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Miosis: Pupil Constriction – Causes & Treatment

Miosis is the constriction of the pupil of the eye to a diameter of less than 2 mm. It can occur naturally in bright light or be caused by medications, drugs, or neurological conditions.

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Things worth knowing about "Miosis"

Miosis is the constriction of the pupil of the eye to a diameter of less than 2 mm. It can occur naturally in bright light or be caused by medications, drugs, or neurological conditions.

What is Miosis?

Miosis refers to the constriction (narrowing) of the pupil to a diameter of less than 2 mm. The size of the pupil is controlled by two muscles within the iris: the sphincter pupillae muscle, which constricts the pupil, and the dilator pupillae muscle, which widens it. Miosis occurs when the sphincter muscle dominates. It can be physiological (normal), pharmacological (caused by drugs or medications), or pathological (caused by disease).

Causes

Physiological Causes

Light exposure: The most common cause of miosis is increased light intensity. The pupillary light reflex automatically constricts the pupil in bright light to protect the retina.
Near response (accommodation): When focusing on close objects, the pupil also constricts as part of the accommodation reflex.
Aging: In older adults, a physiological miosis known as senile miosis may occur due to age-related changes in iris muscles.

Pharmacological Causes

Opioid analgesics (e.g., morphine, heroin): These substances activate the parasympathetic nervous system and produce characteristic pinpoint pupils.
Miotic eye drops (e.g., pilocarpine): Used in the treatment of glaucoma to lower intraocular pressure.
Cholinergic agents and anticholinesterase inhibitors (e.g., organophosphates, physostigmine).
Antipsychotics and certain antidepressants.

Pathological Causes

Horner syndrome: Damage to the sympathetic nerve pathway leads to miosis, drooping of the upper eyelid (ptosis), and sinking of the eyeball (enophthalmos) on the affected side.
Pontine lesions: Hemorrhages or infarcts in the brainstem (pons) can cause bilateral, pronounced miosis.
Iritis / Uveitis: Inflammation in the anterior segment of the eye can reflexively trigger miosis.
Organophosphate poisoning (pesticides, nerve agents): A classic sign of a cholinergic crisis.

Symptoms and Clinical Presentation

Miosis itself is a clinical sign, not a standalone disease. The pupil appears very small and may respond less strongly to light stimuli. Associated symptoms depend on the underlying cause and may include:

Blurred vision or reduced contrast sensitivity in low-light conditions
Ptosis (drooping of the upper eyelid) in Horner syndrome
Pain, redness, or photophobia in inflammatory causes
Neurological symptoms in brainstem lesions (dizziness, double vision, swallowing difficulties)
Altered consciousness in opioid intoxication

Diagnosis

Miosis is diagnosed through a clinical examination of the eye, which typically includes:

Pupillometry: Assessment of pupil size and symmetry (isocoria vs. anisocoria).
Pupillary reflex testing: Evaluation of the direct and consensual light response.
Slit-lamp examination: Assessment of the anterior eye segment for signs of inflammation.
Neurological examination: Performed when a central cause is suspected (brainstem lesion, Horner syndrome).
Imaging (MRI/CT): In cases of suspected cerebral or vascular causes.
Toxicological screening: When poisoning or drug use is suspected.

Treatment

Treatment is directed at the underlying cause of miosis:

Pharmacological miosis due to opioid overdose: Administration of naloxone as an antidote.
Organophosphate poisoning: Treatment with atropine and pralidoxime.
Horner syndrome: Management of the underlying condition (e.g., tumor, vascular injury).
Iritis/Uveitis: Topical corticosteroids and mydriatic eye drops (pupil-dilating agents).
Glaucoma-related miosis from pilocarpine: A desired therapeutic effect to reduce intraocular pressure.

References

Kanski, J. J. & Bowling, B. (2016). Clinical Ophthalmology: A Systematic Approach. Elsevier.
Bhatti, M. T. & Bhatti, A. (2017). Pupillary disorders: Diagnostic approach. Seminars in Neurology, 37(5), 508–518. PubMed PMID: 29207412.
World Health Organization (WHO) (2022). Management of substance abuse: Opioid overdose. Retrieved from https://www.who.int/substance_abuse/information-sheet/en/

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