The normal diameter of the pupil is approximately 3-4 mm in room light. The determination of pupil size is very important for the diagnosis and management of neurosurgical diseases, and pupil regulation is mainly governed by the optic nerve. A pupil smaller than 2 mm in diameter is called a miosis and can be affected by drug intoxication, such as morphine or opioid-containing drugs, and is also seen in cases of levodopa therapy, atherosclerosis, syphilis, diabetes mellitus and increased intracranial pressure, as well as in the elderly, hyperopia and sleep states. Acute brainstem lesions can also present with pinpoint pupils. If the diameter is greater than 5 mm, the pupil is defined as dilated and can be seen in cases of anxiety, fear, pain, hyperthyroidism, coma, and brain death.