Mydriasis: definition, symptoms, causes, diagnosis and treatment

By Cristiano Antonino Last updated May 29, 2023

Mydriasis consists of a dilation of the pupil above 5 millimetres. Usually the pupil, i.e. the central part of the iris, which is black in colour, changes diameter depending on the light

Like in a camera, its purpose is to adjust itself, through the muscles, to let light in: when passing from a very bright environment to a darker one, the pupil naturally enlarges to allow light to enter better and, consequently, to see better.

Characteristics of mydriasis

When mydriasis is a physiological effect of light changes perceived by the eye, it is bilateral: both eyes react in the same way and the pupils dilate to the same extent.

In this case one also speaks of reactive mydriasis, i.e. reacting to changes in light, allowing one to see better in the dark.

In other cases, however, the abnormality may be unilateral, i.e. affecting only one pupil.

In this case we also speak of anisocoria, i.e. when the pupils are asymmetrical and have different diameters.

Mydriasis can also be fixed, when it does not change size according to various lighting situations but always remains larger than it should be.

How large should the pupil measure

A normal pupil has an average size of 5 mm, although in some cases it can be smaller.

Below 2 mm, however, we speak of miosis or punctiform pupils, i.e. the condition whereby the pupils are smaller than normal.

This occurs, just as with mydriasis, in a natural way in reaction to changes in light, when one goes from a very dark environment to a brighter one and the pupil muscles contract to limit the entry of light.

When, on the other hand, the pupil exceeds 5 mm in diameter, we speak of mydriasis.

Causes of mydriasis

As we have already mentioned, mydriasis is an entirely physiological effect of the visual apparatus reacting to different light stimuli.

In some cases, however, a pupil with a diameter of more than 5 mm may be the consequence of pathologies or other conditions, whether unilateral or bilateral.

Generally speaking, one realises one is dealing with an abnormal pupil size because the pupil, despite the fact that light does not change, remains very large.

The causes may be pathological or external, linked to the intake of medicines, drugs or alcohol.

Pathologies linked to an increase in pupil diameter

Mydriasis can be an alarm bell when, just like miosis, it remains fixed and does not fall within parameters considered normal.

Some pathologies cause an enlargement of the pupil: they can be eye diseases or of a different nature.

Eye diseases related to mydriasis include

glaucoma, an optic nerve damage caused by a constant and significant increase in intraocular pressure
Adie’s tonic pupil, also known as Adie’s syndrome, a neurological disorder in which the pupil responds more slowly to light stimuli
iridoplegia, i.e. paralysis of the iris sphincter causing loss of pupillary response
paralysis of the oculomotor nerve, caused by pressure on the nerve, which therefore does not receive enough blood
spasm of the longitudinal iris fibres, i.e. the so-called scleral tensor
amaurosis, the loss of vision in only one eye due to a sudden and non-permanent hypoperfusion of the eyeball
amblyopia, also known as ‘lazy eye’, i.e. reduced vision in one or both eyes

Other causes of mydriasis related to the visual organ are irritation of the sympathetic nervous system (the one involved in the control of involuntary bodily functions, such as the reaction to light) and the abolition of the iris reflex.

Other pathologies

Other pathologies not directly related to visual function that cause a disproportionate increase in pupil diameter include:

botulism, an infection caused by the bacterium Clostridium botulinum, whether food-borne, wound-borne or childhood-borne
cardiac arrest
Reye’s syndrome, a rare form of acute encephalopathy associated with liver dysfunction that particularly affects children and young people under 18 years of age
Serotonergic syndrome, usually caused by the use of certain drugs, intoxication or interactions
cerebral strokes, which have an ischaemic or haemorrhagic basis
mydriasis caused by external factors

In other cases, the increase in the diameter of one or both pupils may be caused by other non-pathological factors, some of which must be considered serious.

Drugs that cause pupil dilation

So-called mydriatic substances, i.e. drugs that cause pupil dilation, are normally used in ophthalmology in the form of eye drops for topical use, e.g. in preparation for certain tests.

Generally, these drugs have a different duration of action:

Tropicamide, used to dilate the pupil and paralyse certain eye muscles, the effect of which is short-lived
Cyclopentolate, also used to dilate the pupil, particularly in the case of diagnostic or surgical procedures on the eyes or to treat certain inflammatory diseases, it has a long-lasting action
Atropine, which not only causes mydriasis but is also capable of paralysing visual accommodation (cyclopegia) and has a very long duration (up to 12 days), so it is only used in specific cases

Drugs and mydriasis

Pupil dilation is also caused by the use of certain drugs other than those that cause mydriasis.

Among the most common drugs that cause mydriasis are:

cocaine
amphetamines
cannabis
ecstasy

Other factors that cause pupillary dilation

Other factors that can cause mydriasis include alcohol abuse, but also head trauma, asphyxia and, in some cases, strong emotions.

Symptoms of mydriasis

Recognising mydriasis is quite simple, as it involves noticing that one or both pupils have an unusual, larger diameter, even in the absence of light changes.

If, after some time, you notice that the situation does not return, it is very likely that you are dealing with mydriasis.

Furthermore, mydriasis causes glare and blurred vision, but depending on the cause you may also have other complaints.

If you have taken medication, drugs or alcohol it could be related to this, but if you have other symptoms such as headaches or loss of vision it could be an ongoing condition.

In any case, it is essential that you go immediately to the nearest ophthalmological emergency room or request an emergency eye examination from your doctor.

How is mydriasis treated

In most cases mydriasis is reactive in nature and therefore physiological: if you notice your pupils larger than usual, it is very likely that they have reacted by dilating to a change in light.

When, on the other hand, with the passage of time the pupils do not regain a regular size, i.e. below 5 millimetres, it is necessary to consult an ophthalmologist to understand the cause and treat it appropriately.

If the mydriasis is due to the intake of drugs, medication or alcohol, discontinuing these will usually lead to an adjustment of the pupil size, which will naturally return to within the parameters considered normal or, at most, the doctor will envisage the use of a topical eye drops to shrink the pupil, e.g. based on pilocarpine, an alkaloid that can cause mydriasis.

If, on the other hand, it is an ophthalmological problem, the doctor will understand the source and choose the most appropriate treatment to cure the condition.

Mydriasis is the dilation of the pupil from its normal size beyond a diameter of 5 mm.

Most of the time it is a reaction: each of us experiences changes in pupil size every day due to light stimuli, but in other cases it can be a symptom of some more or less serious eye, nervous system or other diseases that should not be underestimated.

The pupil also dilates with the intake of certain medications, drugs or alcohol, but in any case it is always a good idea to consult an ophthalmologist if you notice that your pupils, or even just one of them, are larger than usual.

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