Assessment of Diplopia

Introduction

Diplopia, also known as double vision, happens when there is a mismatch in images produced by the eyes.

Binocular vision develops because the brain can fuse the separate images from each eye into a single image. This is aided by the extra-ocular muscles, which ensure both eyes look directly at the same object. One of the most important features of binocular vision is that it indicates the depth of field of view and the ability to judge distances.

If this mechanism fails (due to a problem with the ocular muscles or nerves), diplopia occurs. 

In young children, diplopia will result in the suppression of one image and, if left untreated, can permanently reduced vision (amblyopia) in the affected eye.

Double vision is a common patient presentation and requires accurate assessment to determine the underlying cause of the diplopia. This article will outline an approach to the assessment and diagnosis of diplopia.

Aetiology

Diplopia can be subdivided into monocular or binocular, and horizontal, vertical or oblique.

The initial stage of diplopia workup is to identify whether it is monocular diplopia or binocular diplopia. Binocular diplopia is far more common (89%) than monocular diplopia.

Monocular diplopia is typically due to an ocular cause, while bilateral diplopia is more commonly caused by underlying neurological or other systemic conditions.

Common causes of monocular diplopia include uncorrected astigmatism, corneal irregularities, tear film abnormalities, and cataracts. The hallmark of monocular diplopia from refractive abnormalities is an improvement with pinhole. Binocular diplopia, however, resolves on occlusion of one eye.

A developing cataract can cause monocular diplopia because areas of differing refractive index within the lens disrupts the light reaching the retina. The diplopia improves or resolves as the cataract becomes more dense and less light gets through. Corneal scarring can have a similar effect. 

Strabismus (squint) in young children is often caused by uncorrected refractive error (e.g. hypermetropia), and the more long-sighted eye tends to turn inwards. In this situation, the brain fails to form neural connections with the squinting eye, leading to amblyopia (lazy eye).

Common aetiologies of diplopia are described below in Table 1.

Table 1. Aetiologies of diplopia

Clinical assessment 

Monocular vs binocular

The initial stage of diplopia workup is to identify whether it is monocular diplopia or binocular diplopia: “Does the double vision resolve when closing either eye?”

Monocular diplopia usually persists when the unaffected eye is closed or covered. In other words, the patient still experiences diplopia when viewing with only the affected eye.

Binocular diplopia resolves with either eye being closed or covered, indicating ocular misalignment as an underlying problem.

Approach to binocular diplopia

Assess if the diplopia is horizontal, vertical or oblique (are the images separated horizontally, vertically or diagonally?), as this can indicate the underlying cause:

• Horizontal: medial/lateral rectus muscle pathology or the nerves supplying these muscles (CN III or VI)
• Vertical: thyroid eye disease (inferior rectus muscle is most commonly affected), orbital floor fracture, CN IV palsy
• Oblique: superior and inferior muscle impairment and lateral medullary syndrome

Ask about the speed of onset

• Sudden onset diplopia requires urgent assessment as this may indicate a serious vascular cause

Establish if the diplopia is persistent or intermittent:

• Fluctuation in symptoms throughout the day associated with fatigue suggests myasthenia gravis or decompensating strabismus

Ask if the diplopia is gaze-dependent:

• The worst gaze position will typically represent the field of action of the paretic muscle. However, if there is muscle restriction (e.g. thyroid eye disease, orbital fracture, orbital myositis), the diplopia may be worse in the opposite field of action of the restricted muscle.

Ask about a history of trauma:

• An orbital fracture can lead to diplopia through muscle or nerve impingement. This is an ophthalmology emergency.

Other important areas of the history include:

• Other neurological symptoms
• Previous visual acuity and ophthalmic history (glasses/ contact lenses)
• Previous medical history, including any history of head or ophthalmic trauma, autoimmune or neoplastic conditions

Clinical examination

Before examining the eyes, always start by checking:

• Visual acuity: using a Snellen chart, test for vision improvement with a pinhole test. Assess visual acuity with distance glasses or contact lenses in each eye. Close each eye to assess if the diplopia is monocular or binocular.
• Colour vision

The ophthalmic examination should include:

• Inspection for ptosis or lid retraction/proptosis (thyroid eye disease): ipsilateral ptosis & mydriatic pupil (3^rd nerve palsy); ipsilateral ptosis & miotic pupil (Horner syndrome)
• Pupils: size, shape, symmetry and testing for RAPD (e.g. optic neuropathy); a fixed dilated pupil associated with headache and diplopia is a neurosurgical emergency requiring urgent imaging.
• Red reflex testing
• Corneal light reflex test (to detect any obvious strabismus): in a sixth nerve palsy, the eye will deviate inwards; in a third nerve palsy, it will deviate in a down and out position; and in a fourth nerve palsy, the affected eye will be higher, especially in medial gaze.
• Fundoscopy: optic disc oedema, papilloedema (raised intracranial pressure), or optic atrophy may be present
• Extraocular muscle movements, including cover/uncover test and alternate cover test.

A full neurological examination should also be carried out, including Hirschberg testing and head tilt test.

Investigations

Investigations will be guided by the suspected underlying cause of the diplopia. Acute onset diplopia, or associated neurological signs, requires urgent brain imaging (CT or MRI).

Blood pressure and blood glucose are useful investigations if a vascular cause is suspected. Relevant blood tests may include thyroid function tests (thyroid eye disease), anti-acetylcholine receptor antibodies (myasthenia gravis) and inflammatory markers (temporal arteritis). 

Management

All patients with new-onset diplopia should be advised to stop driving.

Monocular diplopia

A refractive error is probably the most common cause of monocular diplopia, and looking through a pinhole should correct the double vision. This is reassuring, as the patient likely does not have serious neurological pathology and should be referred for refractive correction with spectacles.

Patients with cataracts should be offered surgical intervention if suitable. 

Binocular diplopia

Treatment of binocular double vision depends on the underlying cause.

Strabismus

In refractive cases of strabismus, treatment may involve wearing a patch over the unaffected eye or using filters on spectacles to strengthen the weaker eye, improve neural connections with the brain and reduce squint.

Surgery on the extra-ocular muscles may also be used to correct strabismus, and botulinum toxin injections into the eye muscles can cause a degree of paralysis to correct misalignment as a temporary measure or if surgery is not possible.

Thyroid eye disease

Management of moderate to severe thyroid eye disease usually involves intravenous methylprednisolone. Other options include radiotherapy or surgical intervention (orbital decompression).

Neurological causes

Patients require management of the underlying neurological condition (e.g. surgical intervention for an aneurysmal compressive lesion of the third cranial nerve in its subarachnoid compartment).

Vascular causes

Most microvascular causes of diplopia can be observed if the remainder of the examination is normal as they usually spontaneously resolve within six months. Urgent, same-day imaging should be sought for patients with a fixed dilated pupil, headache and diplopia. An acute medical or rheumatology referral should be made if there is suspicion of temporal arteritis (giant cell arteritis). 

Complications

Each possible cause for double vision has associated complications.

Diplopia itself can cause nausea or vertigo due to altered field of vision/eye strain and sensitivity to light or sounds.

Key points

• Diplopia is a common symptom in neuro-ophthalmology and may be physiological, binocular or monocular
• Clinical assessment of a patient with diplopia should establish if the diplopia is monocular (typically ocular in origin) or binocular (misalignment)
• Acute onset diplopia or associated neurological signs requires an urgent review
• All patients with new-onset diplopia should be advised to stop driving, and management depends on the underlying cause of the diplopia 

Reviewer

Dr Stewart Gillan

Consultant Ophthalmologist

Editor

Dr Chris Jefferies

References

• Kanski JJ, Bowling B. Clinical Ophthalmology: A Systematic Approach. Edinburgh, Elsevier/Saunders; 2015
• EyeWiki. Basic Approach to Diplopia. Available from: [LINK]
• Low, L., Shah, W., & MacEwen, C. J. (2015). Double vision. BMJ: British Medical Journal, 351, h5385.
• Dinkin, M. (2014). Diagnostic approach to diplopia. CONTINUUM: Lifelong Learning in Neurology, 20(4), 942-965.
• Jain, S. (2022). Diplopia: Diagnosis and management. Clinical Medicine, 22(2), 104.