Mydriasis is characterized by a prolonged abnormal dilation of the pupil. The pupil of mydriatic remains dilated or excessively large even in bright environment. Mydriasis is colloquially known as blown pupil.
The condition of prolonged abnormal dilation of the pupil is classified into two; unilateral mydriasis which affects one eye only while bilateral mydriasis affects both eyes where there is total loss of reaction to the light.
Mydriasis is known to have a pupil diameter of about 4mm which is more than 33% of the diameter of iris. Pupil is located in the center of the iris that allows light to enter the retina. The blackness in color of the pupil is due to the absorbed light by the tissue in the retina. Light entering the eye through the retina is regulated by the iris through regulation of the pupil size. The pupil size dilates or gets wider when exposed to dark environment. It gradually increases its diameter from 4mm to 9mm while it is adapting to the dark environment. Exposure to bright lights will constrict the pupil to prevent it from aberration of light rays and attain the expected acuity.
Mydriasis is not a serious health threat except when associated with other condition that might have caused it. This disorder normally goes away or subsides on its own although some people find it a discomfort and can hamper their daily activities.
People suffering from mydriasis will experience sensitivity to the eye and may complain of headache although there is no connection between headache and mydriasis unless there is an existing damage to the optic nerve.
Picture showing Full dilation of pupil (mydriasis)
Mydriasis may occur due to either physiologic or non-physiologic. Trauma, disease and disorders can cause the non-physiologic mydriasis and even some drugs can cause mydriasis.
- Injury of parasympathetic innervations causing paralysis of the pupil sphincter may result non-reaction of the pupil to light ray stimuli.
- Non-working afferent pupillary neurons can result to pupil sphincter dysfunction.
- Sexual arousal can cause a much enlarged pupils which are due to natural release of hormone oxytocin which happens at the peak or sexual excitement while orgasm can lead to pupillary block.
- Traumatic injury involving the head or the orbit can damage the iris sphincter which is responsible for regulating and controlling pupil size when reacting to light.
- Flaccid paralysis of the muscle called botulism can result to mydriasis as the neurotoxin called botulinum toxin paralyzes the nerves resulting to the inability of the muscle to contract.
- Cerebral edema can cause severe brain damage as a result of oxygen starvation to the brain.
- Adie’s syndrome is characterized by tonically dilated pupil that is caused by damage to postganglionic fiber of the parasympathetic innervations as a result of viral or bacterial infection causing inflammation that affects the pupil of the eye.
- Acute angle closure glaucoma pushes or pulls the iris to the walls of drainage channels at the angle of anterior chamber of the eye blocking the aqueous humor resulting to increase intraocular pressure.
There are certain drugs that can cause pupil dilation as a side effect. Anticholinergics provoke pupil dilation by blocking the muscarinic acetylcholine receptors in the eye. Hallucinogens antagonize pupil dilation by antagonizing the serotonergic 5-HT2A receptors in the brain.
There are few drugs that are also use to purposely induce pupil dilation and these are use for routine eye check up following a diagnostic procedure or surgical intervention. A pharmacologic agent called mydriatic is known to cause temporary pupil dilation. Tropicamide is used to dilate the pupil for the purpose of examining the retina and other deep structures of the eye including pain alleviation of ciliary muscle spasm. In surgical intervention where strong dilation is necessary, Phenylephrine is the drug of choice.
Mydriasis vs. Meiosis
Mydriasis is the abnormal prolonged dilation of the pupil in non-physiologic or even in physiologic cause. The exact opposite of this condition is an abnormality of prolonged constriction of the pupil which is called meiosis.
In mydriasis, the diameter of the pupil in dilation is 4mm or more while in miosis, the diameter of pupil in constriction is 2mm or less.
The pupil constricts in bright light exposure to prevent and protect the eye from the aberration of light ray and attain an expected acuity in bright environment.