science

Mydriasis

Mydriasis is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mydriasis rather than just read about it. In short: Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drugs.

Key takeaways

Mydriasis belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mydriasis to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mydriasis from memory before moving on to harder problems.

Reference excerpt

Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drugs. It may also be of unknown cause. Normally, as part of the pupillary light reflex, the pupil dilates in the dark and constricts in the light to respectively improve vividity at night and to protect the retina from sunlight damage during the day. A mydriatic pupil will remain excessively large even in a bright environment. The excitation of the radial fibres of the iris which increases the pupillary aperture is referred to as a mydriasis. More generally, mydriasis also refers to the natural dilation of pupils, for instance in low light conditions or under sympathetic stimulation. Mydriasis is frequently induced by drugs for certain ophthalmic examinations and procedures, particularly those requiring visual access to the retina. Fixed, unilateral mydriasis could be a symptom of raised intracranial pressure. The opposite, constriction of the pupil, is referred to as miosis. Both mydriasis and miosis can be physiological. Anisocoria is the condition of one pupil being more dilated than the other.

Causes

There are two types of muscle that control the size of the iris: the iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The sphincter is controlled by nerves of the parasympathetic nervous system, and the dilator by the sympathetic nervous system. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil. The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which normally constricts the pupil) or overactivity of the sympathetic nervous system (SNS).

Pupil diameter also increases in reaction to cognitive tasks requiring memory and attention, and this phenomenon is used as an indicator of mental activation ('arousal') in psychophysiological experiments.

Drugs A mydriatic is an agent that induces dilation of the pupil. Drugs such as tropicamide are used in medicine to permit examination of the retina and other deep structures of the eye. Mydriatics typically also have a cycloplegic effect, reducing or paralyzing the accommodation reflex which may also be used for certain ophthalmic examinations or treatments, such as reducing painful ciliary muscle spasm. One effect of administration of a mydriatic is intolerance to bright light (photophobia). Purposefully-induced mydriasis via mydriatics is also used as a diagnostic test for Horner's syndrome. Mydriasis can be induced via modulation of adrenergic or cholinergic signalling. Drugs that can cause mydriasis include:

Stimulants (typically monoaminergics) such as amphetamines, cocaine, MDMA, and mephedrone. Anticholinergics such as diphenhydramine, atropine, hyoscyamine, and scopolamine antagonize the muscarinic acetylcholine receptors in the eye. Blocking acetylcholine receptors reduces the pupillary muscles' ability to constrict and causes dilation (which is critical in eye surgery procedures such as cataract surgery which require uninterrupted access to the inner eye via the pupillary aperture, thus requiring that the eye be both paralyzed and anesthetized before the procedure can go ahead). The antimuscarinic, tropicamide, may be used as a mydriastic agent during surgery. Serotonergics such as LSD, psilocybin mushrooms, mescaline and 2C-B. These drugs are typically hallucinogens. Similarly, selective serotonin reuptake inhibitors can cause mydriasis. Dissociatives such as dextromethorphan (an SSRI and sigma-1 agonist). Certain GABAergic drugs, such as phenibut and GHB. Adrenergic agonists, such as phenylephrine and cyclomydril. Adrenergic agonists may be used if strong mydriasis is needed in surgery. Norepinephrine is a hormone and neurotransmitter that regulates the involuntary muscles of the autonomic nervous system, including dilation of the pupil aperture via the muscles of the iris. Hence adrenergic agonists mimic the activity of norepinephrine, which is how they induce mydriasis. Natural release of the hormone oxytocin can cause the pupils to dilate to a greater degree to stimulus but perhaps not directly causative of mydriasis. Long term effects of drugs can also cause mydriasis, for example opioid withdrawal. It is not clear whether Marijuana use causes mydriasis, a survey of several studies produced conflicting findings and suggest the mydriatic effect can be related to other factors occurring with cannabis use.

Autonomic neuropathy Parasympathetic fibers travel with cranial nerve III, the oculomotor nerve, to innervate the circular layer of muscle of the eye (sphincter pupillae). Damage to this nerve typically manifests itself as mydriasis, because the sympathetic supply to the pupil, which causes mydriasis, remains unaffected, and therefore unopposed. Multiple central nervous system disorders e.g. epilepsy, stroke, and impending brain herniation are known to lead to temporal mydriasis as well. A brain catastrophe, or a rapidly increasing brain mass, can cause compression of the oculomotor nerve.

Trauma In cases of head injury or orbit trauma (eye injury), the iris sphincter (the muscle responsible for closing the pupil) or the nerves controlling it can be damaged, reducing or eliminating the normal pupillary light reflex.

References

See also Dilated fundus examination (Ophthalmology diagnostic procedure)

Illustrations

Worked examples

Example 1 — a first encounter with Mydriasis

Start with the simplest possible case. Write down what Mydriasis claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mydriasis before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mydriasis ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mydriasis

In research: Mydriasis appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mydriasis in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mydriasis is common in secondary-school and first-year university syllabi. It links to neighbouring topics Medical signs, Ophthalmology, so understanding it makes those chapters shorter.
In everyday life: Look for Mydriasis outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Mydriasis in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mydriasis means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mydriasis out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mydriasis in simple terms?

Why does Mydriasis matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mydriasis?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mydriasis.