The pupil's size is determined by a balance between the input from the two parts of the autonomic nervous system --- sympathetic and parasympathetic. Sympathetic nerves release adrenaline, which causes the pupil to expand. Parasympathetic nerves release acetylcholine, which causes to pupil to contract. If the sympathetic nerves are damaged, or if there's an excess of parasympathetic activity, the pupil can't expand as it normally would, for example when entering a dark room. Contraction of a pupil, known as meiosis, can indicate the presence of an underlying disease.
Tumors
Lung tumors can invade the sympathetic ganglion, which is a way station in the path of sympathetic nerves to the eyes, causing Horner's syndrome. Symptoms include meiosis, a droopy eyelid, decreased sweating on the same side of the face and a sunken eye. Enlarged lymph nodes from Hodgkin's disease and leukemia can also compress the sympathetic nerves in the chest. Tumors at the base of the skull and tumors of the pituitary glands can damage the sympathetic nerves at different points along their path, as explained by Pacific Oregon University.
Carotid Artery Dissection
The sympathetic nerves travel along with the carotid artery on their way to the eyes. Carotid dissection, which is a tear in the artery's wall, can lead to meiosis as part of Horner's syndrome. According to the UCLA Health's System, hyperextension of the neck is a common cause of carotid dissection. Other causes include chiropractic manipulation and attempted strangulation. Common symptoms of dissection include severe headache on the same side of the artery, neck pain, weakness and loss of vision. Sometimes dissection can be silent, and Horner's syndrome can be the first sign of the problem, as explained in an article published in "PLoS Medicine."
Drugs and Toxins
Drugs that cause meiosis include opiates, such as morphine and heroin. Barbiturate and chloral hydrate are sedatives that can cause meiosis. Pilocarpine eye drops, used to treat glaucoma and other eye conditions will cause meiosis.
Toxins include nerve agents used in chemical warfare, such as sarin, as the U.S. Centers for Disease Control and Prevention Agency for Toxic Substances and Disease Registry explains. Organophosphates, which are used as pesticides, can also cause meiosis, as part of an extensive neurological presentation that can include loss of coordination , double vision, stupor and coma. These toxins and pesticides cause an increase in acetylcholine, which keeps the pupil contracted.
References
- MayoClinic.com: Horner's Syndrome
- Pacific Oregon University, College of Optometry: Pupil Anomalies: Reaction and Red Flags; Weon Jun, OD, FAAO
- UCLA Health System, Neurosurgical Diseases and Disorders: Carotid Dissection
- PubMed.gov/"PLoS Medicine"; Painful Horner Syndrome as a Harbinger of Silent Carotid Dissection; Amit Nautiyal et al.; Jan 2005
- CDC Agency for Toxic Substances and Disease Registry of the Centers for Disease Control and Prevention: Nerve Agents.
