Diplopia occurs when a patient reports seeing two images of a single object in the visual field some or all of the time. The two images may be separated horizontally, vertically or both. It cannot occur unless binocular vision has properly developed.
Binocular vision develops because the brain is able to fuse the separate images from each eye into a single image. This is aided by the ocular muscles that function to ensure both eyes look directly at the same object. One of the most important features of this ability is that it gives an indication of the depth of field of view and the ability to judge distances.
If this mechanism fails, due to anything that interferes with the normal function of ocular muscles or nerves, diplopia usually results. In young children, diplopia will result in suppression of one image and if untreated can lead to a permanent loss of vision (amblyopia) in the affected eye.
The presence of diplopia indicates a mature visual perception system. There are three types of diplopia - physiological, binocular and monocular.
1. Physiological diplopia
Physiological diplopia is a normal phenomenon, and occurs when an object that is outside the point of fixation is seen as double. The brain normally suppresses this effect so people are unaware of it, but some children or adults can start to notice the effect and report it as diplopia. In such cases there is no defect of eye movement and the perception can be induced by persuading the patient to look at a distant object while they focus on a near one.
2. Binocular diplopia
Normal eye movements are organised so that the visual axes remain in the same plane throughout eye movements. The centres for conjugate gaze are the frontal lobe for saccadic movements - very rapid jumps in eye movements that assess a visual target - and the occipital lobe for pursuit movements - smoother movements that follow a moving target. The conjugate movements to the right are controlled from the left side of the brain and vice-versa.
If there is a brain stem lesion, there will be ipsilateral paralysis of horizontal conjugate gaze, that is, there is a loss of the ability to move both eyes in the same horizontal direction. A frontal lobe lesion will cause contralateral paralysis of horizontal conjugate gaze.
Both these situations can occur following a stroke.
Other causes of binocular diplopia may be neurogenic, such as with a paralytic strabismus, or muscular, as with myasthenia gravis - usually accompanied by ptosis - and exophthalmic ophthalmoplegia. Causes may also be mechanical, for example following a fracture or a tumour that displaces the globe of the eye.
Treatment of binocular double vision depends on the underlying cause. In simple cases of strabismus, treatment may simply involve wearing a patch over the unaffected eye, or using filters on spectacles or an opaque contact lens in one eye.
Surgery on the ophthalmic muscles may also be used to correct strabismus, and botulinum toxin injections into the eye muscles can cause a degree of paralysis that may correct misalignment.
Diplopia can be caused by strabismus; if untreated, permanent loss of
vision may occur (Photograph: Dr P Marazzi / Science Photo Library)
Rapid onset of diplopia associated with CNS symptoms that arouses suspicion of a brain tumour requires urgent referral. Such symptoms may include: progressive neurological deficit; recent-onset seizures; mental or cognitive changes; cranial nerve palsy; headaches with vomiting, drowsiness or related to posture; and unilateral sensorineural deafness.
Persistent diplopia with no simple refractive explanation should also be referred, especially if accompanied by headaches of recent onset that persist for at least a month, even in the absence of signs of raised intracranial pressure or neurological deficit.
3. Monocular double vision
Monocular diplopia may occur due to abnormalities in a cornea that is distorted or scarred, multiple openings in the iris, cataract or subluxation of the natural lens or pseudophakic lens implant, vitreous abnormalities, and retinal conditions. Monocular diplopia needs to be distinguished from metamorphopsia, a condition in which objects appear to be misshapen.
A refractive error is probably the most common cause of monocular diplopia, and looking through a pinhole should abolish the double vision. This is a strong indication that the patient needs refractive correction with spectacles.
A developing cataract can cause monocular diplopia because areas of differing refractive index within the lens result in disruption of the light reaching the retina, and diplopia results. As the cataract becomes more dense and less light gets through, the diplopia improves or resolves. Corneal scarring can have a similar effect.
The At a glance guide to the current medical standards of fitness to drive on the DVLA website (www.dft.gov.uk/dvla) states that a patient with diplopia must cease driving on diagnosis, but can resume on confirmation that the diplopia is controlled by glasses or a patch that the patient wears while driving.
If a patch is worn, then the conditions for monocular driving must also be met. There are exceptions - if diplopia persists for six months and there is consultant support that there has been adequate functional adaptation, then driving may be allowed.
However, if there is insuperable diplopia the licence will be withdrawn permanently.
- Dr Barnard is a former GP from Fareham, Hampshire
- NICE. Referral guidelines for suspected cancer. CG27. London, NICE, 2005.