What are the causes of anisocoric pupils (unequal reactive pupils)?

From the Guidelines

Unequal reactive pupils, also known as anisocoria, are most commonly caused by third nerve palsy, which can be due to microvascular disease, compressive lesions, or other conditions such as trauma, tumors, or infections, as noted in the most recent study 1. The causes of unequal reactive pupils can be categorized into several groups, including:

• Neurological conditions, such as third nerve palsy, which can be caused by microvascular disease, compressive lesions, or other conditions, as noted in 1
• Trauma to the eye or head, which can damage the iris or neural pathways controlling pupil size
• Horner's syndrome, resulting from disruption of sympathetic nerve supply, which causes a smaller pupil with partial ptosis
• Adie's tonic pupil, which occurs when the ciliary ganglion is damaged, resulting in a larger pupil with delayed constriction
• Medications, including topical medications like pilocarpine and atropine, and systemic medications such as opioids and anticholinergics
• Structural issues, such as iris damage, inflammation, or previous eye surgery, which can lead to mechanical limitations in pupil movement
• Physiologic anisocoria, a benign condition affecting about 20% of the population, which causes slight pupil size differences without pathological significance, as mentioned in 1 and 1. When evaluating unequal pupils, it's crucial to determine which pupil is abnormal by comparing pupil sizes in both bright and dim light, as the abnormal pupil will maintain its relative size difference regardless of lighting conditions, as noted in 1 and 1. The most recent and highest quality study 1 recommends that if there is a high suspicion for aneurysm despite a normal MRA or CTA, then a catheter angiogram should be considered after brain MR imaging with and without contrast and specific attention to the third nerve is performed. Key points to consider when evaluating unequal reactive pupils include:
• Determining which pupil is abnormal by comparing pupil sizes in both bright and dim light
• Checking for signs of third nerve palsy, such as ptosis and limited extraocular muscle movement
• Evaluating for other neurological conditions, such as Horner's syndrome or Adie's tonic pupil
• Reviewing the patient's medication list for potential causes of pupillary inequality
• Considering structural issues, such as iris damage or previous eye surgery, which can lead to mechanical limitations in pupil movement.

From the Research

Causes of Unequal Reactive Pupils

The causes of unequal reactive pupils, also known as anisocoria, can be physiological, pathological, or pharmacological 2. Some of the possible causes include:

• Underlying disease of the eye, orbit, brain, neck, or chest 2
• Congenital or structural anomalies of the iris or pupil 3
• Lesions in the neural pathways that result in pupillary dysfunction 3
• Compressive third cranial nerve palsy due to an arachnoid cyst 4
• Viral ciliary ganglionopathy 4

Pathological Causes

Pathological causes of anisocoria can be further divided into:

• Ocular pathology, such as congenital or acquired iris structural defects 3
• Neurologic causes, such as lesions in the neural pathways that result in pupillary dysfunction 3
• Systemic causes, such as diseases that affect the brain, neck, or chest 2

Diagnostic Approach

A structured approach to examination of the pupils is crucial in determining the cause of anisocoria 2. This includes:

• Accurate pupil testing 2
• Astute observation of the iris and pupil movements 3
• Directed history and careful observation of the patient's symptoms 3