get the Snellen denominator e.g. logMAR = 0
- MAR = inverse log of 0 = 1
- Snellen denominator = 6 X 1 = 6
Therefore logMAR 0 = 6/6 Snellen
PUPILLARY DILATION:
To obtain satisfactory images of the eye it may be necessary to dilate the pupil. Two muscles control the size of the pupil. The radial muscle fibres of the dilator pupillae pull open the pupil and the circular muscle fibres of the sphincter pupillae, which pulls it shut. The dilator pupillae is innervated by the sympathetic system and the sphincter pupillae is innervated by the parasympathetic system.
In crude terms the sympathetic system is responsible for our “fight or flight” response to danger (wide apart eyelids, big pupils, pounding heart, raised blood pressure, sweating) whereas the parasympathetic system is responsible for our “play dead” response to danger (narrow eye lids, small pupil, slowed heart, low blood pressure, pallor). If the sympathetic nervous system is damaged as part of the autonomic neuropathy seen in patients with long standing diabetes then the pupil will appear smaller than normal and will fail to dilate with pharmacological agents.
Tropicamide 1%
Tropicamide is manufactured by Chauvin Pharmaceuticals Ltd. It is a parasympatholytic agent, which means that it inhibits the action of the parasympathetic nervous system. It does so by competing with acetylcholine, the neuro-humoral transmitter at the receptor site of the parasympathetic nervous system, thus blocking its action. The sphincter pupillae is prevented from working and the unopposed dilator pupillae dilates the pupil.
The manufacturers recommend using 2 drops at 5-minute intervals, with a further 1 to 2 drops after 30 minutes, if required. As with any medication an absolute contraindication is known allergy to the drug itself. The manufacturers state that tropicamide is contraindicated in narrow angle glaucoma and in eyes where the filtration angle is narrow, as an acute attack of angle closure glaucoma may be precipitated. This statement is misleading. Where patients are known to have narrow angles they will have been surgically treated so that it is safe for the pupil to dilate, a normal, everyday physiological occurrence. In other patients it is not possible to know whether their angles are narrow or not without using sophisticated instruments available only to optometrists or ophthalmologists. Although there is a theoretical risk of precipitating acute glaucoma in undiagnosed susceptible individuals, in reality, this risk approximates to zero.
The manufacturers advise caution in an inflamed eye although this is not a concern shared by ophthalmologists. Indeed an inflamed eye often requires dilation, partly so that it can be examined properly but also because dilation can often relieve the pain resulting from an iris in spasm. In children the manufacturers recommend compressing the lacrimal sac at the medial canthus for a minute during and following the instillation of the drops to reduce systemic absorption. As is often the case for pharmaceuticals, there appears to be no evidence as to the drug’s safety in human pregnancy and the manufacturers only recommend its use at the doctor’s discretion. This is an academic point as it is essential that pregnant women with diabetes have their eyes checked during each trimester. More importantly are the effects on the ability to drive and use machines as a result of the effect on accommodation and photophobia. Tropicamide often stings transiently when it is instilled and may cause a dry mouth.
Practical Aspects of Tropicamide Use
Tropicamide drops make the pupils larger than normal. The drops take about 15 to 30 minutes to work and around 6 hours to wear off but sometimes the effects may linger until the next day. The large pupils make you more sensitive to light, and distant and near objects may appear blurred especially if it is sunny.
CONTRAINDICATIONS
The following are contraindications and should be taken into consideration before administration of drops:
- Known allergy to Tropicamide or Phenylephrine or any of its ingredients. (Very rare)
- Intention to drive a motor vehicle before being able to meet DVLA requirements. (approximately 6/10 binocular Snellen equivalent).
The law states that: “A licence holder or applicant is suffering a prescribed disability if unable to meet the eyesight requirements, i.e. to read in good light (with the aid of glasses or contact lenses if worn) a registration mark fixed to a motor vehicle and containing letters and figures 79.4 millimetres high at a distance of 20.5 metres.”
Angle Closure Glaucoma
Normally aqueous fluid is made in the ciliary body and passes between the iris and the lens to reach the pupil. It does not go into the vitreous cavity as its way is blocked by the vitreous gel. The aqueous fluid maintains the shape of the anterior chamber. Too little and the cornea will touch the lens. The fluid is being produced all the time. It is also continuously removed. It passes through the sieve-like trabecular meshwork to enter Schlemm’s canal. The fluid is then drained away by small veins.
Angle closure glaucoma occurs when the iris bunches up into the “angle” and blocks off the sieve-like trabecular meshwork. This means that the aqueous fluid cannot escape into Schlemm’s canal.
This tends to happen in people who are born with a shallow anterior chamber. As everyone ages the lens becomes thicker pushing the iris forward. In predisposed individuals when the pupil dilates the iris closes the angle. The continued flow of aqueous into the pupil keeps the iris up against the angle. The pressure starts to rise and the eye becomes red and extremely painful. Patients often vomit with the pain and may collapse. Untreated the patient may be blinded in that eye. Anything that causes the pupil to dilate may cause this in predisposed patients. Although mydriatics are a well-recognised cause, lying down, a dark room and drinking coffee can all precipitate angle closure glaucoma.
Patients will virtually always report symptoms, as they are so severe. The eye will be red, the cornea cloudy and the patient will be in severe pain. Treatment is initially with intravenous acetozolamide, which switches off the production of aqueous fluid by the ciliary body.
Once the eye has responded to medical treatment then a laser is used to make a hole in the iris of both eyes so that if the iris does occlude the angle fluid can still drain away. This is always done to both eyes so that there is no risk again of the patient ever developing angle closure glaucoma.
TO ENSURE SAFE ADMINISTRATION OF EYE DROPS:
- Identify correct patient.
- Identify correct eye if only one eye is being dilated.
- Check label, strength and expiry date of drug.
- Routine hand cleansing procedures and precautions to avoid cross infection between patients i.e. use of alcohol hand rub.
- There is no need to remove contact lenses.
The following complications and side effects of Tropicamide and Phenylephrine eye drops may occur:
- Transient stinging
- Blurred vision. Patients should be warned not to drive until they are able to meet the DVLA requirements
- Photo phobia – may benefit from wearing sunglasses in daylight.
- Allergic reactions
- Theoretical risk of precipitating angle closure glaucoma (presents several hours later with pain, usually located to brow, nausea, vomiting, red eye).