Bilateral pupil contraction: symptoms and causes

2021-01-26 12:00 AM

Bilateral pupil shrinkage is a symptom of several neurological disorders and toxicity. The most common cause of pupil contraction in patients with impaired consciousness, or coma, or opioid poisoning. 

Description

The pupil is 2 sides, shrinking with a diameter <2mm.

Reason

Common

Opioid poisoning (eg. Morphine, heroin).

Old age pupil.

Less common

Haemorrhage of the brain.

Cholinergic poisoning (eg. Organic phosphate poisoning).

Hernia through the tent pointing up.

Clonidine poisoning.

"Beta-adrenergic antagonistic poisoning (eg. Carvedilol)."

Figure. Two-sided pupil shrinkage is less than 2mm in diameter and symmetrical.

 

Figure. Sympathetic, parasympathetic nerve distribution of iris muscles

Variation in pupil size (horizontal diameter) in the dark at different ages.

Mechanism

Causes of pupil contractions include:

Opioid poisoning.

Haemorrhage of the brain.

Cholinergic poisoning.

Clonidine poisoning.

Cerebral hernias with the pinched brain.

Poisoning to beta-blocker antagonists.

Old age pupil.

Opioid poisoning

The opioid binds to the central kappa-1 (κ1) receptor, causing pupil contraction. Not all opioids induce pupils due to the 1 receptor's attractive affinity. Patients taking meperidine, propoxyphene and pentazocine may not experience pupil contractions.

Haemorrhage of the brain

Meningococcal haemorrhage prevents sympathetic bundles from going down in the brain, leading to an inability to stop the parasympathetic flow and contracting the pupils of both sides. Associated symptoms include bilateral cranial nerve signs (eg, facial paralysis, VI cord paralysis), signs of long motor bundles, coma, and cerebral hernia.

Clonidine poisoning

Choline toxicity causes bilateral pupils to contract due to increased activity of the muscarinic receptors in the neuromuscular junctions. Muscarinic stimulation also causes diarrhoea, excessive urination, bradycardia, bronchial secretion, bronchospasm, rhabdomyolysis, increased tear secretion and digestive disorders. Causes of cholinergic poisoning include organophosphate and carbamate poisoning (eg insecticide poisoning).

Clonidine poisoning

Clonidine is an alpha-2 (α2), central receptor agonist. Inhibition of norepinephrine release decreases sympathomimetic flow, resulting in bilateral pupil contraction.

Cerebral hernias with compression of the brain

Cerebellar almond herniation and upward herniation cause bilateral pupil shrinkage due to compression of the brain. Ventral hernias are typically caused by dilation of the parietal or frontal lobe or a lesion of the occipital lobe. A cerebellar almond herniation is most likely caused by a mass in the cerebellum or by rapid displacement of the brain stem. A typical upward hernia is an enlargement of the posterior fossa lesion.

Beta-blocker toxicity

Beta-adrenergic antagonists relax the muscles of the pupil and cause the pupils to contract.

Old age pupil

With age, the pupil decreases in size and decreases the pupil dilating response under low light conditions.

Meaning

Bilateral pupil shrinkage is a symptom of several neurological disorders and toxicity. The most common cause of pupil contraction in patients with impaired consciousness, or coma, or opioid poisoning.