This prospective study found that the anatomic arrangement of the vitreous is consistent in living eyes with no posterior vitreous detachment and does not correspond precisely to that described from dissection studies of autopsy specimens. Furthermore, the constancy of the specific findings suggests the architectural structure of the vitreous may enhance evolutionary fitness.
The authors write that while dissection has been the mainstay of obtaining anatomic information on the vitreous for millennia, there has never been a way to independently confirm dissection studies of a structure that is very delicate or potentially prone to change while the surrounding structures become damaged. Noninvasive imaging may avoid the high likelihood of artifactual damage to the vitreous structure during dissection. Contact B-scan ultrasonography can visualize some aspects of the vitreous cavity but has low resolution. OCT has much greater resolution but faces several important limitations in imaging the vitreous.
In this study, the authors used a newly developed technique—swept-swept source (SS) OCT with dynamic focusing and windowed averaging –to analyze the posterior vitreous structure of 25 subjects (44 eyes) ranging in age from 23 to 62 years. This method provides a longer range of vitreous imaging than previously used methods and allows undisturbed visualization of the vitreous anatomy in vivo. A focused illumination beam is swept through the scan depth during 96 successive B-scans and the corresponding most highly resolved portion of each scan is used to make an averaged composite image.
All eyes showed an optically empty space above the macula that corresponded to the premacular bursa. Also, a conical space above the optic nerve head was seen that corresponded to the area of Martegiani. These two areas were interconnected in 25 cases (56.8%). Anterior to the premacular bursa was another lacuna, the supramacular bursa, that was separate from the premacular bursa in horizontal scans centered on the fovea and was found in 38 eyes (86.4%). Both the supramacular and premacular bursae coursed anteriorly, and in 21 of the 38 eyes (55.3%) were seen to interconnect.
They write that the imaging from the current study shows the lack of fibrillar material in the bursal cavities, which corresponds to findings from dissection studies. However, this does not mean that the contained material is necessarily a free-flowing liquid, since, from a physical chemistry standpoint, a gel is a semi-rigid colloidal dispersion and is not free-flowing, although the definition does not include the necessity of having any fibrillar components.
They add that the architectural aspects of the vitreous appear to have the potential to reduce stress loading. If the vitreous pockets of fluid form within the vitreous gel through random degenerative effects, one would not expect the cisterns and bursae to be uniformly present from a young age and be stereotypical in terms of size and location. Furthermore, if bursae were bad things in terms of ocular health, one would expect evolutionary pressure to select against individuals with bursae. But, on the contrary, everyone seems to have bursae, even at a very young age.
Bursae may have a larger-scale effect in damping fluid movement. The various-sized cavities inherent in bursa formation alter the resonant frequency of the entire vitreous cavity and restrict wave motion. The cavities themselves are surrounded by the viscoelastic vitreous, and there are interspersed extensions of vitreous that can act as baffles. These would also function to reduce the possibility of resonance and to dissipate kinetic energy of fluid flows. Similar engineering techniques have been used, for example, in waterbeds, which were originally designed to hold a large quantity of water in a single bladder but had the problem of persistent waves when a person moved. Subsequently, this problem was corrected by creating chambers within the bladder and by adding blocks of fiber batting to help dissipate the wave energy.