Pupil size is influenced by a wide range of physiological and pathological factors, including age, ambient illumination, emotional state, exposure to exogenous or endogenous intoxicants, and disorders of the central and peripheral nervous systems.
During pupillary assessment, it is important to recognize that the human body is not perfectly symmetrical and that minor interocular differences in pupil size are common. Clinically insignificant anisocoria—typically defined as a difference of up to 0.5 mm (approximately 4% of the iris diameter)—is observed in nearly 25% of the general population. The prevalence and magnitude of anisocoria increase with age, occurring in approximately 20% of individuals under 17 years of age and in nearly one-third of those over 60.
Anisocoria is a dynamic phenomenon and may fluctuate over time, with alternating dominance between the pupils. When the inter-pupillary difference diminishes under bright illumination and is not accompanied by additional neurological or systemic symptoms, it is considered non-pathological and is classified as simple anisocoria.
By contrast, true anisocoria is frequently associated with pathology involving the central nervous system and, in some cases, internal organs. It may be observed in a variety of neurological conditions, including encephalitis, neurosyphilis, cerebrovascular disorders, intracranial tumors, basal arachnoiditis, craniocerebral trauma, and multiple cervical spinal cord lesions. True anisocoria may also be associated with certain visceral conditions, such as apical lung pathology, calculous cholecystitis, urolithiasis, and appendicitis.
In visceral disease, pupillary dilation is thought to result from reflexive activation of the sympathetic trunk. Accordingly, unilateral changes in pupil size may serve as an indicator of pathology in ipsilateral internal organs, provided that primary neural pathology has been excluded.
