Pupillary light reflex refers to the shrinkage of the pupil when exposed to light, including direct light reflex and indirect light reflex. Direct light reflex means that when the light shines on one side of the pupil, it can cause both pupils to shrink in response; indirect light reflex means that when the light shines on one side of the pupil, the pupil of the other side will also shrink. Clinical observation of pupillary light reflex is helpful in the diagnosis of the disease. 1. When one side of the retina or optic nerve has a lesion and the vision is dark, the affected pupil is dilated, there is no direct light reflex to the affected eye, and there is no light reflex to the healthy eye. This condition is called blackout pupil constriction. 2. Pupil dilation, but the direct and indirect light reflexes still exist, and accompanied by mild ptosis, eyeball depression, facial flushing, etc., can be seen in cervical sympathetic paralysis syndrome. 3. For example, unilateral pupil dilatation, disappearance of light reflex, accompanied by eyeball fixation and ptosis, can be seen in temporal lobe sulcus hernia. Pupil to light reflex for the eye and intracranial diseases have good diagnostic significance, if abnormal, it is recommended to consult a doctor in time.