Ultrastructure of Retino-choroidal coloboma
Dr. Shashank Somani, Dr Samyak Gupta, Dr Prabhat Nangia, Dr. Sarang Lambat, Dr. Vinay Nangia
Suraj Eye Institute, 559, New Colony, Sadar, Nagpur- 440001.
A female, 44 years of age, came with a complaint of diminution of near vision in both eyes since 3 years. Her best corrected visual acuity(BCVA) was 6/6, N6 in right eye (OD) and 6/6, N6 in left eye (OS). Anterior segment examination of both eyes was within normal limits. Intraocular pressure recorded by Goldmann applanation tonometer was 20 mm Hg in both eyes. Fundus examination showed a vertical cup disc ratio of 0.75 and inferior rim thinning in the right eye. Left eye fundus examination showed a vertical cup disc ratio of 0.8 and inferior rim thinning along with a coloboma which was inferior to the disc, adjacent to the infero-temporal vessels.
Retinochoroidal coloboma is a rare malformation caused by defective closure of the embryonal fissure.. Colobomas have been associated with upto a 40% risk of rhegmatogenous retinal detachment.1 One of the main factors causing retinal detachments is the presence of retinal breaks. Majority of the breaks have been observed to occur within 2 disc diameters of margin of coloboma.
Tissue overlying the coloboma is known as the intercalary membrane (ICM) and it is an extension of the retina which has lost its layered anatomy over the coloboma. This layer is known as the inter-calary membrane.(ICM) Transition from normal retina to ICM can be abrupt or gradual. ICM detachments indicate elevation of the ICM from the coloboma floor.
Retinal detachments occuring due to a choroidal coloboma are often treated with pars plana vitrectomy and silicon oil tamponade.
Prophylactic laser around the margins can also be done to separate the coloboma from the rest of the retina to prevent breaks and retinal detachment.
Our patient presented with complaints of near vision in both eyes, but was generally unaware of the presence of the coloboma, since the coloboma had neither involved the optic disc nor the macula and hence the distance vision was normal. It is of interest to note the abrupt absence of the choroidal tissue in the area of the coloboma ( Fig.7b yellow arrow) it is of interest to note that there is disorganisation of the retinal layers, with the formation of the intercalary membrane, which in our case consists of three layers. 1. inner hyperreflective layer, 2. middle hyporeflective layer and 3. outer hyperreflective layer. This organisation of the intercalary membrane continues all along the coloboma. It is of interest to note that the outer nuclear layer along with the other inner layers reaches up to the edge of the coloboma, whereas the other layers stop short of the edge. ( Fig. 7 b. blue arrows) The intercalary membrane may therefore be considered to be a continuation in our patient of mainly the inner retinal layers.
It is of significant interest that the coloboma involves not only the choroid, but also involves the bruch’s membrane, retinal pigment epithelium, and the photoreceptors layers including the outer nuclear and outer plexiform layer. While the inner retinal layers are present but become disorganised in the area of the coloboma. While the choroid- when present- does not occupy the entire space created in the area of the choroidal coloboma, this tells us that the choroid and perhaps also the Bruch’s membrane are very important in maintaining the spherical shape of the eye. That in their absence, the sclera tends to fall back, almost like an ‘out-pouching’ that is is localised with significant depth.
The intercalary membrane continues to show the presence of larger blood vessels coursing across the coloboma. However their appearance is not very healthy. ( Fig. 4 yellow arrow, Fig. 11 b yellow arrow, and Fig. 12 b yellow arrow. They appear to be attached to the floor of the coloboma and their attachments can be seen to be stretched as the ICM detaches from the floor ( Fig 11 b red arrow. This indicates that they have strong adhesions to the floor of the coloboma and this tissue (Fig. 13 red arrow) may play a role in keeping the ICM attached securely to the floor of the coloboma. The general appearance of the coloboma is pale and that is an obvious indication of the relatively avascular status, perhaps with the absence of small vessels that are essential for the health of the retina. One may note the silhouette of the closed vessel (Fig 4 black arrow). The closed vessels seen on the colour photograph are seen much better on the infra-red images. ( Closed vessel in Fig 4 black arrow and same vessel in Fig 13a yellow arrow) In addition there are multiple vesicles that are seen on the retinal surface (Fig 4 blue arrow) These may be construed as a sign of avascularity. These vesicles are so small that they were not visible on the OCT but only through the retroillumination effect in the intercalary membrane. The relatively high likelihood of developing a hole in the ICM may be largely related to the closure of small vessels, the resulting avascularity, which may lead to a localised melt of the retina associated with hydration and loosening of the underlying tissues which hold the ICM to the sclera. There is also a possibility that once the ICM detaches, it will stretch, the hole will enlarge and because of its shortening it may be difficult to reattach it. This may require retinotomy during surgical management.
The detachment of the ICM over the coloboma was maximum near the hole, and became less towards the anterior part of the coloboma.( see video of coloboma) The presence of the coloboma also led to the RNFL thinning as seen in Fig 6 b,c and d (red arrows). This thinning is related to the loss of a significant area of inferior RNFL in the area of the coloboma. Also note the much larger cup with the absence of inferior neuroretinal rim. ( Fig. 2 yellow arrow)
- Gopal L, Badrinath SS, Sharma T, Parikh SN, Biswas J. Pattern of retinal breaks and retinal detachments in eyes with choroidal coloboma. Ophthalmology.https://www.sciencedirect.com/science/article/abs/pii/S0161642095308883
- Lingam G, Sen AC, Lingam V, Bhende M, Padhi TR, Xinyi S. Ocular coloboma—a comprehensive review for the clinician.https://www.nature.com/articles/s41433-021-01501-5#citeas
- Jain S., Kumar V., Salunkhe N., Tewari R., Chandra P. & Kumar A., Swept Source Optical Coherence Tomography Analysis of the Margin of Choroidal Coloboma : New Insights,Ophthalmology Retina (2019), doi: https://doi.org/10.1016/j.oret.2019.08.010
Dr Vinay Nangia
MS, FRCS, FRCOphth
Suraj Eye Institute
Email – email@example.com
Q 1. Regarding blood vessels and the presence of a retinochoroidal coloboma the following is/are true
1. There are no blood vessels in the intercalary membrane
2. There blood vessels are present in the intercalary membrane
3. The blood vessels in the intercalary membrane may be closed.
4. One should be cautious in preserving the blood vessels in the intercalary membrane during retinal surgery
Q 2. Regarding the Retino-choroidal coloboma the following is/are true.
1. The coloboma involves the choroid
2. The coloboma does not affect the inner retinal layers structure
3. The coloboma is associated with lack of the Bruch’s membrane
4. The intercalary membrane maintains the retinal architecture of the inner retinal layers.