|The 3D OCT-2000 from Topcon features intuitive FastMap software that can provide 3D, 2D and fundus images simultaneously.|
|The 3mm scan depth and 40Â° scanning of the Avanti Widefield OCT from Optovue provide the detail and clarity to assess the retina.|
|The Cirrus HD-OCT from Carl Zeiss Meditec with ChamberView image provides an expansive 15.5 mm wide view of the entire anterior chamber.|
|Heidelberg’s SPECTRALIS OCT integrates confocal scanning laser ophthalmoscopy to provide unique views of the structure and function of the eye.
Advances in OCT technology will help diagnose and better manage patients with glaucoma.
Ever since optical coherence tomography (OCT) was first introduced in 1991, it has offered a standardized, objective method of quantifying subtle nerve tissue loss in a quick and non-invasive way. Although the gold standard of glaucoma management is still optic nerve head (ONH) assessment and visual field perimetry, the OCT provides a more sensitive approach to glaucoma management for earlier diagnosis and treatment.
The top four companies that produce OCTs are Carl Zeiss Meditec, Inc. (Cirrus HD-OCT), OptoVue, Inc. (Avanti Widefield OCT), Topcon Medical Systems, Inc. (3D OCT-2000) and Heidelberg Engineering, Inc. (Spectralis SD-OCT). Each company has its own proprietary analysis software, scan methods and display formats, but most standard OCTs contain a basic assessment of the circumpapillary retinal nerve fiber layer (RNFL) thickness.
However, new advances in OCT software can now perform macular thickness scans to assess ganglion cell complexes, obtain large-area scans that incorporate both optic nerve and macula in a single display and include progression analysis software to compare baseline and follow-up scans.)
BEYOND THE GOLD STANDARD
Though ONH assessment and visual field perimetry are the foundations in management and detection of glaucoma, the OCT is a valuable adjunct. OCT offers an objective and quantitative measurement that largely bypasses patient error. The shift from time domain to spectral domain OCT has enhanced resolution two- to three-fold and increased scan speed 60- to 100-fold, allowing a great deal of information to be obtained in a short amount of time. Given the ease of use, valuable quantitative measurements and short scan duration, OCT can be regularly performed to establish a baseline for glaucoma suspects and to assess for change in glaucoma management.
However, the OCT is not a perfect system; there are a number of limitations when interpreting the data. Patient data is compared to a normative database that is unique to each company. If a patient falls outside of the norm, such as with a refractive error beyond the +12.00D to -8.00D range, the RNFL can give false negative and false positive results; similarly so with anomalous optic nerves.
Also, inter-machine comparisons of the RNFL measurements are inconsistent, so retinal and optic nerve measurements should only be compared within each instrument. OCT artifacts need to be viewed carefully and can come from a variety of sources such as patients’ micro-saccades, clinicians’ off-center/inverted/cut-off measurements, and/or disease-related epiretinal membranes and peripapillary atrophy.
CURRENT USES OF OCT
Currently, OCTs are most commonly used to help confirm glaucoma suspicions and diagnosis by directly measuring circumpapillary RNFL thickness. The RNFL thickness scan is a circumpapillary circular scan centered over the optic nerve to measure the exact thickness of the RNFL surrounding the nerve. This data is compared to a normative database to help highlight any focal thinning and suspicious areas. The RNFL thickness scans can also be used to monitor glaucoma progression by manually comparing serial OCT scans to look for subtle change.
ADVANCES IN OCT TECHNOLOGY:
Ganglion Cell Complex: The macula is gaining the spotlight in early detection of glaucoma due to the presence of 50% of the retinal ganglion cells and the RNFL constituting 30% to 35% of the macular thickness. In glaucoma, the first area to be affected is the ganglion cell complex (GCC), which is comprised of the nerve fiber, ganglion cell and inner plexiform layers. Studies have shown that measuring the GCC has improved diagnostic accuracy when compared to measuring the overall macular thickness.
Macular thickness can also be analyzed in the posterior pole asymmetry analysis that assesses the intra-eye and inter-eye asymmetry. Early glaucoma is often detected based off asymmetry analysis. RNFL, ONH, ganglion cell analysis and macular thickness can also be integrated into a single widefield display to visualize retinal and optic nerve damage. This holistic view may assist in analyzing the pattern of glaucomatous damage.
Progression analysis: Some manufacturers have new advances in OCT software that generate progression analysis of RNFL and/or GCC scans by allowing side-by-side comparison of baseline and follow-up scans. OCTs use either event-based analysis or a trend-based analysis. Event-based analysis compares a baseline scan to a subsequent follow-up scan and flags any changes that surpass a pre-established threshold value. A trend-based analysis uses multiple scans over time to produce a linear regression line to map out disease progression and extrapolate the rate of change to predict future trends.
Pachymetry: Central corneal thickness is another piece of the puzzle in understanding a glaucoma patient’s risks and management. This function is standard on all OCTs, obviating the need for the traditional, more invasive technique.
With advances in scanning resolution and software, OCTs have taken a pivotal role in the diagnosis and management of glaucoma. OCTs are equipped to quantitatively measure RNFL thickness and ganglion cell complex and to provide progression analyses in an objective and highly repeatable method. These important measurements are unique to OCT and cannot be obtained with direct observation of the optic nerve or with visual fields. Therefore, using OCTs in conjunction with visual fields and direct ONH assessments can profoundly improve the care and management of glaucoma patients in your practice.
Amiee Ho, OD, and Hannah Shinoda, OD, are assistant professors at Pacific University College of Optometry, Forest Grove, OR.
WHERE TO FIND IT:
Carl Zeiss Meditec, Inc.
800.342.9821 | meditec.zeiss.com/usa