27th December 2023

Background

Epidemiology

Diplopia (seeing double) is a common presentation estimated to represent 1.4% of ocular emergencies. There are many potential causes, which could be neurological or ophthalmological. Differentiating between diagnoses relies on thorough and careful history-taking and clinical examination.

Pathophysiology

Diplopia is the symptom of seeing double, either in the horizontal, vertical or diagonal plane, and can be constant or intermittent. It can be monocular or binocular, each with its own set of possible underlying causes. Characterising if the diplopia is monocular or binocular is the first step towards identifying a cause. To provide an idea of how many causes there could be for this symptom, consider the complexity of binocular vision. Each eye is moved by six extraocular muscles, each controlled by one of three cranial nerves which arise from the midbrain and pons. These cranial nerves, via the medial longitudinal fasciculus, carry information about the direction in which the eye should move, all of which are coordinated by the supranuclear gaze centres. In normal sight, each eye sends a slightly different image to the visual processing centre, where complex mechanisms converge the two images together. When images from the eyes are too dissimilar, diplopia is experienced.

Monocular diplopia

This is double vision, which resolves only when the affected eye is covered. The majority of monocular diplopia presentations are intraocular, and common pathologies include uncorrected refractive errors, cataracts, corneal irregularities such as dry eye and keratoconus, vitreous opacities or abnormalities of the iris. Rarely disorders of the primary or secondary visual cortex can cause monocular diplopia.

Binocular diplopia

This is double vision, which resolves by covering either eye. Binocular diplopia is due to misalignment of the visual axes, the cause of which is many. There are three general categories which cover the causes of binocular diplopia relating to nerves, muscles and those conditions which may affect the neuromuscular junction.

1. Nerve: supranuclear, nuclear or infranuclear causes, infarct, infection, compression, inflammation, demyelination, degeneration
2. Muscle: pathologies within the extraocular muscles, such as thyroid eye disease
3. NMJ: primarily myasthenia gravis

Physiological diplopia

Normal double vision is when objects behind or in front of a visual target are seen in double. This is normal, where true diplopia has been excluded. In true diplopia, whatever the patient is fixating on will be in double. A simple test is to ask the patient to fixate on something in the distance, with a target in near vision, such as a thumb. When looking at an object in the distance, the thumb will be in double, and when looking at the object in near vision, the items in the distance will be in double. Everyone has an element of double vision, but the visual centres amalgamate images from each eye into one, so you will likely not appreciate it in normal vision. Some patients are very astute to this normal double vision and may present for evaluation.

References

• Walker, J. R. & Earl Corley, J. (2021) ‘Diplopia’, in Ferri’s Clinical Advisor 2021. [Online]. pp. 477.e11–477.e14.
• Danchaivijitr C, Kennard C. Diplopia and eye movement disorders. Journal of Neurology, Neurosurgery & Psychiatry. 2004 Dec 1;75(suppl 4):iv24-31.
• Iliescu DA, Timaru CM, Alexe N, Gosav E, De Simone A, Batras M, Stefan C. Management of diplopia. Romanian journal of ophthalmology. 2017 Jul;61(3):166.