WHAT IS A CATARACT?
A cataract is a cloudy or opaque area in the normally transparent lens of the eye. As the opacity thickens, it prevents light rays from passing through the lens and focusing on the retina, the light sensitive tissue lining the back of the eye. Early lens changes or opacities may not disturb vision. But as the lens continues to change, several specific symptoms including blurred vision; sensitivity to light and glare; increased nearsightedness; or distorted images in either eye, may develop.
The lens is located behind the iris, the coloured portion of the eye, and the pupil, the dark centre of the eye. Tiny ligaments, called zonules, support the lens capsule within the eye.
The lens has three parts, the capsule, the nucleus and the cortex. The outer membrane, or capsule, surrounds the cortex, which in turn surrounds the centre or nucleus of the lens. If you imagine the lens as a piece of fruit, the capsule is the skin, the cortex is the fleshy fruit, and the nucleus is the pith.
Types of Cataracts
There are three main types of cataracts:
- Nuclear cataract – A nuclear cataract occurs in the centre of the lens and causes the lens to become yellow, then brown. It is difficult to visualise but makes the image blurred, despite being in focus with the retina. Common symptoms include blurring or dimming of vision, glare and visual distortion. A nuclear cataract can induce myopia, or near-sightedness, a temporary improvement in reading vision sometimes referred to as “second sight”. Unfortunately, “second sight” disappears, as the cataract gets worse.
- Cortical Cataract – The cortical cataract begins as wedge-shaped spokes in the cortex of the lens. The spokes extend from the outside of the lens to the centre. When the spokes reach the centre, they interfere with the transmission of light and cause glare and loss of contrast. Many people with diabetes develop this type of cataract. Although a cortical cataract usually develops slowly, it may impair both distance and near vision so significantly that surgery may be suggested at a relatively early stage. It is easily seen against the red-reflex.
- Posterior Subcapsular Cataract – A subcapsular cataract develops slowly and starts as a small opacity under the capsule, usually at the back of the lens. Significant visual symptoms may not appear until the cataract is well developed. Typical symptoms are glare and blur. A subcapsular cataract is often found in people with diabetes or high myopia, adults with retinitis pigmentosa and in people taking steroids.
Pseudophakia
Nowadays at the time of cataract surgery the affected lens is replaced with an intraocular lens. This lens sits in the remnants of the capsule or “bag” that held the original lens in place. It is not uncommon for the capsule or “bag” to thicken some time after surgery leading to a “second cataract”.
A hole can easily be made in this thickening using a YAG laser thus improving vision again.
How To Photograph A Cataract
The following applies to the Canon CR5/6:
- Change magnification from 45 degree to X2
- Pull out + lens diopter (+10 diopters on Canon)
- Click button to enable fundus view
- Line up view
- Turn the viewing light up
- Focus manually on pupil margin/lens opacities
- Fire as normal
If focussing isn’t accurate try taking the Infra Red filter off, re focus and re-take.
Stereoscopic Photography
Retinal photographs can infer the presence of oedema by revealing hard exudates or microaneurysms/haemorrhages within a one disc diameter of the geometric centre of the macula. Not all patients with the above features will actually have retinal thickening/oedema leading to “false positive” referrals to ophthalmology. It is possible, however, to perform stereo fundus photography and actually visualise the actual oedema/ retinal thickness using a stereoviewer
The technique described by Allen (Allen L.)9 can be used for taking stereo fundus photographs. An Allen stereo separator or manual lateral movement of the camera may be used to obtain the required, non-simultaneous stereo pairs.
If the manual method is used, the camera should not be rotated; instead, it should be moved from left to right with the joystick (or by sliding the camera base on its table, if preferred). It is customary to take the left member of the pair first, but this is optional (for angiograms to be viewed in strips, the right member is taken first). The first member of the pair is taken as far to one side of the pupil as possible, while maintaining good illumination and a clear image. If the separator is used, it is then flipped to the other side and the second photograph is taken if its quality is good. If the quality is not good, refocusing with spherical or astigmatic correction and/or slight vertical movement of the camera (to avoid lens opacity) may be needed. Such vertical movement will not impair the stereoscopic effect. Somewhat less than optimal focus and clarity is acceptable, if necessary, in the second member of the pair in order to maintain the stereoscopic effect. The same principles apply when the manual technique is used. If the stereo separator is used, it should be set between 2.25 and 2.75mm. About 2mm is the minimum separation between members of the stereo pair to be aimed for when moving the joystick or sliding the camera.