Miosis is characterized by a decrease in the diameter of the pupil. If this is a normal phenomenon, it can also be indicative of certain pathologies. How to make the difference ? What are the symptoms and treatments? State of play with Dr Nathalie Butel, medical and surgical ophthalmologist.

Miosis is a narrowing of the pupil. “Our pupils have a photomotor reflex, which explains why, when there is light, we are in miosis, and when it is night, we are in mydriasis, that is to say that our pupils dilate. », Says Dr Nathalie Butel.

This reduction in the diameter of the pupil is a reflex and physiological contraction which occurs in the face of too much light. Miosis can be a telltale sign of pathology when the pupil remains tight in the dark since the pupil normally dilates in the dark. If it doesn’t expand at this point, it stays tight and that’s not normal.

Several causes can be at the origin of miosis:  

• A physiological mechanism: the pupil adapts to light by shrinking. This contraction of the iris muscles is caused by the ocular parasympathetic system. This is a natural phenomenon;
• Taking certain medications: pilocarpine (eye drops used to treat glaucoma), as well as morphine and its derivatives are likely to cause miosis;  
• Inflammation or infection of the eye;
• Neurological pathology;
• A birth defect called microcoria;
• Claude Bernard Horner syndrome: miosis is then associated with ptosis (sagging eyelid) and enophthalmos (tiny eye).

Miosis as such does not cause any symptoms. 

“You can completely miss a miosis, it’s not easy. Either the patient will realize it himself, or it is his entourage who will point out to him that his pupils are not symmetrical, one being very small and the other larger. But in terms of feeling, there is nothing at all, the fact of having the pupil tight does not involve sensory modification. The miosis in itself is not associated with a loss of vision or the like, or else it is the underlying cause which generates it, but the miosis in itself is completely asymptomatic ”, details the ophthalmic surgeon .

Diagnosis of miosis is primarily clinical. The ophthalmologist will observe an asymmetry at the level of the pupils and, depending on the ophthalmological examination, he will be able to look for the cause, and determine whether it is pathological or not. 

“If the ophthalmological examination does not make it possible to make the diagnosis, additional neurological and vascular examinations will be carried out to understand the origin of this miosis”, specifies the specialist.

Miosis as such does not require treatment. It is not painful or embarrassing, you have to treat the problem that caused it.

“For example, one of the most serious causes of miosis is what’s called a carotid dissection, which is a dissection of one of the large vessels in the neck. This leads to miosis associated with ptosis (lower eyelid), ”underlines Dr Nathalie Butel.

If carotid dissection is suspected, CT angiography is performed. In milder cases, miosis may be due to Adie’s pupil which results in having a less responsive pupil. There is a slow contraction and dilation of the pupils. This is linked to aging and is not serious.