During a routine eye examination, near point of convergence will be measured (usually less than 10cm). Convergence is where the eye muscles pull the eyes inwards towards the nose smoothly when looking at an object close up. This normally occurs without effort and allows comfortable use of both eyes “binocular vision”.

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Weakness of these eye muscles may cause problems. This is called convergence insufficiency. There are many causes of convergence insufficiency for example, increased close work, worry, recent illness, and whiplash. Sometimes the reason is simply unknown.

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Typical symptoms can be headache at the front/forehead, around the eyes, eyestrain and difficulty in changing focus.

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Treatment for convergence insufficiency usually involves convergence exercises that can be practised at home. Improvement is usually quite rapid if these exercises are carried out regularly. Suggested convergence exercises can include push up pen exercise and jump convergence exercise. Push up exercises uses a pen or a small picture on a stick as a target to look at. This is then moved slowly toward your nose, making sure the target stays in focus for as long as possible. Jump convergence exercise is where the target is initially held at arm’s length in front. Then a distant target is chosen across the room. The patient then looks from the distance to the near target then after a few seconds back to the distant target. The near target is moved 2cm closer and then refocused onto this nearer target. This is repeated several times until the target is about 6cm from your nose and still in focus.

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Other visual stability tests exist such as Fixation Disparity test. A fixation disparity exists when there is a very small misalignment of the eyes when both eyes are used together (binocular vision). This misalignment is much smaller than a squint which can cause a breakdown in binocular vision. Fixation disparity can however reduce a patient’s level of stereopsis.

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To measure this, a Mallet Fixation Disparity test is generally used. This consists of a small central “X” letter surrounded by two “O” letters, one on each side of the “X”. These letters are viewed using both eyes (binocularly). Two coloured polarised bars are also viewed either side of the “X” (usually red for distance and green for near vision). These bars will be viewed separately. Any misalignment of the eyes will be indicated by the misalignment of the polarised bars in relation to the letter “X”. Special lenses called prisms can be used to realign the polarised bars and hence realign the eyes.

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To indicate the level of binocular vision, we can measure stereoscopic acuity. Generally if visual acuity is good then stereoscopic acuity should be good. Poor stereoscopic acuity may indicate poor ocular motor balance.

Examples of tests we use for this are Titmus Fly Test which uses polarized spectacles and the TNO test which uses red and green spectacles for this test.

 

Anterior eye examination

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It is also important to examine the front of the eye, which includes the eyelids, lashes, corneas and conjunctiva. For this, a slit lamp is used. The slit lamp is a table mounted microscope with special adjustable illumination with a stereoscopic view. The illumination has a choice of filters for example cobalt blue. This gives an enhanced view especially when used alongside a yellow fluorescein die. There is also a red free (green) filter to give added definition.

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The slit lamp can also be combined with specialised lenses which allow a stereoscopic view of the posterior eye including the vitreous and retina. Slit lamp and “Volk” lens is an important tool in the examination of those patient experiencing flashes and floaters.

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Flashes and floaters are thought to occur because of the natural shrinkage of the jelly like substance “vitreous” that fills the space behind the lens and the retina in the eye. This shrinkage as you get older causes the jelly to pull away from the inside surface of the eye. Usually this shrinkage causes no damage and is known as a posterior vitreous detachment. Flashes are thought to be as a result of the pulling or rubbing on the retina. The shrinkage causes clumps to form in the jelly which appear as the floaters.

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When patients experience this phenomenon it is recommended that they have a full examination using the slit lamp and Volk lens with a dilated pupil. Drops are instilled to cause the pupil to open and remain large whilst being examined with a reasonably bright light. After, the vision may remain slightly blurry and your eyes may be sensitive to light for a few hours, which is, why not driving is recommended for a few hours afterward.