Anatomy

There are 6 external ocular muscles, of which the superior oblique is supplied by the trochlear nerve (IV), the lateral rectus is supplied by the abducent nerve (VI), and all other external muscles, the internal muscles and the eyelid are supplied by the oculomotor nerve (III).

The 4 rectus muscles pull directly on the globe so that they move the eye in the direction of their name: superior, inferior, medial and lateral. The oblique muscles hook round a 'pulley' so that they move the eyes the opposite way. Hence, the superior oblique moves the eye down and out (the tramp's muscle) and the inferior oblique moves it up and out. The superior and inferior recti are not placed centrally so they have a tendency to move the eye medially but this is opposed by the obliques.

Aetiology

The cause may be a space-occupying lesion in the orbit or skull, a vascular lesion, trauma or a neurological, or muscle disorder.

Presentation

Paralysis of a muscle means that the eye will not move fully in the direction in which it pulls. At rest, when the other muscles are unopposed, the eye may deviate in the opposite direction from the pull of that muscle.

See separate article Squints, dealing with eye movements and squint. Strabismus occurs in children when there is an imbalance between the external ocular muscles. It may be noticed on routine developmental checks or the parents may present with the complaint. Children with squints often tilt the head to one side. Strabismus will cause diplopia but the immature brain will suppress one of the images and this is why early diagnosis and treatment are important.

Symptoms

The patient will often complain of double vision. Ask when it occurs. There may be diplopia on looking to the right but not to the left. This could be due to paralysis of either the right lateral rectus or the left medial rectus. It may be intermittent and associated with tiredness. A droopy eyelid may also be noticed. Is there pain in the eye?

Signs

Look at the eyes.

• Ptosis is often the first sign of weakness and it suggests the affected side.
• Are both pupils equal and do they react to light and accommodation? In Horner's syndrome the pupil on the affected side will be smaller because of reduced sympathetic tone.
• Move your finger along the skin from forehead to cheek on both sides. In Horner's syndrome the finger moves less freely on that side of the face, as there is loss of sweating.
• Look at the patient looking straight at you. Does one eye seem to be deviated?
• Hold the head still with one hand and hold up the index finger of the other hand about 40 to 50 cms from the eyes. Get the patent to follow your finger to the left and the right, up and down, and to note when diplopia occurs.
• If it is not obvious that one eye is failing to move as far as the other, repeat the test but, instead of getting the patient to follow a finger, shine a pen torch in their eyes. The light should be reflected in the centre of the pupil. When diplopia occurs the reflection will appear eccentric in one pupil and that is the side with the pathology.
• Check the retina and ascertain that the disc is not obscured by papilloedema.
• Check the other cranial nerves as described in the separate article Neurological History and Examination.

Children

In children, especially very small children, getting the child to follow a pen torch is much easier than asking the child to follow your finger and to report diplopia. Another test that is used is the cover test. Get the child to look at your face and then suddenly cover one of their eyes. If there is a squint, the eye with the pathology will move to look directly at you when the other is covered.

Differential diagnosis

There are many possible causes of diplopia and this list is not exhaustive but gives some leads. From the history and examination it should be possible to decide which muscle or muscles are affected. Isolated palsy of just one of the muscles supplied by the oculomotor nerve (III) is unusual.

Common causes

• The intracranial course of the abducent nerve (VI) is long and so it is vulnerable at many sites. Hence, abducent nerve paralysis is important but it is a poor localising sign for a space-occupying lesion.
• Examination may fail to show any abnormality where fatigue is important, as in myasthenia gravis. About half of all patient with myasthenia gravis will present with ocular symptoms such as diplopia and 50 to 60% will progress to the full disease. A Cochrane review was unable to find good evidence as to the best medical or surgical management.^[2]
• Do not be too eager to assume hysteria. That is a diagnosis of exclusion. Fluctuating weakness can also occur in encephalopathy and sepsis.^[3]
• Horner's syndrome is a good lateralising sign but no use to localise a lesion.
• Inability to look up or down, called vertical gaze palsy, affects both eyes, pupils are often unequal but fixed and there is usually no diplopia. Think of Parkinson's disease and progressive supranuclear palsy. Inability to look down can lead to falling down stairs.
• Multiple sclerosis (MS) often presents with optic neuritis in which there is pain and loss of vision. Diplopia can occur.^[4] In MS there will usually be nystagmus and an extensor plantar response too.

Rarer causes

• Ophthalmoplegic migraine is a rare condition, thought to represent a variant of migraine; however, clinical presentation and MRI findings have suggested that it might be an inflammatory cranial neuropathy.^[5]
• Temporal arteritis, also called giant cell arteritis, can present with diplopia.^[6]
• Sarcoidosis can present as an isolated cranial nerve lesion. Chest X-ray may show bilateral hilar lymphadenopathy (BHL).
• Diplopia may be a complication after refractive surgery.^[7]
• Ocular lesions, including diplopia, can be amongst the presenting features of Creutzfeldt-Jakob disease (CJD), especially new variant CJD.^[8]
• Chronic progressive external ophthalmoplegia (CPEO)^[9] is a rare disorder of slowly progressive paralysis of the extraocular muscles. It is usually bilateral and symmetrical with progressive ptosis followed by external ocular paresis months to years later. Ciliary and iris muscles are spared.
• Kearns-Sayre syndrome (KSS)^[9] is very rare mitochondrial myopathy related to CPEO but starting before age 20 and with pigmentary retinopathy.

Investigations

• Diagnosing which muscles are affected is fairly easy. A final diagnosis is unlikely to be reached in primary care, so referral is usually required. Before referral, check for diabetes if it is not already diagnosed and check blood pressure in case of a vascular disorder.
• MRI scan may show a tumour, an area of infarction or even an arterial aneurysm pressing on a nerve. It can also show demyelination.
• Chest X-ray may reveal malignancy or sarcoidosis with bilateral hilar lymphadenopathy (BHL).

Management

Clinical management depends upon the cause. In childhood strabismus, surgery may be required but not always. An alternative that shows potential is botulinum toxin.^[10]

Driving

People with diplopia must not drive. They may resume driving on confirmation to the licensing authority that the diplopia is controlled by glasses or by a patch which the licence holder undertakes to wear whilst driving. There will probably be permanent revocation of an LGV or PCV licence.

Exceptionally, a stable uncorrected diplopia of 6 months' duration or more may be compatible with driving if there is consultant support indicating satisfactory functional adaptation.

Prognosis

A sixth nerve palsy of vascular or unknown causes typically resolves within 6 to 8 weeks. If resolution does not occur within 2 to 3 months, the condition progresses or, if additional neurological signs or symptoms develop, imaging studies are required.^[11]