Anisocoria is a condition, characterized by unequal pupil sizes. The causes can range from benign to life-threatening.
Anisocoria is a condition characterized by an unequal size of the pupils. It is a common disorder with causes ranging from benign to life-threatening. It is defined by a difference of more than 0.4mm between the pupils. A systematic approach is required to identify patients with serious conditions.
Structural defects can cause anisocoria. Congenital defects such ectopic pupils and lenses, Rieger’s syndrome and other defects may cause the pupils to be unequal. Other causes may be trauma and glaucoma.
Physiologic anisocoria is seen in about twenty percent of the population. The anisocoria is usually less than 0.4mm, and there is no lag in the dilation or constriction. The physiologic anisocoria is usually persistent.
If the small pupil is abnormal it means the pupil is not dilated well. The cause include:
If the large pupil is abnormal, this occurs when the pupil doesn’t respond well to light when there is an ipsilateral parasympathetic pathway. Causes include:
This condition is common, but the true prevalence is unknown. Physiological anisocoria is noted in over 20% of the general population. The mortality depends on the underlying causes. These causes may be life threatening like Horner’s syndrome which may be due to neck injury or carotid dissection, and herniation .
The pathophysiology depends on the underlying cause and an injury in either the dilator or constrictor pathway may cause anisocoria.
Pupillary size is determined by a balance between the dilator and sphincter muscles and by reflex actions that are responsive to light. Other factors such as the sympathetic tone and may also influence the pupil size.
Constriction is mediated by increasing light and near vision. This is primarily transferred via parasympathetic fibres that originate from the Edinger-Westphal subnucleus of cranial nerve III in the midbrain. Dilatation is mediated by sympathetic outflows that originate from the hypothalamus. The first order neuron descends down to the spinal cord to the T2 level. The second order neuron travels upward the cervical spine via the brachial plexus and over the apex of the lung to the cervical (superior) ganglion, near the bifurcation of the carotid. The third order neuron then ascends with the carotid artery to the lateral sellar compartment, where it travels near the sixth nerve. The fibres then travel with the fifth cranial nerve and innervate the dilator muscle and the Mueller muscle   .
Prognosis is dependent on the cause and which may be totally benign or life threatening.
The presentation is dependent on the cause of the anisocoria. It seldom causes any symptoms (the symptoms are usually of the underlying pathology). Many times it is found incidentally and examination of old photographs may assist.
A pupil exam is required to identify the pupil with the problem. First the pupils have to be examined in dark and light rooms. The small pupil is abnormal if the anisocoria is worse in the dark and the larger pupil is abnormal if the anisocoria is worse in the light when compared to dark conditions.
History of any associated conditions and drugs is necessary to find the cause. A full eye exam is required to find the cause and other signs such as ptosis and double vision  .
First the affected pupil is identified by using the method as detailed above. If the anisocoria is greater in the dark, then the dilation lag cocaine test is done. Cocaine dilates the normal pupil but not the affected pupil, in syndromes such as Horner’s syndrome. If the test show no lag and there are no other features then it is most likely diagnosis is physiological.
If the anisocoria is greater in light then the number of differentials is increased. Conditions such as third nerve palsy have to be examined. The pharmacologic pupil is larger than most other causes of anisocoria and there is poor response to light. The rest of the exam may be normal. Possible exposures should be found. Mechanical cause have an obvious history such as trauma and surgery.
Imaging studies will dependent on the possible cause. For suspected Horner’s syndrome a magnetic resonance imaging (MRI) or angiography may be required. Third nerve palsies should be presumed compressive if there is a pupillary involvement. The imaging modality will depend on the suspected etiology    .
This is highly dependent on the cause of the anisocoria. The benign causes do not warrant treatment. Life-threatening causes will require immediate attention.
Due to the large number of differential diagnosis, the prevention strategies will be determined by the underlying etiologies. In many of the cause such as trauma, protective gear and eye wear should be worn in potentially dangerous environments.