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Pinpoint pupils (also known as abnormal miosis) or unusually small pupils often have one of three causes: excessive use of opiate class drugs, a hemorrhage at the brain stem, or exposure to chemicals such as organophosphates or nerve agents. These are the most commonly seen causes of the condition. In few cases, pinpoint pupils could also be linked to Horner's Syndrome or neurosyphilis.
In normal settings, the pupil is round and medium sized when compared to the rest of the iris. It controls the amount of light that gets into the eye by constricting or dilating, and three different types of photoreceptors in the eye—ganglion, rod and cone cells—convert the available light to electrical impulses so a person can see. In a pitch-black room, when a light is shone directly into it, it can constrict to the size of a pinpoint. As it adjusts to the light, however it should dilate, though not quite to its normal size.
Abnormal miosis refers to constriction less than 0.0079 inches (2 mm) in normal light settings. The term miosis is from ancient Greek, meaning “to close the eyes.” Scientists think the condition is related to ganglion cells in the eye reacting too slowly compared to rod and cone cells.
The use of opioid drugs is a common reason for vision issues. These substances are psychoactive, meaning they can cross the blood-brain barrier and, through the central nervous system, affect mental processes. They include well-known opiates that come from the poppy plant such as morphine and oxycodone.
Medically, opioids are used to control pain. Many people also use them for fun because of the fact they can alter states of consciousness and perception. The appearance of constricted pupils is a symptom of overdose and can alert others to opioid use. In some cases, people who have abused opioids heavily develop drug tolerance and might have this symptom even if they aren’t overdosing.
Benzodiazepines, better known on the street as benzos, are another class of psychoactive drugs. Although they aren’t considered safe during pregnancy, doctors often prescribe them for problems like anxiety, panic disorders, insomnia and seizures. Similar to opioids, they can affect the nervous system and how the eye works.
The term myotic drug covers any drug that makes the pupils constrict, so it includes opioids and benzodiazepines. Some myotic drugs do not fall into these categories, however. A good example is carbachol, which eye doctors routinely use in the treatment of conditions such as glaucoma. It usually comes in the form of eyedrops.
Exposure to certain chemicals, including organophosphates, is another source of eye problems. The chemicals overexcite the parasympathetic system. These types of substances appear as ingredients in pesticides. When a person gets too much exposure to them, he might experience symptoms such as muscle cramps, an increase in body fluid and pinpoint pupils. Someone who knows they have been using pesticides with these symptoms should go to a doctor to be examined for pesticide poisoning.
Brain hemorrhaging and pontine infarctions (death of tissue in the pons area of the brain due to lack of blood supply) are additional causes of extreme constriction of the pupils. These can happen with traumatic brain injury, stroke or illness. If the issue is at the front of the brain stem, both typically will be constricted. If the injury is in other areas of the brain, they may be different sizes, with one dilated and the other constricted. When a person has a history of stroke or other brain trauma, it’s best to get checked for these conditions.
Sometimes bacterial or viral infections cause pinpoint pupils. An example is uveitis, which affects the middle part of the eye including the iris. Depending on the type of infection and how bad it is, a person can develop inflammatory adhesions between the lens and the iris. These essentially cause tissue in the eye to stick together, which makes it difficult for expansion or contraction to happen as it normally would.
Not all infections that lead to this condition are concentrated in the eye itself. In neurosyphilis, for instance, the brain or spinal cord is affected, which changes how well a person can handle sensory information and react to light. This condition is caused by Treponema pallidum, the same bacteria that cause syphilis.
Eye conditions occasionally are due to a rare physical disorder such as Horner syndrome, which affects the nerves in the face and eye. Horner syndrome is usually the result of an underlying medical issue such as a stroke, but sometimes it occurs alone. Pinpoint pupils are a classic symptom of the disorder, along with drooping eyelids and a reduced ability to perspire on the affected side of the face. Babies born with the syndrome will have one iris lighter than the other.
A lot of information has been collected here. Since I am a neurology researcher, it is very useful.