Physiological Anisocoria: This condition is characterized by a small, benign difference in pupil size that is constant in different lighting conditions. It is very common and often discovered incidentally.

Iris Damage or Trauma: Previous eye injury or surgery can cause scarring or damage to the iris, leading to uneven pupil sizes.
Pupil-Sparing Third Cranial Nerve Palsy: Although typically associated with other symptoms like ptosis and diplopia, a pupil-sparing third nerve palsy can present with anisocoria without these additional symptoms, especially if the cause is ischemic (e.g., diabetes).
Adie's Tonic Pupil: A condition affecting the nerves of the eye, leading to a dilated pupil that reacts slowly to light. It often presents unilaterally and can be associated with diminished or absent deep tendon reflexes.

Third Cranial Nerve Palsy with Compression: If caused by an aneurysm or tumor compressing the third cranial nerve, it can initially present with isolated anisocoria before other symptoms develop. Early detection is crucial for preventing serious complications.
Horner's Syndrome: Characterized by miosis (small pupil), ptosis, and anhidrosis. Although it typically presents with a smaller pupil on the affected side, it's essential to consider in the differential due to its potential causes, including carotid artery dissection or stroke.
Oculosympathetic Paresis (Horner Syndrome): Similar to the above but can sometimes be confused with anisocoria due to the relative difference in pupil sizes.

Argyll Robertson Pupil: Pupils that constrict when focusing on a near object but do not react to light, often associated with neurosyphilis.
Holmes-Adie Syndrome (with areflexia): While Adie's pupil is considered in the likely diagnoses, the full syndrome with areflexia is less common.
Congenital Anisocoria: Present at birth, this condition can be due to various anomalies in the development of the iris or nerves controlling pupil size.