Thursday, February 21, 2013

The Story of Vitreous Circulation

Several months back my father noticed black spots in his vision in the right eye. After a thorough check-up at a premier eye hospital in Chennai, he was diagnosed with a 'retinal tear'. Read more in wikipedia, under retinal detachment. Why does this happen? Remedy? What has this got to do with the title story of vitreous circulation?

The human eye is an imperfect sphere, about 24 mm (just about an inch) thick in the major axis, from the cornea on the front side to the retina on the back side. In between we have the lens supported on the front side by two tubular 'hooks' containing the aqueous humor fluid. As the name implies, this fluid is aqueous and can even circulate within the tubular 'hooks'. The retina, on which the lens focuses the images of what we see, is separated from the lens by a vitreous humor. This vitreous humour, by default, is not aqueous; it is jelly-like, what is known technically as a Non-Newtonian fluid. It maintains the shape of the eye, keeping the distance between the lens and the retina. As we age, the vitreous gel loses its aqueous nature and hardens. Unfortunately, as it hardens and contracts, confined between the retina and the lens, it 'pulls' on the walls. In effect, for some of us as we age, the retina is pulled in -- a retinal 'tear'.

The remedy is to stick the retina, figuratively, back in its place. For this, first we need to slice open the eye behind the lens and scoop off the jelly-like vitreous humour. Then, the retina is stuck back in its place by cauterizing it with targeted laser heating. Then the eye is stitched in place and closed. Within few weeks, bodily fluids ooze into the region between the retina and lens to replace the original hardened vitreous humour that was scooped off. Hopefully, when the eye patch is taken off, the vision should be minus those black spots and become normal. These things went on fine and let us say my father regained his normal vision.

But all of this is only background information for a short story that took off in between, which (obviously?) involves me and my research.

From the experience I had earlier with studying retinal laser surgery using computer simulations of predictions from relevant mathematical equations, I knew most of what to expect in the laser cauterization briefed above. The interesting difference in this situation is, after the surgery, the vitreous region is filled by bodily fluids that is not as hard as the original jelly-like humour. This fluid can circulate, just like the aqueous humour on the front side of the eye. It can circulate, but will it? Under what conditions? Curiosity often kills the cat but sometimes allows a Columbus to discover 'India'.

When I put this idea and a related application to my research student, he wasn't convinced. Already eye is small; and this circulation is a fleeting phenomenon; and you are asking me to check for conditions when it would happen; and use it to do something that is remote, if not vague. Such went his reasoning; a path I have also traveled several times, then as a graduate student and even now as a graduated student. So I persisted. Every research student has this important period in the course of a research degree. She has to put the faith in something that is beyond her grasp; be it the adviser's experience or the vague promise a risky problem offers or in her abilities. This should be discussed elaborately and separately. Back to our story. Thankfully, my student mustered belief; in me and eventually in the problem at hand.

Circulation can be mechanically started, like sloshing a fluid in a container with a stick. This is not possible in the vitreous region, for obvious reasons`. Heating a fluid region can induce circulation due to local buoyancy differences set in by the density reduction caused by the heating. This circulation called the natural convection, requires a certain temperature for the local fluid mass to move and circulate. Higher the temperature, stronger the circulation. Such a circulation could be possible in the vitreous region subjected to heating. What sort of heating? Why not laser heating again? Wait a minute, are we saying we heat the retina again, once it is cauterized anyway, during which the vitreous was filled by bodily fluids that could circulate in the first place? No, we are checking for a heating situation that could arise once this retinal tear is plugged. Is there such a situation? Yes, there is one.

There is a process, mouthful at that, called Trans-pupillary thermotherapy or TTT. Simply put, this is a thermal massage for the retinal portions of the eye. Instead of laser heating that involves strong energy doses and high temperatures, the TTT uses laser with diffused heating -- heat energy spread over a larger area, hence lower temperature. TTT sets a temperature difference of about five to ten degree Celsius across the eye, from the relatively colder cornea (front side) to the hot retina, where the laser heating falls. The vitreous region in between is in this thermal gradient. The buoyancy force differences set in during a TTT process can induce natural convection of the fluid in the vitreous region. Only, this TTT should have been performed on a human eye anytime after it underwent a retinal laser treatment that replaced the original jelly-like vitreous. Do we encounter such situations? Consulting with eye doctors from the premier eye institute in Chennai convinced us; several patients as they age, first go through the retinal tear closure treatment and then, if required in the subsequent years, the TTT process.

Having identified a convincing situation the rest was just hard work. Hard work shared equally between us, the student and the teacher; i.e. the (staying) 'hard' part by me and (doing) the 'work' part by the student. We were able to employ a mathematical model of the situation, which involves equations that state the condition of the vitreous fluid in terms of its velocities and temperatures at all points in the region at all time and solving them to know of these conditions at particular locations at a required time. About an year and few months after the day my father complained about the retinal tear, we now have our research results of the vitreous humour circulation (and related stuff) published in a relevant journal. Here is an excerpt from the abstract of the paper:
During retinal surgical treatment often the gel-like vitreous humor is replaced by aqueous substitutes. A two-dimensional computational model is developed for simulating transpupillary thermotherapy (TTT) process in a human eye under post-retinal treatment. [...] results for steady state and at the end of 60 s of the laser irradiated TTT process show that flow in vitreous humor is significant. The velocity contours indicate strong natural convection on the upper half of the vitreous chamber. Compared with the stationary vitreous case, the peak temperature in retina during TTT, drops by 15 K and 12.5 K due to natural convection flow in the vitreous humor under steady and transient states, respectively. [...] The vitreous humor convection enhances heat transfer in the regions adjacent to the laser spot. The temperature rise and the associated thermal damage in the neighboring regions resulting from the flow of vitreous humor is presented.
 That is, for about a five degree temperature difference across the vitreous region, a steady circulation can be observed in the fluid, resulting in the hot spots to move up along the surface a few millimeters from the middle of the retina. That is, when compared to the case of no circulation, TTT heats more of the retinal regions due to circulation of the vitreous humor due to convection. This could lead to thermal damage in unexpected spots of the retinal surface.

There is an exciting sequel to this story, which is currently pursued by the student. This time, I didn't need to inspire him. All about this in a future write-up.