Diplopia

Abstract

Patients frequently present to medical facilities and emergency rooms with symptoms of diplopia, where a single object is observed as two items. It is important for the clinician to know the clinical presentation of diplopia to be able to determine its aetiology and management after establishing important historical information. The aetiology of diplopia can vary from common ocular entities, such as corneal and lens problems, binocular vision anomalies, and uncorrected astigmatism; to life-threatening conditions such as intracranial anomalies. The management options of diplopia depend on several factors, including its specific cause and prognosis. This paper reviews the common causes and characteristics of diplopia, to assist in appropriate management strategies.

Keywords:

• diagnosis
• diplopia
• extra-ocular muscles
• management
• ocular motor palsy

Introduction

The word diplopia, derived from the Greek words “diplous” and “ops”, meaning “double” and “eye”, respectively,¹ is a common visual complaint in which a single object is perceived as two.² The patient sees two images of a single object either all the time (constant) or some of the time (intermittent), and this may be horizontally, vertically, or diagonally. Diplopia may present monocularly or binocularly, and may be worse at distance or near. It develops from a wide variety of causes, ranging from benign conditions with spontaneous resolution to serious life-threatening conditions.³ It can also be an ocular manifestation of systemic conditions such as myasthenia gravis and dysthyroid eye disease.³

Monocular diplopia is less common, and the patient sees double with only one eye open, with the double vision coming from one eye only, and continues when the unaffected eye is covered.^4,5 This may occur for a number of reasons, such as an extra hole in the iris from an iridotomy, refractive error, corneal disease (irregular astigmatism), iris injury, cataract, media opacity, macular disease, and primary or secondary visual cortex disorder, and usually resolves with the use of a pinhole.^4,5 If it does not resolve with a pinhole, it may be due to a lesion in the visual cortex, which is usually associated with visual field defects.^4,5

Physiological diplopia is a normal phenomenon, has no serious underlying neurological disorder, and often goes unnoticed because the patient’s brain usually compensates for this type of diplopia.⁶ It is common in children and usually occurs when the objects that the person is not focusing on (i.e. background objects) appear doubled.⁶ Binocular vision therapy is used to align the eyes of patients with strabismus (squints) via special eye exercises, which include the Brock string, eccentric circles, lifesaver cards, cat stereogram, and physiological diplopia exercises.⁶⁻⁸ Physiological diplopia is employed in these eye exercises as feedback from the patient to ensure that the brain is not suppressing the image from the strabismic eye.⁶⁻⁸ Binocular diplopia is a breakdown in the fusional capacity of the binocular system resulting in the normal neuromuscular co-ordination not maintaining correspondence of the visual objects on the retinas of the two eyes.⁹ With binocular diplopia, the patient sees double only if both eyes are open and resolves it with closure of either eye.⁹

Diplopia has a wide variety of possible causes, each with its own natural history and associated morbidity.¹⁰ Diplopia itself has significant morbidity in terms of confusion with orientation and loss of depth perception.⁵ Death is a potential serious complication of this condition, and prompt diagnosis facilitates appropriate management which may reduce the risk of fatal complications. The most common causes of binocular diplopia are discussed below, followed by an outline of the available diagnostic tools and the most appropriate management strategies.

Common causes of diplopia

The most common causes of diplopia include conditions such as refractive, binocular vision disorder, ocular motor palsy, post-operative diplopia, post-traumatic diplopia, neurologic conditions, and myogenic and other causes, each of which will be reviewed.

Refractive

Inadvertent displacement of optical centres on prescription spectacle lenses may cause diplopia or worsen an existing heterophoria, and the patient will experience prismatic power and other aberrations.¹¹ This is commonly seen in over-the-counter reading glasses that are not assessed by clinicians.¹² The edge effects of high-prescription spectacles, prismatic effect of ill-fitting spectacles, and contact lens edge effects may cause diplopia. Anisometropia, which is a marked difference (usually >1.5 D) in refractive error between the two eyes, produces diplopia, especially in extremes of gaze. This condition can also be created by removing a cataract from one eye only and is commonly referred to as post-operative, iatrogenic diplopia; this is usually resolved after the removal of the “second” cataract.⁵ Similarly, large differences in refractive error that are corrected by spectacles may result in diplopia due to unequal image size and shape in the visual cortex (psychic image).⁷ This visual condition is referred to as aniseikonia, and may be largely eliminated by correction with contact lenses.⁷

Binocular vision disorder

Decompensating heterophorias and vergence problems, such as convergence insufficiency, are the most common causes of intermittent diplopia at near only.¹³ This type of diplopia is normally associated with asthenopic symptoms such as headaches and tends to occur following a specific visual activity. In convergence insufficiency, the eyes have a reduced ability to converge at near, this being a common cause of horizontal diplopia that manifests with prolonged near vision tasks, such as reading, typing, and computing.^5,6 Among the elderly, it is more common in patients who have Parkinson’s disease. ⁵ Convergence insufficiency may also be associated with an uncorrected refractive error, and exacerbated by dry eyes, as the normal blink rate is reduced with near vision tasks.⁵ Antidepressants drugs can exacerbate the symptoms of convergence insufficiency due to their anticholinergic properties on the accommodative system of the eye.^5,6

Ocular motor palsy

These are very common causes of diplopia in the elderly due to microvascular ischemia to one of the ocular motor nerves (third, fourth, or sixth cranial nerves).⁴ Microvascular ischemia is usually associated with atherosclerosis, diabetes, and hypertension, and an ocular motility test will confirm under-action of the affected muscle.⁴ The most common cranial nerve palsy involving the eye with serious vascular disease influence is third nerve palsy.³ As the oculomotor nerve innervates the majority of the muscles controlling the eye, its damage leads to strabismus and, as a consequence, double vision.¹⁴ In a classic full ophthalmoplegia the patient presents with a divergent squint, dilated pupil, droopy eyelid, and diplopia; clinicians remember it as the four Ds. An interesting point with third nerve palsy is that vascular aetiologies have pupil sparing.^5,6 In case of a sixth nerve palsy, abduction (outward movement of the eye) is restricted, while a third nerve palsy may cause restrictions in any combination of adduction, elevation, and depression because it is the innervation to the extra-ocular muscles that is involved in these gaze positions.¹⁵ A sixth nerve palsy of vascular or unknown origin typically resolves within six to eight weeks.¹⁵ The fourth cranial nerve innervates the superior oblique muscle which is responsible for intorsion and some vertical movement. A fourth nerve palsy usually presents with complaints of a vertical diplopia, especially when looking away from the lesion.¹⁶ However, many patients adopt a compensatory head position away from the affected side in order to compensate for the underacting superior oblique muscle to reduce eye strain and prevent double vision.⁶ These three cranial nerves are noted for the ocular aetiologies in diplopia. The prognosis of diplopia caused by microvascular ischemia in the elderly is usually favourable, as the majority resolve spontaneously within one to four months.^5,6 However, if it does not resolve within four months and is associated with neurological signs and symptoms, papilledema, dilated pupil (as in the case of third nerve palsy), variable ptosis, and diplopia, further investigations are required.⁵

Useful clinical tests to aid in the diagnosis of ocular motility palsies include, but are not limited to, the prism cover test, ocular motility testing, and the Bielschowsky head tilt test. These three tests will be reviewed briefly below.

The cover test

If the patient has an ocular misalignment such as an exotropia, esotropia, hypertropia, hypotropia, or a combination of these, performing the unilateral (cover/uncover test), helps to characterise the deviation.⁷ In conjunction with the prism bar, the magnitude of the deviation or ocular misalignment can also be determined. This test is important in identifying the paretic muscle. However, if there is no obvious misalignment, the alternate cover test can be used to diagnose a phoria, particularly if the patient has symptoms of intermittent diplopia.⁷

Ocular motility

This test is administered at near and uses the nine diagnostic action fields to assess binocular versional eye movements and monocular ductions where there appear to be points of restriction.⁶⁻⁸ In case of a sixth nerve palsy, abduction is restricted, while a third nerve palsy may cause restrictions in any combination of adduction, elevation, and depression.⁶⁻⁸ Fourth nerve palsy usually presents with no restriction, but the patient may have ipsilateral inferior oblique over-action.⁶⁻⁸

Bielschowsky head tilt test

This test is part of a broader Parks three-step test and is a recommended diagnostic test that can be used to detect the paretic muscle if vertical diplopia is present.⁶⁻⁸ This test is useful in recent-onset single muscle paresis and is sensitive to diagnosis of cyclovertical muscles involvement and does not include horizontal rectus muscles.⁶⁻⁸ It is useful when a hypertrope is clinically present. The examiner together with the patient tilts the head to each shoulder and notes the tilt that reveals the greatest hypertropic deviation of the eyes.⁶⁻⁸

Post-operative diplopia

The causes of post-operative diplopia range from damage to the extra-ocular muscles or nerves, to dislocated intraocular lens.⁵ Details of the case history will indicate the onset of diplopia after a surgical procedure, such as cataract, strabismus, or retinal detachment surgery. Damage to the extra-ocular muscles or nerves will cause binocular diplopia, and a dislocated intraocular lens will lead to monocular diplopia.⁶

Post-traumatic diplopia

Head injuries involving the orbit can sometimes result in damage to the extra-ocular muscles or the nerve supply to the muscles, both of which can result in diplopia. This can result in a strabismus, which can cause binocular diplopia. The angle of deviation also varies, depending on which eye is fixating, and may be due to cranial neuropathy, supranuclear disorder, ocular myopathies, neuromuscular junction disorder, all being due to a recent ocular, facial, or head trauma, especially orbital fractures.^5,6 Acquired fourth nerve palsy is most commonly trauma-related: because the trochlear nerve “emerges dorsally from the medullary velum and decussates, it is in a vulnerable position from a blow to the forehead”.⁶

Neurologic conditions

Diplopia may develop from diseases of the cerebral cortex, deep white matter, thalamus, brainstem, vestibular apparatus, neuromuscular junction, and the cerebellum.⁵ For example, neurological conditions such as strokes, myasthenia gravis, Parkinson’s disease, and intracranial lesions are common causes of diplopia in the elderly and may involve a specific cranial nerve or cause multiple cranial nerve palsies, as indicated below.⁵

Stroke (cerebral vascular accident)

The diplopia is due to the infarction of the brainstem, resulting in cranial nerve palsies and supranuclear ophthalmoplegia (skew deviation). Clinical presentation includes oscillopsia (oscillating vision), which is caused by nystagmus and homonymous visual field defects.¹⁷

Myasthenia gravis

This is an autoimmune neuromuscular disorder that is associated with general systemic weakness and rapid fatigue, and may mimic almost any ophthalmoplegia.¹⁸ It presents with signs of eye lid fatigue and worsening ptosis on prolonged up gaze. The diagnosis is confirmed following a rapid but short-term improvement in muscle function following an injection of an anticholinesterase (edrophonium chloride [Tensilon^™]), and treatment includes pyridostigmine and/or immunosuppressive agents.⁵ A cheap, reliable, bedside test called the ice pack test can aid in the differentiation of myasthenia gravis from other conditions.¹⁹ The test consists of the application of ice to the eyes for 2–5 min, ensuring that the ice is covered to prevent ice burns. If positive, the patient no longer has the characteristic myasthenic ptosis following application of an ice cube, wrapped in a towel or surgical glove, applied to the levator muscle of the eyelid for at least two minutes.⁶ The physiological theory behind the test is that cold improves neuromuscular transmission by decreasing the activity of the acetylcholinesterases.^20,21

Parkinson’s disease

This is a neurodegenerative condition associated with degeneration of the basal ganglia of the brain and a deficiency of neurotransmitter dopamine and often results in an inability to look up or down, called vertical gaze palsy.^5,6 Convergence insufficiency is very common in patients with Parkinson’s disease; this is aggravated with anticholinergics, and the vertical gaze palsy and the associated decrease in the blink rate worsen its symptoms.^5,6

Intracranial lesions/masses

The location of a tumour may involve or be in close proximity to the ocular motor nerves, resulting in diplopia. Comitant strabismus can be a presenting symptom for retinoblastoma or intracranial tumours, and all children with a comitant squint/strabismus of unknown aetiology should therefore have a thorough fundus examination.⁹ Comitant squint/strabismus can be the presenting symptom for retinoblastoma²² or intracranial lesions.²³ Neuroimaging studies, such as computed tomography (CT), magnetic resonance imaging (MRI), and sometimes spinal fluid analysis, play an important role if an intracranial tumour is suspected in patients presenting with diplopia.⁴

Myogenic causes

Systemic conditions that directly affect the extra-ocular muscles, such as Graves’s disease, frequently cause mechanical type restrictions of movement. Typical clinical features such as proptosis, extra-ocular muscle edema, and lid retraction in Graves’s disease usually occur before the onset of diplopia.²⁴ Ocular forced duction testing, orbital CT imaging, and thyroid function testing can help in the diagnosis of Graves’s disease.²⁵ The forced duction test is performed in order to determine whether the absence of movement of the eye is due to a neurological disorder or a mechanical restriction. In this test, the anaesthetised conjunctiva is grasped with forceps and an attempt is made to move the eye ball in the direction where the movement is restricted. If a mechanical restriction is present, it will not be possible to induce a passive movement of the eye ball.²⁵

Other causes

Other causes of binocular diplopia may include orbital cellulitis, carotid venous fistula, cavernous sinus thrombosis, Miller Fisher syndrome, thiamine deficiency, epiretinal membrane, and Whipple’s disease. Medication such as anti-epileptic and anti-convulsant drugs may induce diplopia due to an unwanted side effect.⁶

Due to its broad aetiology, the diagnosis of binocular diplopia requires further information than a standard office visit in order to determine its exact cause. Some of the pertinent questions to establish the cause of binocular diplopia^5,6 are shown in Table 1. Comitant ocular deviations (also referred to as decompensated deviation or phoria) are characterised by the angle of the deviation being the same in all directions of gaze. Patients with comitant deviations usually do not present with diplopia, due to long-standing sensory adaptations. However, suppression (cortical inhibition of one image binocularly when diplopia presents) is associated with congenital strabismus, which on occasion can “break down” or change, causing diplopia.^5,6 An incomitant deviation is characterised by the angle of the deviation being different in the various directions of gaze. Incomitant deviations are usually due to a weakened, restricted, or paralysed extra-ocular muscle, and patients who have incomitant deviations usually complain of diplopia.^5,6

Table 1: Pertinent issues to establish cause of binocular diplopia^{5,26,27,29–31}

Item	Response	Possible cause/aetiology
Alignment of images	Horizontal diplopia	Impaired neural control or function of medial or lateral rectus muscles, sixth nerve, or both
	Vertical diplopia	Weakness of inferior oblique and superior rectus muscles. Weakness of superior oblique and inferior rectus muscles. Fourth nerve palsy
	Oblique diplopia	Weakness of superior oblique or inferior oblique. Vertical recti weakness. Third nerve palsy cause. Orbital processes (e.g. orbital pseudotumour, orbital cellulitis, fractures) can cause horizontal, vertical, or oblique diplopia
Frequency	Intermittent	Decompensating phorias and vergence problems, ocular myasthenia, multiple sclerosis, internuclear ophthalmoplegia, superior oblique myokymia, transient ischemia involving the vertebrobasilar system, ischemia of extra-ocular muscles (as in temporal arteritis)
	Constant	Physiological or non-organic conditions
Diplopia worse at distance or near	Horizontal diplopia Worse at distance	Esotropia, lateral rectus muscle problem (as in sixth nerve palsy)
	Worse at near	Convergence insufficiency, medial rectus muscles problem
Is it worse with different gaze positions	Yes	Restriction/weakness of one or more extra-ocular muscles
	No	Decompensated deviation or phoria
Is there any fatigability/ variability	Yes	Myasthenia gravis
Onset	Sudden	Ocular motor palsy, neurologic
	Long-standing
Associations	Intermittent lid droopiness (worsens later in the day)	Myasthenia gravis
	Mild ipsilateral eye pain (subsides within a week)	Ischemic ocular motor cranial neuropathy
	Severe pain	Temporal arteritis, subarachnoid haemorrhages
	Painful third nerve palsy	Aneurysms
	Diabetes, hypertension, hyperlipidemia	Ischemic ocular motor cranial neuropathy
	Vertigo, speech difficulty	Vertebrobasilar ischemia
	Lid, pupil, or eye movement involvement	Horner’s syndrome (ptosis with small pupil), third nerve palsy (ptosis with big pupil), inflammatory disease (e.g. Guillain–Barré or Miller Fisher variant)

Diagnostic tools

The presentation of diplopia can be obvious and pose no diagnostic challenge, or it may be complex and require neuro-ophthalmic assessment. A comprehensive systemic management approach that includes a proper history and the characteristics of the diplopia (such as knowledge of clinical presentation, signs and symptoms) is essential in determining its aetiology and management. A number of testing tools have been developed to diagnose both monocular and binocular diplopia, and, although they will not be reviewed in detail here, Figure 1 gives an overview of the general practical approach to diagnosing diplopia and its possible causes.⁴

Figure 1: A summary of the approach to diplopia⁴

Management

The most appropriate clinical management plan is to treat the underlying cause of the diplopia. Immediate relief of binocular diplopia involves occlusion of one eye⁹, while if the diplopia is due to optical causes (such as misalignment of optical centres, anisometropia, and uncompensated phoria) alignment of the optical centres and adjusting the optical prescriptions are necessary (Table 2)^5,9,26,27. Both passive and active treatments of binocular vision disorders such as convergence insufficiency and uncompensated phoria are used.⁶⁻⁸ Passive therapy includes the use of compensating prisms, especially in uncompensated phorias, and Fresnel prisms in cases of manifest large incomitant diplopia is beneficial. In addition, convergence insufficiency and uncompensated phoria respond particularly well to active visual/orthoptic therapy such as the Brock string, eccentric circles, lifesaver cards, cat stereogram, and physiological diplopia 1, 2, and 3 tests.⁶⁻⁸ Rigid gas-permeable lenses are effective in monocular diplopia associated with corneal astigmatism.⁹ Although there is no specific medication to relieve diplopia, specific treatments are available for given conditions, such as steroid pulse for multiple sclerosis and pseudotumour cerebri.²⁸

Table 2: A summary of suggested treatment for the most common causes of diplopia^5,9,26,27

Cause	Management
Refractive
Presbyopic spectacle	Bifocal adjustment
Misaligned optical centres	Alignment of optical centres
Corneal astigmatism	Rigid gas permeable lenses
Refractive anisometropia	Provide appropriate prescription
Iatrogenic anisometropia	Cataract surgery
Binocular vision disorder
Convergence insufficiency	Eye exercises, prisms, strabismus surgery (rarely)
Ocular motor palsies	Spontaneous resolution (usually), prisms (temporary)
	Strabismus surgery (non-resolving)
Post-operative diplopia
Dislocated intra-ocular lens	Lens explantation/implantation
Retinal detachment surgery	Prisms
Post-traumatic diplopia
Head and orbital injury	Surgery, prisms
Neurologic conditions
Stroke	Spontaneous resolution, prisms, strabismus surgery (rarely) and systemic control of vascular condition.
Myasthenia gravis	Pyridostigmine, immunosuppressives, head turn, prisms (rarely), strabismus surgery (rarely)
Parkinson’s disease	Separate reading glasses, treat convergence insufficiency, treat associated dry eyes
Intracranial lesions	Neurosurgical intervention, prisms, strabismus surgery
Medication	Seek alternative medication/drug

Computed tomography (CT) scan or magnetic resonance imaging (MRI) of the skull and orbits can be ordered to rule out intracranial masses or other pathologic processes, such as carotid cavernous fistula, aneurysm of intracranial carotid artery, blow-out fractures, and orbital tumours.^26,27 Symptomatic treatment for disabling diplopia may be appropriate in certain non-organic causes of diplopia and can include unilateral eye occlusion therapy, eye patch, opaque tape over the lens of glasses, and Fresnel prisms.²⁸ A simple pinhole test can help to determine that the cause of the monocular diplopia is not neurological. If the pinhole is used and the second image disappears, this suggests that the patient has no brain tumour. Surgical treatment may be indicated if complete recovery is not achieved. Botulinum toxin treatment has been reported to be effective when injected into the antagonist of a paretic muscle.⁹

Conclusion

Some causes of diplopia are relatively minor, but others need urgent medical attention because they may be life-threatening. Specific investigations regarding onset and frequency, progression, and changes with gaze or head position, as well as previous similar episodes (especially if associated with neurologic symptoms, history of strabismus or amblyopia, loss of vision, pain) and/or spontaneous resolution, are critical to appropriate diagnosis and management.^5,32 Many patients whose diplopia is due to optical causes and binocular vision disorders can be treated with appropriate optical prescriptions and eye exercises, respectively. Diplopia due to trauma and neurologic conditions may require neurosurgical intervention.

Conflict of interest

The authors declare no conflict of interest.

Acknowledgements

The authors thank Professor OA Oduntan and Ms Carrin Martin of the University of KwaZulu-Natal for commenting on the manuscript.