Unilateral typical incomplete iris coloboma and optic disc coloboma with bilateral optic disc pit: A rare occurrence

A 51-year-old male who experienced diminution of vision in his right eye (RE) was referred to glaucoma clinic for a suspicious optic disc. His distance visual acuity was 6/24 improving to 6/6 (-1.50 cylinder 130°) in the RE and was 6/6 in the left eye (LE). On examination, RE had a horizontally oval 4 mm pupil, inferonasal wedge-shaped hypopigmented iris patch extending from the pupillary margin towards the iris root (iris coloboma) with transillumination [Fig. 1a and b], enlarged optic disc with deep inferior excavation (optic disc coloboma) and inferotemporal optic disc pit [Fig. 2a]. The LE examination was unremarkable [Fig. 1c] except for the presence of a temporal optic disc pit [Fig. 2b]. Both eyes (BE) had intraocular pressure 12 mmHg and open angles. Posterior segment optical coherence tomography showed a marked retinal nerve fiber layer thinning in the RE [Fig. 3] and a normal macula in BE [Fig. 4]. Bilateral enlargement of blind spot with a bi-arcuate scotoma (more pronounced superiorly and nasally) in the RE was present on visual field examination [Fig. 5a and b]. Anterior segment optical coherence tomography (oblique scan) of the RE displayed the absence of the anterior limiting layer and the anterior and posterior pigmented layer with stromal thinning of the iris in the inferonasal hypopigmented iris patch region [Fig. 6].Figure 1: Slit lamp photograph (a) Diffuse illumination of RE showing inferonasal iris coloboma. (b) Retro illumination of RE showing transillumination defect through the inferonasal iris coloboma. (c) Diffuse illumination of LE showing a normal eyeFigure 2: Colour fundus photograph of (a) RE showing inferior optic disc coloboma with embedded inferotemporal greyish optic disc pit (black arrow), generalized retinal nerve fibre loss and a normal macula. (b) LE showing temporal greyish optic disc pit and a normal maculaFigure 3: Optical coherence tomography of the optic nerve head and retinal nerve fibre layer (RNFL) (Topcon 3D optical coherence tomography DRI OCT triton plus, Japan) of BE showing generalized RNFL loss and decreased average, superior and inferior RNFL thickness in RE and normal RNFL thickness in LEFigure 4: Optical coherence tomography (5Line cross report) (3D, optical coherence tomography Topcon, Japan) of BE.Figure 5: Standard automated perimetry (Humphrey field analyzer, 24-2 test grid, SITA-Standard, Carl Zeiss meditec, Dublin, CA USA) of (a) RE and (b) LEFigure 6: Anterior segment optical coherence (oblique scan) tomography (ASOCT) (CASIA 2, Swept source ASOCT) of the RE through the inferonasal iris colobomaDiscussion Coloboma (plural: colobomata) refers to a notch, gap, hole, or fissure in any of the ocular structures.[1,2] Coloboma can be classified as typical or atypical, depending on their location in the iris or fundus. Typical colobomata are caused by defective closure of the fetal fissure and hence can the occur anywhere in the globe along the line extending from optic nerve to the inferonasal border of the pupillary margin. Coloboma located anywhere other than the inferonasal quadrant of the globe is termed “atypical”[2] and does not represent defects in embryonic fissure closure.[1] Hence, only those iris colobomata that are inferonasal in position are “typical."[1] They are frequently associated with coloboma of the ciliary body, zonule, choroid, retina, and optic nerve but may be an isolated phenomenon.[2] On the other hand, iris defects located elsewhere are considered “atypical.”[1] Atypical iris colobomata are not associated with chorioretinal or optic nerve coloboma[1] and are usually restricted to the iris. In our case, the RE iris coloboma was typical that extended upto the inferonasal border of the pupillary margin. It was also associated with an isolated optic disc coloboma (without any chorioretinal coloboma). Iris coloboma may be complete, that is, a full thickness defect involving both the pigment epithelium and the iris stroma.[1,2] Complete iris coloboma may be total, extending to the iris root and giving rise to the “keyhole pupil,” or partial, involving only the pupillary margin and causing a slightly oval pupil.[2] On the other hand, incomplete iris coloboma is usually partial thickness, involving either the pigment epithelium or the iris stroma. It tends to be wedge-shaped and best demonstrated by iris transillumination. The iris coloboma, in our case did not extend to the iris root and was incomplete partial thickness with focal wedge-shaped transillumination but had pigment epithelial loss and stromal thinning both [Fig. 6]. Lack of significant history of trauma, absence of symptoms such as redness, pain, photophobia, and signs such as pigment dispersion and old keratic precipitates ruled out other causes of focal iris atrophy like trauma and herpes. It is well known that congenital optic disc anomalies like optic disc coloboma or disc pit sometimes have a benign presence, may remain asymptomatic, and are often diagnosed as incidental findings during fundus examination. They can mimic glaucomatous cupping with visual fields simulating glaucomatous defects is well known. Our case too, had no symptoms and was unaware of his optic disc anomalies. The RE optic disc coloboma simulated glaucomatous optic disc which was the cause of referral and had a typical bi-arcuate glaucomatous visual field defect. An inferonasal wedge-shaped mild stromal defect or pigment epithelial defect manifest by focal iris transillumination are minimal expressions of typical iris coloboma.[1] Such typical but incomplete iris coloboma with wedge-shaped transillumination per se is a rare observation. Its occurrence together with an ipsilateral isolated optic disc coloboma and bilateral optic disc pit as seen in our case, is not a commonly encountered phenomenon. Authors' contributions’ Author 1: Concept and design, acquisition of data, drafting of article, manuscript writing and editing, final approval. Author 2: Data collection. Author 3: Manuscript editing. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published, and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship: Nil. Conflicts of interest: There are no conflicts of interest.