Diplopia, or Double Vision is a distressing thing to have as an adult. There are problems doing most every day tasks- particularly driving, where knowing the position on an oncoming car is critical. Most adults need to close or cover one eye to alleviate their symptoms. By contrast, young children can very quickly learn to ignore one eye to avoid the double vision and confusion, which can be helpful in the short term, but causes a lazy eye if not addressed.
Diplopia causes two problems, double vision (obviously) and confusion- double vision arises at any time the eyes fail to point in the same direction, confusion occurs because, as there are two images in different positions, the sufferer loses their knowledge about which is actually the true position. Critical to avoid an oncoming car!
Double vision may arise because there has always been a tendency to squint, which until now has been able to be compensated. Sometimes, during ill health, periods of stress, tiredness or older age the individual may lose the ability to cope with the tendency- this is called decompensation. Decompensation may be improvable with eye exercises or spectacles with a prism correction to restore single vision. In these situations, the double vision will usually be fairly similar on all gaze directions- the gap between the two remains the same when looking right and left, or up and down.
Incomitant squints, where the angle varies depending on where the individual is looking are not so easily corrected with exercises or spectacles. Affected individuals sometimes develop abnormal head turns or tilts to allow them to turn their eyes towards the direction which is least troublesome, sometimes without even being aware they are doing it. More importantly incomitancies can indicate a weakness in one or more of the muscles controlling one of the eyes- there are six muscles moving each eye around and they must work together in complete co-ordination between themselves and the other eye’s six to achieve single vision in every direction.
A weakness in an extra ocular muscle may be due to direct muscle damage, or an interruption to the blood supply or nerve supply of the muscle. Any of these causes are not good news, but particularly nerve or blood vessel damage within the head would always be looked upon with suspicion, until proven otherwise.
There are some causes of incomitant squint which can be developmental anomalies, or congenital problems which have always been there, and as such do not require urgent investigation, but mostly these people are either aware of the issue, as it was detected years ago, or have just learned to live with it- I had a 14 year old girl in recently who was having a routine test. When asked “do you ever get double vision?” responded “Only when I look up, but it has always been like that, and I don’t look up with my eyes very often, I just lift my chin to do it.” This was a developmental anomaly called Brown’s Superior Oblique Tendon Sheath Syndrome, and while it can sometimes be improved with surgery, as she only felt it was an issue when blow drying her hair, she declined further investigation and treatment.
Most incomitancies detected when investigating double vision will require referral to Ophthalmology, either via the GP if longer standing (weeks or more), or perhaps direct to casualty if of recent onset. Optometrists can easily determine which squints can be dealt with in practice, and which ones require onward referral, often by doing some very simple tests such as a motility test, where we ask you to follow a light around without moving your head.
Hospital investigation will involve trying to find the cause of the problem- and the best possible outcome is that no cause can be found. Tumours, of the brain or the orbit (the eye socket), Neurological problems such as stroke and MS, and blood vessel problems – haemorrhages or blockages all require investigation to be ruled out. Once the cause, if any is determined, often treatment of the double vision will be attempted by prism correction, to try to get at least the straight ahead vision to be single. Over time often the problem reduces or resolves- if one muscle remains weaker, the others can compensate by adjusting their own strength, so that the extent of single vision usually increases over the first few weeks or months. This is assuming that the original cause doesn’t fully resolve in itself.