Spark ImageWise 19 – Glaucoma

Morphology of a Bleb

Dr Swati. Mishra, Dr Prabhat Nangia, Dr Sarang Lambat, Dr Vinay Nangia 
Suraj Eye Institute, 559 New Colony, Nagpur 

Case Description
A male, 50 years of age, presented to SEI with complaints of progressive visual loss in both eyes (BE) since 1 year. His best corrected visual acuity (BCVA) was 6/24, N18 in the right eye (RE) and 6/18, N18 in the left eye (LE) with a correction of +2.50 DS with +0.50 DC in RE and +1.75 DS in LE. Anterior segment examination in BE showed a posterior subcapsular cataract. The IOP was 18 mmHg in BE. Fundus examination of both eyes showed non-proliferative diabetic retinopathy (NPDR) with macular edema. He was advised intravitreal bevacizumab and triamcinolone acetonide in both eyes for diabetic retinopathy. Post RE Injection he developed an IOP of 40 mmHg. He was started on eye drops brimonidine (0.25%), eye drop timolol (0.5%) and tablet acetazolamide. Since he did not respond well to the antiglaucoma therapy and in view of significantly raised IOP secondary to intravitreal steroid injection he was advised to undergo trabeculectomy on RE. Post trabeculectomy he had an IOP of 14 mmHg with a well raised bleb. 

Figure 1: Diffuse illumination image of right eye showing  a  large pale bleb. The walls of the bleb (black dots) are avascular with vessels at its edge (red arrow). Note the leash of blood vessels along the nasal edge of the bleb reaching to the limbus (blue arrow). The cut edge of the scleral flap is visible through the bleb (green arrow).
Figure 2: Slit lamp image of right eye showing a diffuse elevated bleb. 
Figure 3 shows retinal nerve fibre layer (RNFL) thickness map from SD-OCT, which shows significant thinning superotemporally and superonasally and early thinning inferiorly (blue arrow).
Figure 4 shows SD-OCT RNFL thickness map, which shows normal thickness of RNFL in all the quadrants.
Figure 3: A vertical line scan passing through the bleb showing  intraconjunctival fluid space (red arrow), supra-scleral space (pink arrow), sclera (yellow arrow), a thick bleb (blue arrow) with an ultrastructure of a meshwork showing flow channels (green arrow). 

Discussion

The appearance of the bleb is near ideal with very good control of IOP.  There is the possibility that the presence of intravitreal steroids tends to enhance the response of glaucoma surgery and the formation of a pale functional bleb as in this case. Steroids are known to inhibit healing and perhaps that was a contributory factor in this patient. The bleb is rather large in size and there is evidence of a possible ring of steel, as indicated by the presence of blood vessels around the margins of the bleb. It is important also to note the presence of some blood vessels on the surface of the bleb, which in this case may be considered a sign of good health. When there are no blood vessels at all on the surface of the bleb it may indicate unhealthy conjunctival tissue. The presence of significant vascularity of the bleb is of course not in the best interest and is mostly an indication of a failed bleb or of imminent failure. The presence of blood vessels around the bleb is also a healthy sign as long as these vessels are not congested and aggressive looking. Therefore in many patients, the presence of a clinically insignificant ring of steel may be considered as a good sign in a patient who has had a mitomycin -C trabeculectomy. This preferred balance of healing following trabeculectomy is hard to predict. In favour of the bleb is also the fact, that it is sufficiently posterior. 

When we look at the blebs on slit lamp, they often give the appearance of having a thin wall and perhaps lead us to imagine that there is an empty cavity between the conjunctiva and the sclera. Nothing could be further than the truth in a functional bleb.  With the advent of high resolution anterior segment OCT (swept source AS-OCT); one can understand the structure of a classical filtering bleb better. Multiple channels giving the appearance of a conjunctival meshwork (conceptually, much like the trabecular meshwork) enables flow of the aqueous in the bleb (green arrows). The ultrastruture of the bleb shows in a vertical section, the presence of irregular channels, which one may presume criss cross throughout the tissue.  The hypo-reflective spaces in these channels, probably indicate the aqueous flow pathways in the bleb. 

In our patient, we describe the clinical appearance and the ultrastructure of a functioning bleb post trabeculectomy surgery. The higher scanning resolution in swept source AS-OCT images allows us to identify the sub-bleb (supra-scleral) fluid space when present and visualise the ultrastructure of the bleb, thus enabling us to understand the clinical, functional and in-vivo relationships following glaucoma surgery.

ReadWise

  1. Leung CK, Yick DW, Kwong YY, Li FC, Leung DY, Mohamed S, Tham CC, Chung-chai C, Lam DS. Analysis of bleb morphology after trabeculectomy with Visante anterior segment optical coherence tomography. Br J Ophthalmol. 2007 Mar;91(3):340-4. doi: 10.1136/bjo.2006.100321. 
  2. Paulaviciute-Baikstiene, D., Vaiciuliene, R., &Januleviciene, I. (2016). Filtering blebs structure and function evaluation using optical coherence tomography. Modeling and Artificial Intelligence in Ophthalmology, 1(2), 10–19. https://doi.org/10.35119/maio.v1i2.15
  3. Khamar MB, Soni SR, Mehta SV, Srivastava S, Vasavada VA. Morphology of functioning trabeculectomy blebs using anterior segment optical coherence tomography. Indian J Ophthalmol. 2014 Jun;62(6):711-4. doi: 10.4103/0301-4738.136227. 

Correspondence 

Dr Vinay Nangia
MS, FRCS, FRCOphth
Director 
Suraj Eye Institute
Nagpur
Email –education@surajeye.org

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