By chance covered the other eye: the aetiology may originate from the eye, e.g. a cataract or macular degeneration.
If the visual acuity is impaired only in one eye, it is important to examine the pupillary reactions before dilating eye drops are administered: relative afferent pupillary defect (RAPD) is the only objective sign of unilateral or asymmetric optic nerve lesion.
Assessment of RAPD using pupillary reflex (swinging flashlight test): when the examination light is moved to the healthly side, the constriction of the pupil is stronger than on the side of the lesion.
Problems in the afferent pathway may be caused, for example, by inflammation (optic neuritis), compression (tumour) or circulatory problems.
Physiological anisocoria is very common: in 20% of individuals the pupils are not totally equal in size.
Ask initially about eye surgery or trauma, eye drops? For how long have the pupils been unequal in size (are there old photographs)?
Examine both pupils, do they react normally? Usually the one with weaker reaction to light is abnormal.
If both pupils react normally, the patient has either Horner's syndrome or physiological anisocoria.
Horner's syndrome (a disturbance of the sympathetic innervation): ptosis and miosis (occasionally impaired sweating ipsilaterally); picture 1.
An attempt to localise the site of the problem may be made using special eye drops (hydroxyamphetamine), but such examination has reduced significantly after imaging studies have become readily available.
In acute Horner's syndrome with ipsilateral facial/neck pain or a history of trauma, suspect carotid artery dissection.
Emergency imaging studies of the carotid arteries
In young individuals (women), one pupil may show intermittent (hours-days) dilatation: a migraine variant
Unilateral impairment of pupillary response: disturbance of the parasympathetic innervation
One pupil dilated, symmetric and round with no reaction to light: oculomotor nerve palsy induced by an aneurysm or other compression. Almost always diplopia (may be mild).
Oculomotor nerve palsy + fixed dilated pupil: emergency magnetic resonance imaging (MRI) and magnetic resonance or computed tomographic angiography (MRA or CTA)
In injured or unconscious patients, a dilated pupil may be indicative of increased intracranial pressure and imminent herniation.
If the only sign is a fixed and dilated pupil, administer 1% pilocarpine eye drops into each eye (the unaffected eye will act as a control).
If the pupil constricts: an aneurysm of the basilar artery tip?
If the pupil does not constrict: a pharmacological blockade? An atropine-like drug has been administered to the eye, either by accident or knowingly.
Has the patient been in contact with plants, does he/she work in a hospital, does he/she have a family member with iritis or has he/she used antinausea patches?
If one pupil is slightly irregular, reacts poorly to light and flashes of bright light induce vermiform movements at the edge of the pupil, the patient is likely to have a tonic pupil (Adie's pupil).
The affected pupil is initially larger than the contralateral pupil but will become smaller over the years; however, the pupillary reactions will remain poor, slow and tonic.
More common in young women
In 20-30% of cases the condition becomes bilateral. The patellar reflexes are absent in one third of patients.
A tonic pupil constricts with 0.1% pilocarpine drops (denervation supersensitivity)
A tonic pupil is a benign condition and needs no further investigations.
Diplopia (double vision)
A careful history is important: binocular or monocular diplopia?
If diplopia persists when the patient covers one eye, the origin of diplopia is in the uncovered eye: cornea, lens, macula.
A referral to an ophthalmologist
Binocular diplopia: true double vision
Intermittent diplopia, more when tired: the patient may have heterophoria.
Cover test is indicated.
Examine the range of ocular movements: any deficits? Are the findings in line with a paralysis of any of the cranial nerves that move the eye or with some other neurological cause of an eye movement disorder?
Ophthalmoplegia
Oculomotor nerve (the 3rd cranial nerve)
The oculomotor nerve innervates the majority of the muscles responsible for eye movement as well as the levator muscle of the upper eyelid and the muscle controlling pupil constriction.
Complete oculomotor nerve palsy: ptosis with poor elevation, depression and adduction of the eye, the eye is turned outward (+ slightly downward) and the pupil is dilated and not reacting to light. Emergency imaging warranted; an aneurysm or other nerve compression is likely.
If the oculomotor nerve palsy is otherwise complete but there is no pupil involvement, the likely diagnosis is a microinfarct involving the nerve itself.
Will resolve within a few months.
If the patient is elderly and has vascular risk factors, imaging studies are not indicated, but the patient does need regular follow-up.
The possibility of temporal arteritis must be borne in mind.
A normal pupil does not exclude nerve compression in incomplete oculomotor nerve palsy.
Trochlear nerve (the fourth cranial nerve)
The trochlear nerve innervates the muscle that allows the depression, adduction and intorsion of the eye.
Trochlear nerve palsy causes diplopia that is worse when looking down; spiral staircases, reading, eating etc.
Often present at birth, but noticeable diplopia may not develop until around the age of 20-30 years. Old photographs are useful in diagnosis: characteristic slight head tilt.
Because the trochlear nerve exits from the posterior aspect of the brain stem, it is the nerve most likely to be injured by trauma, e.g. by a fall backwards; a sudden jolt may result in damage.
Trochlear nerve palsy may also be caused by a microinfarct. More rarely, nerve compression is the cause.
Abducens nerve (the sixth cranial nerve)
The abducens nerve innervates the lateral rectus muscle. In abducens nerve palsy, the inability of the affected eye to abduct results in esotropia.
In elderly patients the most common cause is a microinfarct of the abducens nerve, which will resolve spontaneously within a few months.
Other aetiologies include nerve compression and lesions within the cavernous sinus.
If the intracranial pressure is increased the patient may exhibit unilateral or bilateral weakness of the abducens nerve function.
Nuclear damage within the brainstem is often also accompanied by other neurological signs and typical paresis.
Damage to the abducens nucleus does not produce isolated abducens palsy but causes gaze palsy in which neither of the eyes turns horizontally in the direction of the damaged nucleus.
Internuclear ophthalmoplegia (INO)
INO is the most common eye movement disturbance in multiple sclerosis.
Damage occurs in the medial longitudinal fasciculus with the result that a message does not cross over from the abducens nucleus to the medial rectus subnucleus.
For example, with a gaze to the left, the right eye fails to adduct and the left eye may demonstrate jerky movements (nystagmus).
In elderly patients, the most common aetiology for INO is vascular.
Myasthenia gravis
If the patient has intermittent diplopia that is worse when tired, and either unilateral or bilateral ptosis, the possibility of myasthenia gravis should be borne in mind.
Myasthenia gravis may mimic any eye movement disorder, including oculomotor nerve palsy and INO.
The condition is (almost always) bilateral (pictures 23).
Loss of spontaneous venous pulsation
The visual acuity is usually good.
Signs of increased intracranial pressure (headache, nausea, reduced level of consciousness, diplopia caused by abducens nerve palsy)
The cause for papilloedema must be identified quickly: imaging studies.
Differential diagnosis: an optic nerve head with an unusual appearance: optic disc drusen (ODD)?
If the patient does not have symptoms of increased intracranial pressure, an ophthalmologist must be consulted.
Examinations available to ophthalmologists
Optic Coherence Tomography (OCT): more accurate examination of the ocular fundus (especially the macular area) and the structures of the optic nerve head
Visual field tests (e.g. Goldmann, Octopus, Humphrey): detection of possible visual field defects
Electrophysiological examinations: VEP (Visually Evoked Potentials) and ERG (Electroretinogram) to detect functional abnormalities of the optic nerve and retina