BIOMECHANICS

New imaging technologies that allow visualisation of the lamina, sclera and choroid are providing new insights into the anatomical aspects of glaucomatous optic neuropathy, said Claude Burgoyne MD, Devers Eye Institute, Portland, Oregon, US.

“The challenge is to predict the level of engineering stress and strain within a particular optic nerve head for a given level of eye pressure. The amount of that engineering stress and strain can either be very high or very low, depending on the geometry of the connective tissues and their material properties,” Dr Burgoyne said at the 11th Congress of the European Glaucoma Society in Nice, France.

He said that the sharper pictures the technologies provide, not only of the optic nerve head, but also of surrounding tissues and structures, will contribute to a more precise and detailed distinction between the anatomical characteristics of normal eyes and of eyes prone to or affected by glaucomatous damage.

New parameters becoming available

There is an increasing amount of research into the assessment differences of lamina cribrosa thickness between diseased and non-diseased eyes. Although the consistency of such imaging is controversial, recent studies with emerging developments in imaging technology, such as spectral domain optical coherence tomography with enhanced depth imaging (EDI-OCT), adaptive optics compensation, swept-source OCT and OCT at new wavelengths (1050-nm), are yielding encouraging findings.

Another development has been the use of anatomical landmarks such as axis between the entrance to the optic nerve head (Bruch’s membrane opening or BMO) and the fovea (FoBMO axis) for optic nerve head, retinal nerve fibre layer and macular phenotyping.

It is now possible for BMO and the foveal pit to be identified in real time allowing consistent datasets for the optic nerve head and the peripapillary retina and the fovea to be acquired in every human eye.

These new approaches show promise in identifying other elements of laminar anatomy, such as where and how deeply the lamina inserts into the sclera.

Researchers are now extending FoBMO phenotyping to the peripapillary sclera, scleral flange and even to the posterior ciliary arteries. Such imaging capability may in the future shed light on the underlying pathological mechanisms of glaucoma.

“It is our working hypothesis that peripapillary atrophy of the RPE in ageing and glaucoma is a manifestation of the posterior ciliary artery blood flow changes within the sclera that also affect laminar capillary outflow,” he explained.

Researchers are now also using adaptive optics imaging to characterise changes in beam diameter and pore diameter and evaluate macular choroidal thickness in different subsets of glaucoma.

Detection of early disease

Dr Burgoyne noted that, in studies he and his associates carried out, FoBMO phenotyping with SD-OCT imaging reveal-
ed deformation of the lamina cribrosa in the eyes of monkeys with modest unilateral experimental intraocular pressure (IOP) elevation very early on in the course of their neuropathy – well in advance of detectable retinal nerve fibre alterations (Invest Ophth-
almol Vis Sci. 2014 Jan 29; 55(1):574-86).

In addition, a team in South Korea using EDI-OCT have been able to show that disc haemorrhages in humans are related to changes in how the lamina inserts into the sclera (Lee et al, Invest Ophthalmol Vis Sci. 2014 Apr 28;55(4):2805-2815).

Researchers using EDI-OCT have also been able to detect the presence of lamina cribrosa defects and pits earlier than was previously possible. The research also suggests that the micro-lesions are related to adjacent rim-thinning and retinal nerve fibre layer defects (Tatham et al Ophthalmology. 2014 ;121:110-118). Others, using swept-source OCT, have linked lamina cribrosa defects and pits to optic disc haemorrhages (Takayama et al Invest Ophthalmol Vis Sci. 2013 ;54(7):4798-807).

Dr Burgoyne said the increasingly detailed visualisation and quantification of optic nerve head’s anatomy combined with clinical estimates of material properties should furnish the data necessary to build engineering finite element models of the tissues involved.

“If we can achieve this kind of modelling we could then predict, in an individual eye, where to look for pathologic events which would follow from a given combination of stresses and strains and the level of IOP at which they might occur,” he said.

Claude Burgoyne:
cfburgoyne@deverseye.org