pupildiameter

Pupildiameter is the width of the pupil, measured in millimeters. In clinical practice it is assessed with a pupillometer or infrared video pupillography; manual measurement with a ruler or calipers can be used but is less precise. Normal resting diameter depends on ambient light: in bright conditions the pupil is typically about 2 to 4 mm; in darkness it commonly ranges from about 4 to 9 mm, varying with age and iris characteristics.

Pupil size is controlled by two opposing muscles of the iris. The sphincter pupillae constricts the pupil under parasympathetic control, while the dilator pupillae expands it under sympathetic control. The pupillary light reflex engages a pathway from the retina to the pretectal area and Edinger–Westphal nuclei, which drive constriction via the oculomotor nerve; dilation is mediated by sympathetic fibers reaching the iris via the superior cervical ganglion.

Many factors influence pupil diameter beyond light level. Age-related reduction in size is common (senile miosis). Medications and substances can markedly alter size: opioids cause miosis; anticholinergic or antimuscarinic drugs and sympathomimetics cause mydriasis. Systemic illness or neurologic injury can also change diameters or the reactivity of the pupil.

Anisocoria is a difference between the eyes in pupil size. The clinical pattern across lighting conditions helps localize a defect. Anisocoria that is greater in bright light suggests a parasympathetic disturbance in the eye with the smaller or less constricted pupil; anisocoria greater in dim light suggests a sympathetic disturbance limiting dilation.

In medicine, pupildiameter and pupillary reactivity are used to assess neurological status, pharmacologic effects, and ocular health. Standard practice uses consistent lighting, tests the direct and consensual light reflex, and tracks changes over time or after drug administration.