Imaging of the Lamina Cribrosa
The lamina cribrosa is challenging to image because of its location deep within the ONH. However, advances in OCT, including enhanced depth imaging OCT (EDI-OCT) and swept-source OCT (SS-OCT), have provided a means of imaging deeper ocular structures, including the lamina cribrosa. In EDI-OCT, the OCT source is placed closer to the eye than in typical practice, thereby producing an image in which the most tightly focused illumination is located more posteriorly, at the level of the choroid and inner sclera. The wavelength of light used in an OCT system affects image resolution, and when penetration depth increases, the image resolution and signal strength decrease. Commonly used SD-OCT devices employ wavelengths in the range of 840–880 nm. However, SS-OCT uses wavelengths of 1000 nm, allowing higher penetration with minimum light absorption and dispersion by the vitreous, which provides improved imaging of deeper ONH structures. Figure 5-19 shows imaging with EDI-OCT and SS-OCT of the right eye of a glaucoma patient with a clinically visible lamina cribrosa defect.
Another technology used to evaluate the lamina cribrosa is adaptive optics. This system uses a wave-front sensor to measure ocular aberrations, for example, those induced by the lens and cornea. A deformable mirror or a spatial light modulator is then used to compensate for measured aberrations and improve image quality. Adaptive optics can correct ocular aberrations in real time; its use can be combined with the use of OCT or scanning laser ophthalmoscopy.
Imaging has enabled the identification of general and localized configurational changes in the lamina cribrosa of glaucomatous eyes, including posterior laminar displacement, altered laminar thickness, and focal laminar defects that have spatial association with conventional structural and functional losses. In addition to changes in the depth and thickness of the lamina cribrosa, studies have suggested that focal lamina cribrosa defects may be important structural features in glaucoma and could potentially serve as biomarkers for glaucomatous visual field loss. There is growing evidence of an association between lamina cribrosa structure and other structural and functional measures of glaucoma, suggesting that evaluation of the lamina cribrosa may become a useful addition to clinical imaging options in the detection of glaucoma and in monitoring glaucoma progression. Lamina cribrosa imaging also has the potential to improve understanding of the mechanisms of glaucomatous RGC injury; in addition, although the temporal relationship between lamina cribrosa and neural changes remains uncertain, lamina cribrosa changes may be a useful biomarker of increased risk of neural losses.
Figure 5-19 Images from the right eye of a patient with glaucoma showing a lamina cribrosa defect. A, Optic disc photograph. B, Radial scans from an enhanced depth imaging with a SD-OCT device (Spectralis, Heidelberg Engineering) showing an unmarked image and a manually marked focal defect in the superior region of the optic disc. C, Axial scans showing the same defect on the superior region of the optic nerve using a swept-source optical coherence tomography device.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.