REMEMBERING DR DALJIT SINGH

Prof Daljit Singh (11 October 1934 – 27 December 2017)

Prof Daljit Singh was a versatile researcher, clinician and innovator. In my interactions with him, I often found him discussing about lymphatics. This is an innovative way of looking at glaucoma. 

He also published an article on corneal and conjunctival lymphatics in the Ukraine Archives of Ophthalmology. This blog post is based on his article.

 CORNEAL AND CONJUNCTIVAL LYMPHATICS

Singh et al in 2003 presented their study of conjunctival lymphatics. It was suggested that lymphatics play an important role for aqueous drainage, normally and especially after a filtration surgery. 

We could demonstrate conjunctival lymphatics by a variety of ways. The conjunctival lymphatics enter the cornea at the limbus, in great numbers, as single channels. Proximally they join each other and merge with the wide conjunctival network. A question arose, where do they disappear in to the cornea? What is happening in the cornea? The current paper is a continuation of the same work in greater depth. 

The lymphatic network has been traced in the cornea. They exist as a network of sinusoidal channels that are connected to a large circular sinusoidal channel in the periphery of the cornea. The location of this channel corresponds to the “lucid interval” that is often seen in cases of arcus senilis. The Lucid interval /channel of Singh is situated not far from canal of Schlemm. The two are connected through “Aqueducts of Singh”. The Lucid interval channel has multilayered lymphatic connections both on the corneal as well as on the conjunctival side. 

Arcus senilis is an interesting condition in which the cornea becomes semi-­opaque and wherein it is possible to observe channels. The following picture (Fig. 3) in 3 D shows wide arcus senilis with a network of channels. In the periphery, the network merges with the “lucid interval”. In every case of arcus senilis, the “lucid interval” on optical section, appears as roughly triangular, irregularly edged, optically empty space, the apex of the triangle being directed towards the center. There is an anterior and a posterior wall. Towards the apex/corneal side, it appears to be continuous with the corneal channels.

“Lucid interval” is indeed a sinusoidal canal that serves to move fluid in the cornea. Histology of the cornea shows a huge number of large and small slits in the stroma (Fig.6). These slits are either ignored or they are labelled as artifacts. It is because of lack of awareness about the lymphatics in the cornea. The huge number of slits seen in cornea histology pictures seem to match the large number of big and small corneal lymphatics that we have been able to see and photograph through the slit lamp microscope. It must be emphasized that the clinically visible channels are only a fraction of the real number, since they are being visualized in one plane only. 

The next question was if lucid interval is a channel can we pass a wire in to it? We tried and found that a 100-micron blunt steel wire could be easily pushed for considerable distance. We could also pass a straight 240-micron blunt cannula for a shorter distance 

Anterior segment OCT images often show lucid interval channel connected to Schlemm canal via aqueduct connect. We have studied hundreds of OCTs of our patients. We frequently come across images that clearly show the triple relationship of lucid interval channel, aqueduct and Schlemm canal. This relationship is uneven/unequal around the circumference. Much more amazing has been the fact that sometimes the OCT image showing the triple connection might change appearance after local medication with pilocarpine. 

What happens with every pulse beat? This is how I visualize. With every pulse beat, aqueous gets pushed from the ciliary body in to the posterior chamber, from where it is pushed towards the anterior chamber and further towards the angle of the anterior chamber. There is a positive phase when the heart is beating and there is a negative phase when it is relaxing. The aqueous is pushed through trabecular meshwork in to the canal of Schlemm. A little less goes back than gets in, since the fluid has to move on. From the Schlemm canal it pulses through the aqueduct of Singh in to the lucid interval/ Singh canal. From Singh canal the fluid pulses in to and out of the corneal lymphatic network. The to and fro movement is equal in volume. However, with every pulse beat, some fluid escapes from Singh canal to conjunctival lymphatics, through limbal connections, from where it gets in to general circulation. 

The formation of topography waves is explained by to and fro limbal capillaries. But these sources do not relate to the presence of a huge network of channels in the cornea and conjunctiva and the presence of a junction between the two in pulsatile movement of fluid between canal of Singh and corneal channels. Similar fluid movement is also occurring on the uveo-­scleral side. The fluid ultimately passes through the scleral network and episcleral network in to the conjunctival network. The lymphatic network exists as a three dimensional network.

CONCLUSION. On the basis of our slit lamp microscopy studies we strongly believe that the parallel formations as seen at the limbus that are actually a part of the lymphatic system. These lymphatics can join the cornea in only one pattern -­ the palisade. Once away from the limbus they join each other and merge in to the conjunctival lymphatic system. The ‘lymphatic palisades’ have important function to perform, i.e. they are a conduit for the movement of the aqueous. The corneal, limbal and conjunctival network is one system. It explains their functions of providing nutrition to the cornea. It serves as a fast channel for the movement of inflammatory cells in cases of corneal infections. It explains the movement of the aqueous to drain the aqueous normally and after filtration surgery.