Anatomy, Head and Neck, Eye Iris Sphincter Muscle


Article Author:
Jeffrey Bloom
Mahsaw Motlagh


Article Editor:
Craig Czyz


Editors In Chief:
Jasleen Jhajj
Cliff Caudill


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Frank Smeeks
Kristina Soman-Faulkner
Radia Jamil
Patrick Le
Sobhan Daneshfar
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Pritesh Sheth
Hassam Zulfiqar
Navid Mahabadi
Steve Bhimji
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Mark Pellegrini
James Hughes
Beata Beatty
Nazia Sadiq
Hajira Basit
Phillip Hynes
Tehmina Warsi


Updated:
6/6/2019 3:31:30 PM

Introduction

The iris sphincter muscle, also known as the pupillary sphincter or sphincter pupillae, is a muscle located in the colored part of the eye called the iris. The sphincter muscle fibers are located near the pupillary margin and are slightly anterior to the pigmented epithelium of the iris. It encircles the pupil of the iris and functions to constrict the pupil in bright light via the pupillary light reflex or during accommodation. By controlling the diameter of the pupil, the iris controls the amount of light that reaches the back of the eye at the retina.

Structure and Function

The iris is a circular, pigmented diaphragm that lies anterior to the lens in a coronal plane. It is attached peripherally to the ciliary body, with its unbound central border forming an aperture known as the pupil. The iris divides the space between the cornea and the lens into two chambers: anterior and posterior. The anterior chamber is bound by the cornea and iris. The anterior chamber communicates through the pupil with the posterior chamber, which is bound by the iris, ciliary processes, zonules, and lens. A fluid substance known as aqueous humor fills both chambers.

The iris, via the sphincter pupillae, is a part of the accommodative reflex. The accommodative reflex of the eye is an involuntary response when visual focus switches from an object at a distance to one that is closer. The reflex is controlled by the parasympathetic nervous system, and involves three actions - alteration of pupil aperture, lens shape, and convergence. The sphincter pupillae causes narrowing of the pupil in order to prevent diverging light rays from the corneal periphery creating a blurred image.

Embryology

The vertebrate eye is made up of tissue from various embryonic origins. The lens and the cornea originate from the surface ectoderm. The retina and the epithelial layers of the iris and ciliary body originate from the anterior neural plate.[1] Within the stroma of the iris, the sphincter pupillae and dilator pupillae muscles develop from optic cup neuroectoderm, along with the iris epithelium.  In contrast, the ciliary muscle, which is responsible for changing the shape of the lens, is derived from overlying mesenchyme. The optic cup is neuroectodermal in origin and gives rise to the neuronal retina, the retinal pigmented epithelium, the epithelial layers of ciliary body and iris, and the iris musculature.[2]

Blood Supply and Lymphatics

Ocular blood flow is controlled both via direct autonomic influences on the vasculature of the optic nerve, choroid, ciliary body, iris, and indirect influences on retinal blood flow.[3] The iris is supplied by the anterior ciliary arteries, the long posterior ciliary arteries, and anastomotic connections from the anterior choroid. The majority of the venous drainage from the anterior segment of the eye drains into the choroid posteriorly and then into the vortex veins.

Nerves

The iris sphincter muscle receives its parasympathetic innervation via the short ciliary nerves which lead to pupillary constriction (miosis) and accommodation.[3] The parasympathetic fibers that serve the sphincter muscle originate from the Edinger-Westphal nucleus of cranial nerve III. The signal synapses in the ciliary ganglia become terminated on muscarinic receptors of the muscle fibers.

Reciprocal function is largely the responsibility of sympathetic innervation, which comes from the superior cervical ganglia. The sympathetic fibers help promote sphincter muscle relaxation, which is critical for periods of low-light or night vision. Sympathetic innervation from the superior cervical ganglion project to the dilator muscle of the iris via long ciliary nerves to control pupil dilation. The sympathetic and parasympathetic systems work in opposition to constantly adjust pupillary aperture.[3]

Muscles

There are two muscles of the iris and they both consist of smooth muscle. The sphincter muscle contracts the pupil in a circular motion while the dilator muscles enlarge the pupil by pulling the iris radially. An area called the iris collarette is the thickest portion of the iris where the sphincter muscle and dilator muscle overlap. While the collarette is typically flat, there are cases in which this structure can be rather prominent.[4]

Physiologic Variants

Aniridia is characterized by iris hypoplasia and has been shown to be associated by mutations in the PAX6 gene.[5] Usually involving both eyes, it can be congenital or caused by a traumatic injury to the eye. Isolated aniridia is a condition that can be associated with low vision, macular or optic nerve hypoplasia, cataracts, and cornea changes. Aniridia also can be associated with WAGR syndrome or Gillespie syndrome. [6][7]

Surgical Considerations

Surgery to the iris sphincter is done for functional or cosmetics issues. In general, surgery is warranted if the patient is having photophobia or blurry vision that cannot be controlled with drops. Patients can use pilocarpine drops to decrease the size of their pupils if some function of the sphincter remains. Reading glasses may also be necessary if the patient cannot accommodate. If inflammation is present after a trauma, the patient should be treated with steroid drops until resolved and re-assessed for sphincter function.

Clinical Significance

Adie tonic pupil is a neuro-ophthalmological disorder distinguished by a lack of response to light and a pupil that is tonically dilated but is able to respond to accommodation.[8] It is usually a benign entity. A variety of causes (usually infectious) lead to Adie tonic pupil and result from damage to postganglionic fibers of the parasympathetic innervation to the eye which results in an inability to constrict the iris sphincter muscle. It is more common in women and manifests unilaterally in 80% of cases.[9] Adie’s tonic pupil has been reported in the literature to be associated with a number of associated syndromes, including Adie syndrome, which involves diminished deep tendon reflexes.

Iridoplegia is the paralysis of the sphincter muscle of the iris. Many times, it is a result of direct trauma to the orbit but can also result from inflammation or even from eyedrops. There are three types of iridoplegia:

  1. Accommodative iridoplegia: The inability of the pupil to constrict during accommodation
  2. Complete iridoplegia: Will not constrict regardless of stimulation
  3. Reflex iridoplegia: Also known as Argyll Robertson pupil; will not constrict to light, but retains the ability for accommodation reflex

It is important to differentiate the ptosis in Horner syndrome from damage to the oculomotor nerve. In Horner syndrome, the ptosis occurs with a constricted pupil due to loss of sympathetic innervation to the eye (inability to dilate leads to constriction). This is compared to damage to the oculomotor nerve where the ptosis is accompanied by a dilated pupil (due to damage to the parasympathetics and inability to constrict via the sphincter pupillae).

Other Issues

An iris coloboma is a hole in the iris usually due to incomplete closure of the fetal fissure during organogenesis. Colobomas can range from a simple hole in the iris to more severe involvement of the posterior pole, a coloboma of the optic nerve of the choroid of the retina.[10] Colobomas also have a clinical association with CHARGE syndrome, which is a rare heritable disease cause by mutation in the CHD7 gene that regulates chromatin remodeling.[11][12]

Traumatic mydriasis is usually the result of blunt injury to the eye and can present with iris sphincter tears that alter the shape of the pupil in the affected eye. Treatment initially involves cycloplegia to prevent posterior synechiae in patients with iritis. Patients are then observed clinically for the return of function. If the function does not return, and the patients are symptomatic, surgical intervention can be considered. Ocular trauma can also cause pupillary miosis though this is less common than mydriasis and tends to be related to anterior chamber inflammation.


  • Image 8581 Not availableImage 8581 Not available
    Image courtesy S Bhimji MD
Attributed To: Image courtesy S Bhimji MD

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Anatomy, Head and Neck, Eye Iris Sphincter Muscle - Questions

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Which of the following does not supply parasympathetic innervation to the sphincter pupillae muscle?



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Which of the following is not typical of a smooth muscle fiber found in the sphincter pupillae muscle of the eye?



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Which muscle(s) do not develop from mesenchyme?



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From which embryologic layer does the iris sphincter muscle arise?



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A 37-year-old man received a blow to the orbit during a fight. A few hours later in the emergency department, the patient's pupils constrict to light but cannot constrict while focusing on near objects. What is the most likely diagnosis?



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Which of the following is not part of the blood supply of the iris?



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A newborn is admitted to the NICU for cyanosis that is most prominent during feeding. When the feed is withdrawn, and the baby cries, he appears to pink up with resolution of cyanosis. Attempts to pass a suction catheter through the nose fail. On exam, there is missing tissue from the iris. Which of the following is also most likely to be present on examination?



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A 14-month-old girl is brought to the clinic by her mother, who recently felt an abdominal mass on her daughter during a bath. On further questioning, the mother notes that the patient’s cousin was also recently found to have an abdominal mass. Physical exam shows an absence of the irises. Abdominal examination reveals a palpable mass on the left flank that does not cross the midline. Which of the following in the most likely diagnosis?



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A 68-year-old man presents to the emergency department with severe left eye pain associated with nausea and vomiting. His symptoms began a few hours ago and have progressively worsened. Past medical history is unremarkable. Initial assessment reveals markedly diminished visual acuity of the left eye. Which of the following is most expected on the ocular exam of the patient?



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A 68-year-old male presents to the emergency department with right eyelid drooping. Past medical history is significant for hypertension and chronic bronchitis. The patient smokes a pack per day and describes himself as a social drinker. Physical exam shows right eyelid drooping and an abnormally small pupil in the right eye. The left side of the face appears normal. Which of the following is the most likely location of the suspected lesion?



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A 37-year-old male presents to the emergency department following a bar fight. Initial imaging confirms an infraorbital fracture. During the examination, it is noted that the patient’s pupils constrict to light but cannot constrict while focusing on near objects. Which of the following in the most likely diagnosis?



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Anatomy, Head and Neck, Eye Iris Sphincter Muscle - References

References

McDougal DH,Gamlin PD, Autonomic control of the eye. Comprehensive Physiology. 2015 Jan     [PubMed]
Graw J, Eye development. Current topics in developmental biology. 2010     [PubMed]
Tamm ER,Ohlmann A, [Development of the human eye]. Der Ophthalmologe : Zeitschrift der Deutschen Ophthalmologischen Gesellschaft. 2012 Sep     [PubMed]
Karadžić J,Jaković N,Kovačević I, Unilateral Adie's Tonic Pupil and Viral Hepatitis - Report of Two Cases. Srpski arhiv za celokupno lekarstvo. 2015 Jul-Aug     [PubMed]
Jaja Z,Laghmari M,Lazrek O,Daoudi R, [Isolated Adie pupil: about a case]. The Pan African medical journal. 2015     [PubMed]
Yahalom C,Blumenfeld A,Hendler K,Wussuki-Lior O,Macarov M,Shohat M,Khateb S, Mild aniridia phenotype: an under-recognized diagnosis of a severe inherited ocular disease. Graefe's archive for clinical and experimental ophthalmology = Albrecht von Graefes Archiv fur klinische und experimentelle Ophthalmologie. 2018 Aug 30     [PubMed]
Zerkaoui N, [Bilateral iris coloboma]. The Pan African medical journal. 2018     [PubMed]
Li S,Liang L, Protruding Iris Collarette. The New England journal of medicine. 2017 Mar 16     [PubMed]
Fischbach BV,Trout KL,Lewis J,Luis CA,Sika M, WAGR syndrome: a clinical review of 54 cases. Pediatrics. 2005 Oct     [PubMed]
Hall HN,Williamson KA,FitzPatrick DR, The genetic architecture of aniridia and Gillespie syndrome. Human genetics. 2018 Sep 22     [PubMed]
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