Differential Diagnosis for Unequal Pupil Size
Single Most Likely Diagnosis
- Physiological Anisocoria: This condition is characterized by a normal, benign variation in pupil size between the two eyes, often noticed under certain lighting conditions. It's the most common cause of unequal pupil size and usually doesn't indicate any underlying serious condition.
Other Likely Diagnoses
- Iris Damage or Trauma: Injury to the iris can cause the pupil to become irregularly shaped or dilated due to damage to the iris sphincter or dilator muscles.
- Third Cranial Nerve (Oculomotor Nerve) Palsy: This condition can cause the affected pupil to dilate due to paralysis of the iris sphincter muscle, which is responsible for pupil constriction.
- Adie's Tonic Pupil: A condition where there is damage to the nerves that control the pupil, leading to a dilated pupil that reacts slowly to light.
Do Not Miss Diagnoses
- Mydriatic Pupil Due to Intracranial Hemorrhage: A sudden, severe headache accompanied by a dilated pupil on the same side can be indicative of an intracranial hemorrhage, such as a subarachnoid hemorrhage, which is a medical emergency.
- Pupillary Dysfunction in the Context of a Horner's Syndrome: While Horner's syndrome typically presents with miosis (pupil constriction), in the context of comparing pupil sizes, it's crucial to recognize that a smaller pupil could be indicative of this condition, which can be associated with serious underlying causes like carotid artery dissection or a stroke.
- Ocular or Orbital Malignancy: Though rare, certain cancers can affect the eye or orbit, potentially causing unequal pupil sizes due to mass effect or nerve invasion.
Rare Diagnoses
- Argyll Robertson Pupil: A condition where the pupils are bilaterally small and do not react to light but constrict during accommodation. It's often associated with neurosyphilis.
- Holmes-Adie Syndrome: A rare neurological disorder affecting the autonomic nervous system, characterized by a tonic pupil (a pupil that reacts slowly to light) and reduced or absent deep tendon reflexes.
- Congenital Anomalies: Certain congenital conditions can affect the development of the iris or the nerves controlling the pupil, leading to unequal pupil sizes from birth.