Iris: physiological and pathological phenomena

The iris represents the front visible part of the uvea – the middle covering of the eye. The iris has a pupillary opening in its central part – the pupil. Through the pupillary opening, aqueous humor passes from the posterior to the anterior chamber, envelops the cornea and is filtered through trabecular beds in the iridocorneal angle (Figure 1). In addition, the iris regulates the amount of light that enters the eye through the pupil opening – the role of the diaphragm – aperture.

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 1. Transverse section of the lens, ciliary body; relationship with other structures of the anterior segment of the eye; flow of aqueous humor

Inside the stroma of the iris, which is made up of loose connective tissue, there are: chromatocytes-pigment cells, muscle fibers, blood vessels and nerves.

The radial direction of blood vessels within the loose stroma provides a drawing of the anterior surface of the iris.

There are two muscles in the stroma of the iris: circular with concentric fibers around the pupillary opening (m. sphincter pupillae) and radial fibers (m. dilatator pupllae). M. sphincter papillae is innervated by the parasympathetic and by contracting leads to the narrowing of the pupil – miosis. M. dilatator pupillae is innervated by the sympathetic and by contracting these fibers the pupil expands – mydriasis (Figure 2). Both muscles are of smooth fibers and of ectodermal origin, which is a unique case in the organism.

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 2. Function of the iris muscle, miosis and mydriasis

The diameter of the pupil is usually 3-4 mm, but it can vary depending on the amount of light from 1.5 to 8 mm. In conditions of reduced amount of light, the pupil dilates, while in conditions of strong light, the pupil contracts. Young people’s pupils are wider, while babies’ and older people’s pupils are narrower, so-called senile miosis. We meet widened pupils in affective states, due to the predominance of sympathizers, we remember the saying “in fear, big eyes”. The width of the pupil also changes when taking some medicines. The different size of the pupil openings in the same person is called anisochoria.

Color of the iris and pigmentation disorders

The color depends on the looseness of the iris stroma as well as the amount of melanin pigment in the chromatocytes of the iris stroma and the RPE cells lining the posterior surface of the iris. The looser the stroma, the brighter the iris – the blue color of the eyes in small children – babies. With a smaller amount of pigment, the iris is lighter – blue, a slightly larger amount of pigment is found in green iris, then dark and black color indicates a greater amount of pigment (Figure 3).

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 3. Pigmentation of the iris depends on the amount of melanin pigment in the stromal chromatocytes as well as in the RPE layer – schematic representation

Albinism is a metabolic disorder characterized by the absence of melanin pigment due to a disorder at the level of melanin synthesis, it can be ocular (when only the eyes are affected) and oculocutaneous (eyes, hair and skin are affected). The iris is light blue in color, retroillumination shows the complete absence of melanin pigment in the layer of neuroepithelial cells, the iris is transparent. Due to the lack of pigment, light enters the eye through the entire surface of the iris, and patients have photophobia. (Figure 4).

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 4. Retroillumination of the iris in albinism

Pigmentation disorders can also be focal accumulations of pigment that we call moles – naevus iridis. Each nevus must be monitored at least once a year due to possible maligma alteration. More accumulations of pigment make up the tiger’s iris – iris tigrata. The iris can have sectors with different colors, two-color iris – iris bicolor. A different color of the iris in the same person is called herochromia iridis, it can be congenital – heterochromia iridis congenita as well as acquired. Acquired heterochromia is a sign of a pathological process, e.g. iridocyclitis heterochromica -Fuchs. In the case of Fuchs’ iridocyclitis, the iris becomes washed out, discolored – hypochromic (Figure 5). Heterochromia in the sense of a lighter – washed out iris can be found in sympathetic ophthalmia, due to damage to the sympathetic innervation. The opposite of heterochromia is the state of hyperpigmentation of the iris – melanosis.

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 5. Heterochromia of the iris as part of Fuchs’ Heterochromic Iridocyclitis

The condition of the surface network of pathological newly formed blood vessels on the iris is called rubeosis iridis. Rubeosis occurs as a result of the release of vascular endothelial growth factor (VEGF – Vascular Endothelium Growth Factor) as part of ischemic diseases of the posterior segment of the eye (ischemic proliferative diabetic retinopathy, Central Venous Occlusion). The shape of the iris is also changed by changes at the edge of the pupil that deform the pupillary edge – iridolental growths – synechiae posteriores, (Figure 6) as well as post-traumatic rupture of the pupil sphincter – iridorrhexis.

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 6. Posterior synechiae

The root of the iris – Margo ciliaris is the point of attachment of the iris to the ciliary body, in that part the iris is the thinnest and it easily breaks during eye trauma – split of the root of the iris – iridodyalisys (Figure 7). Changes in the root part of the iris also change the configuration of the iris, e.g. in tumors of the ciliary body (Figure 8).

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 7. Traumatic iridodialysis

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 8. Malignant melanoma of the ciliary body that becomes visible only when its growth disrupts the configuration of the iris

Colobomas are defects of the iliac tissue. Colobomas can be congenital or acquired (post-operative and post-traumatic). Congenital colobomas arise as a result of incomplete physis of the embryonic optic cup, which usually occurs in the 6th week of embryonic life. Coloboma – the defect of the lens tissue is most often positioned nasally and below, it is often bilateral, and extends to the ciliary body, zonules, choroid as well as the optic disc (Figure 9).

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 9. Congenital iris coloboma

Surgical coloboma – basal iridectomy is performed as part of cataract and angle glaucoma surgery (Figure 10). There are also post-traumatic colobomas that are very irregular (Figure 11).

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 10. Surgical basal iridectomy at 12h

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 11. Post-traumatic coloboma – lack of lens tissue, subluxation of the lens with the development of traumatic cataract

The complete absence of the iris is called aniridia, if it is an inherited condition (autosomal dominant inheritance) then it occurs bilaterally. Unilateral aniridia can occur as a result of a severe penetrating injury, then there are usually tissue remnants in the root part. Aniridia is accompanied by photophobia, low vision – amblyopia, as well as nystagmus (Figure 12).

Iris: physiological and pathological phenomena, dr Sandra Jovanović

Figure 12. Aniridia, by retroillumination the ciliary processes give peripheral defects in the red reflex

Iris: physiological and pathological phenomena, dr Sandra Jovanović