A consistently small pupil, or constricted pupil, is controlled by the parasympathetic nervous system. This condition, known as pupillary miosis, can be caused by various factors, including the use of opioids or narcotics, antihypertensive medications, Horner’s syndrome, head injuries, anterior uveitis, and exposure to pesticides.

Pupillary constriction, or miosis, occurs due to damage or irritation of the autonomic innervation of the pupils. This condition is characterized by a pupil diameter of 2.5 mm or less (less than 21% of the iris diameter). Miosis can be categorized into two types: paralytic, resulting from damage to the pupil dilator due to sympathetic tract disruption, and spastic, caused by spasms of the pupil sphincter from parasympathetic tract irritation.

Physiological miosis can be influenced by several factors:

  • Constitution (such as iris hyperpigmentation, parasympathicotonia, and the balance of cholinergic and adrenergic systems).
  • Age (related to the deterioration of the body’s adaptive-protective mechanisms, like iris atrophy).
  • Vagotonic factors (temporary parasympathicotonia due to mental or physical fatigue, hyperventilation, sleep, or postprandial states).
  • Psycho-emotional state (e.g., during an aggressive attack or high stress).
  • Refraction (pupils are generally narrower in hypermetropes compared to emmetropes and narrower in emmetropes compared to myopes).

Pathological miosis can result from various factors affecting the central vegetative centers, including:

  • Exogenous intoxications (pharmacological agents like carbocholine, pilocarpine, eserine, pyrophose, morphine; alcohol; carbon monoxide).
  • Endogenous intoxications leading to comatose states (uremic, diabetic, alimentary, dystrophic).
  • Astheno-depressive psychosis.
  • Acute cerebrovascular disorders.
  • Hypofunction of the autonomic nervous system, which can affect other organs, such as the digestive tract.

Unilateral miosis is particularly significant for diagnosing central nervous system injuries, especially in the vertebro-basilar region.

Key primary and secondary syndromes associated with such injuries include Bernard-Horner’s syndrome, Dejerine-Klumpke’s syndrome, Pancoast’s syndrome, Babinski-Naugeot syndrome, and alternating bulbar Wallenberg-Zakharchenko syndrome.