Differential Diagnosis for Pupil Not Reactive to Light

• Single Most Likely Diagnosis
  • Third cranial nerve (oculomotor nerve) palsy: This is a common cause of a pupil not reactive to light due to the nerve's control over the muscles that constrict the pupil.
• Other Likely Diagnoses
  • Traumatic brain injury: Head trauma can cause damage to the nerves or brain structures responsible for pupil reactivity.
  • Stroke or cerebral vasculature issues: Vascular events in the brain can affect areas controlling pupil function.
  • Diabetes: Long-standing diabetes can lead to neuropathy, including third cranial nerve palsy.
  • Hypertension: Severe hypertension can cause damage to the blood vessels supplying the nerves, leading to palsy.
• Do Not Miss Diagnoses
  • Aneurysm (especially posterior communicating artery aneurysm): Compression of the third cranial nerve by an aneurysm can lead to pupil dilation and lack of reactivity. Missing this diagnosis can be fatal.
  • Pituitary apoplexy: A sudden hemorrhage or infarction in the pituitary gland can compress nearby structures, including the third cranial nerve.
  • Increased intracranial pressure: Conditions causing increased pressure within the skull can compress or damage the nerves controlling pupil function.
• Rare Diagnoses
  • Syphilis: Late-stage syphilis can cause neurosyphilis, which may affect the nerves controlling pupil function.
  • Lyme disease: Neurological manifestations of Lyme disease can include cranial neuropathies.
  • Sarcoidosis: This autoimmune disease can cause inflammation in various parts of the body, including the nerves.
  • Tumors (e.g., brainstem glioma, pinealoma): Although rare, tumors in certain locations can compress or invade the nerves responsible for pupil function.