Miosis: definition, symptoms, diagnosis and treatment

By Cristiano Antonino On May 29, 2023

What is miosis? It is the decrease in the diameter of the pupil and is a completely physiological condition of the eye, the functioning of which involves a narrowing or dilation of the pupil depending on the light

When, for example, we enter a very dark room, the pupil dilates (mydriasis) to allow the eye to see better.

When, on the other hand, we are in the dark and suddenly turn on the light, the pupil constricts.

But miosis is not only a physiological issue that everyone experiences several times a day: it can also be a symptom of certain diseases or a consequence of taking certain medications.

How the eye works

The eye is made up of several parts, has six muscles, an optic nerve and several layers covering it.

The pupil is the central and front part of the eye, the famous black dot in the centre of the iris, and it is from here that, like a camera, light passes.

In fact, light passes through the cornea, the transparent film that covers the eye, and then continues through the pupil, crystalline lens and vitreous, finally arriving at the retina, from where visual stimuli depart and, transformed into electrical impulses, are communicated to the brain via the optic nerve.

The pupil is, therefore, fundamental for managing light and thus the images we see.

How much should the pupil measure

Generally speaking, a pupil measures 5 mm on average, but can also be as small as 2 mm.

Due to the effect of light, it dilates and shrinks continuously, a physiological condition that we are not normally aware of.

In the dark, due to the effect of dilation, the pupil can measure as much as 8 mm, while conversely, when passing from darkness to light, the pupil shrinks to measure as much as 1 mm.

Anisocoria

Anisocoria is the difference in the diameter of the two pupils. In some cases, especially if it has always been present, it can be physiological.

At other times, particularly if observed during gaze activity or light changes, it may be a symptom of brain diseases, such as aneurysm and brain tumour, but also meningitis or epilepsy, or be a consequence of trauma or intoxication.

Symptoms of miosis

If miosis is not of physiological origin, a chronic narrowing of the pupil, which can be as small as one millimetre, can be observed.

The only noticeable symptom of miosis is thus a change in the size of the pupils (or pupil in the case of anisocoria), which become smaller.

Miosis, if associated with other symptoms such as headaches, diplopia, blurred vision, nausea and vomiting, photophobia, eye pain and loss of vision, should be treated immediately by going to an ophthalmological emergency room.

Pathological miosis

When miosis is triggered by factors unrelated to light, i.e. the pupil suddenly appears smaller than normal, the cause may be pathological.

In particular, miosis is present in paralysis of the cervical sympathetic system, in brain haemorrhage, in encephalitis, in lesions of the eyeball or orbit.

It can be a symptom of various neurological lesions, such as destructive lesion of the sympathetic pathways, irritated lesion of the parasympathetic pathways of the third cranial nerve or destructive lesion of the inhibitory pathways that lead from the cortex and hypothalamus to the Edinger-Westphal nuclei.

Diseases that cause miosis

Among the most common diseases that cause miosis are:

uveitis, an inflammation of the eye involving its middle layer, including the iris
iridocyclitis, an inflammation involving the iris and ciliary body
Horner’s syndrome, a rare disease caused by lung cancer, stroke or brainstem damage
hypoglycaemic coma, the most severe phase of hypoglycaemia, i.e. the presence of very low blood sugar often linked to taking diabetes medication

Other causes of miosis

The cause of punctiform pupils is not only due to physiological or pathological factors, but may have to do with external factors such as intoxication, drug taking or exposure to harmful substances.

They cause miosis:

exposure to nerve gas and other organophosphates, such as those in some insecticides
intoxication from medicines and drugs, especially opiates and derivatives such as heroin or morphine
side effects of antispasmodics or antihistamines
mycotic intoxication, such as Muscarinic syndrome from ingestion of toxins present in mushrooms
use of miotic drops for glaucoma

How miosis is treated

Physiological miosis, of course, does not need to be cured but rather is synonymous with the proper functioning of the eyes that dilate and contract in response to light stimuli, just as a camera does.

When, on the other hand, miosis appears suddenly and does not disappear with light changes, an ophthalmologist should be consulted immediately.

When possible, go to the nearest emergency room specialising in ophthalmology.

Only in this way will it be possible to trace the origin of the problem and figure out how to treat it.

In some cases, particularly if caused by inflammation of the eyes, miosis is treated with eye drops containing atropine, a medicine that dilates the pupil.

If, on the other hand, intoxication is involved, a broader evaluation is carried out, which may also include other medical specialisations.

If the cause is medication, it may be necessary to discontinue it, but this is an indication that only a team of doctors can assess.

Miosis is a narrowing of the pupil, also called punctiform or pinpoint pupil.

It is a physiological condition of the eyes, which react to light by reducing its diameter, but it can also be the symptom or consequence of certain pathologies affecting the neurological system, certain diseases and, finally, the effect of intoxication or the intake of certain medications or drugs.

It is important to seek medical advice if bilateral or asymmetric myosis does not fit, in order to ascertain the cause and proceed immediately to the most appropriate treatment.

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