Editing Mydriasis

{{short description|Excessive dilation of the pupil}}
{{Infobox medical condition (new)
| name          = Mydriasis
| synonyms      = Blown pupil<ref name=AANS>{{cite web|title=Traumatic Brain Injury|url=http://www.aans.org/Patient%20Information/Conditions%20and%20Treatments/Traumatic%20Brain%20Injury.aspx|publisher=American Association of Neurological Surgeons|access-date=27 March 2012}}</ref>
| image         = Dilated pupils 2006 (cropped 2).jpg
| caption       = Dilated pupils caused by mydriatic drops instilled for a [[dilated fundus examination]]
| pronounce     = {{IPAc-en|m|ɪ|ˈ|d|r|aɪ|.|ə|s|ɪ|s|,_|m|aɪ|-}}<ref>{{cite Dictionary.com|mydriasis}}</ref>
| field         = [[Ophthalmology]], [[neurology]]
| symptoms      = 
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'''Mydriasis''' is the [[Pupillary dilation|dilation]] of the [[pupil]], usually having a non-physiological cause,<ref>[http://medical-dictionary.thefreedictionary.com/mydriasis Mydriasis] in Farlex medical dictionary. In turn citing:
* The American Heritage Medical Dictionary (2007)
* Mosby's Dental Dictionary, 2nd edition.</ref> or sometimes a physiological [[pupillary response]].<ref>[http://medical-dictionary.thefreedictionary.com/mydriasis Mydriasis] in Farlex medical dictionary. In turn citing: Mosby's Medical Dictionary, 8th edition.</ref> Non-physiological causes of mydriasis include [[disease]], [[Physical trauma|trauma]], or the use of certain types of [[drug|drugs]]. It may also be of unknown cause.

Normally, as part of the [[pupillary light reflex]], the pupil dilates in the [[dark]] and [[miosis|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 [[Ophthalmology|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==
{{More citations needed section|date=August 2014}}
[[File:Mydriasis due to LSD usage.jpg|thumb|300x300px|Both pupils are dilated (mydriasis) because of hyper-activation of the [[sympathetic system]] caused by the psychedelic drug [[LSD]]]]
[[File: Human eye in dim light.jpg|thumb|300x300px|Mydriasis with the pupil dilated to 9mm in diameter as a result of dim lighting. Most individuals are not able to achieve pupillary dilation to this extent without mydriatic agents]]
There are two types of [[muscle]] that control the size of the [[Iris (anatomy)|iris]]: the [[Iris sphincter muscle|iris sphincter]], composed of circularly arranged muscle fibers, and the iris [[Iris dilator muscle|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  [[Muscle contraction|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).
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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.<ref>Koss MC. Pupillary dilation as an index of central nervous system alpha 2-adrenoceptor activation. J Pharmacol Methods. 1986;15:1–19. [PubMed]...Peavler WS. Pupil size, information overload, and performance differences. Psychophysiology. 1974;11:559–566. [PubMed]...Laeng B, Sirois S, Gredebäck G. Pupillometry: a window to the preconscious? Perspect Psychol Sci. 2012;7:18–27. [PubMed]...Kloosterman NA, Meindertsma T, van Loon AM, Lamme VA, Bonneh YS, Donner TH. Pupil size tracks perceptual content and surprise. Eur J Neurosci. 2015;41:1068–1078. [PubMed]</ref>

=== Drugs ===
A '''mydriatic''' is an agent that induces [[Pupillary dilation|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 [[Cycloplegia|cycloplegic]] effect, reducing or paralyzing the [[accommodation reflex]] which may also be used for certain ophthalmic examinations or treatments, such as reducing painful [[Spasm of accommodation|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 [[norepinephrine|adrenergic]] or [[acetylcholine|cholinergic]] signalling.

Drugs that can cause mydriasis include:
* [[Stimulant]]s (typically monoaminergics) such as [[amphetamine]]s, [[cocaine]], [[MDMA]], and [[mephedrone]].
* [[Anticholinergic]]s such as [[diphenhydramine]], [[atropine]], [[hyoscyamine]], and [[Hyoscine hydrobromide|scopolamine]] antagonize the [[muscarinic acetylcholine receptor]]s 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.<ref name="Saenz-de-ViteriGonzalez-Salinas2016">{{cite journal|last1=Saenz-de-Viteri|first1=Manuel|last2=Gonzalez-Salinas|first2=Roberto|last3=Guarnieri|first3=Adriano|last4=Guiaro-Navarro|first4=María Concepción|year=2016|title=Patient considerations in cataract surgery – the role of combined therapy using phenylephrine and ketorolac|journal=Patient Preference and Adherence|volume= 10|pages=1795–1801|doi=10.2147/PPA.S90468|issn=1177-889X|pmc=5029911|pmid=27695298 |doi-access=free }}</ref>
* [[Serotonin|Serotonergic]]s such as [[LSD]], [[psilocybin mushrooms]], [[mescaline]] and [[2C-B]]. These drugs are typically [[hallucinogens]]. Similarly, [[selective serotonin reuptake inhibitor]]s can cause mydriasis.
* [[Dissociative]]s such as [[dextromethorphan]] (an SSRI and sigma-1 agonist).
* Certain [[GABAergic]] drugs, such as [[phenibut]] and [[GHB (drug)|GHB]].
* [[Adrenergic agonist]]s, such as [[phenylephrine]] and [[cyclomydril]].<ref>{{cite web|url=https://www.drugs.com/pro/cyclomydril.html|title=Cyclomydril - FDA prescribing information, side effects and uses}}</ref> Adrenergic agonists may be used if strong mydriasis is needed in surgery.<ref>"Common eye diseases and their management", Galloway/Amoako/Browning, Springer science 2006, 3rd edition, p196</ref> [[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 (anatomy)|iris]]. Hence adrenergic agonists mimic the activity of norepinephrine, which is how they induce mydriasis.

Natural release of the hormone [[oxytocin]] can cause mild to moderate mydriasis.{{Citation needed|date=April 2017}}

Long term effects of drugs can also cause mydriasis, for example [[opioid]] withdrawal.

Marijuana can cause mydriasis.

===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 nervous system|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 [[eye injury|orbit trauma (eye injury)]], the [[Iris sphincter muscle|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==
{{Reflist}}

==See Also==
* [[Dilated fundus examination]] (Ophthalmology diagnostic procedure)

== External links ==
{{Medical resources
|   ICD10       = {{ICD10|H|57|0|h|55}}
|   ICD9        = {{ICD9|379.43}}
|   DiseasesDB  = 8603
|   OMIM        = 159420
|   OMIM_mult   = {{OMIM|159410||none}} {{OMIM|106240||none}}
| MeSH=D015878
}}
{{Eye pathology}}

[[Category:Medical signs]]
[[Category:Ophthalmology]]