The dimensions of pupils depend on the age, extent of eye illumination, emotional state, exogenous and endogenous intoxications, diseases of the central and peripheral nervous systems, and many other factors. It should be taken into consideration at examination that our organism is not perfectly symmetric: the small difference in the dimensions of pupils is quite usual. Almost one quarter of people have clinically significant anisocoria (up to 0.5 mm, that is up to 4 % of iris diameter). This phenomenon become more pronounced with the age: such anisocoria takes place in 1/5 people up to 17 years and in 1/3 people older than 60 years. The extent of anisocoria is not constant, it can be changed (increased or decreased) in time, in so doing the pupil size can be bigger either at one or the other side.
If the difference between pupil sizes is decreased in bright illumination, is not a manifestation of some disease, is not accompanied by the other symptoms, then, this difference is not a pathological symptom and is called 'simple' anisocoria. True anisocoria in most cases is the symptom of the injury of not only the central nervous system, but also of many internal organs. Anisocoria is observed in different diseases of the central nervous system (encephalitis, neurosyphilis, cerebral blood circulation disorders, brain tumors, basal arachnoiditis, cranio-cerebral traumas, multiply injuries in the cervical part of the spinal cord) and in internal diseases (the injuries of lungs apex, calculous cholecystitis, urolithiasis, appendicitis). In case of visceral diseases, dilation of the pupil is associated with the reflector action on the boundary sympathetic trunk. Thereby, changes of the pupil are indicative of possible pathology of internal organs of the same side if there is no neural pathology.