Spectral-domain optical coherence tomography (SD-OCT) is now an established diagnostic tool for optic nerve diseases such
as glaucoma. Since the first studies on glaucomatous eyes were published a decade ago using time-domain OCT (TD-OCT), remarkable
advances have been realized.
Today ophthalmologists can use SD-OCT to investigate several parameters in cases with any degree of glaucoma. These are derived
from the peripapillary retinal nerve fibre layer (RNFL), the optic nerve head (ONH) and the macular ganglion cells (GC). Comprehensive
analysis of these data help the clinician in making the diagnosis and following patients up.
The RNFL thickness has been traditionally the OCT target for glaucoma diagnosis. Since the time of TD-OCT, a 3.4 mm-diameter
circular scan has been positioned around the optic nerve and the RNFL thickness along this line has been measured. All currently
available SD-OCT instruments still follow this approach, which calculates RNFL thickness values in 12 clock hours and 4 quadrants.
The measured values are then compared to a normative database.
Given the already good diagnostic capabilities of TD-OCT, it is not surprising that most studies performed with SD-OCT failed
to detect any significant improvement when conventional circular peripapillary scans are used.1–4 The main advantage of SD-OCT is rather the remarkably improved repeatability of RNFL thickness measurements in both healthy
and glaucomatous eyes, as shown by a test-retest variability which is half compared to that of TD-OCT.1,5 The main reason for such an improvement is related to the new acquisition protocols.
In the case of Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, California, USA), for example, the instrument does not just scan
a circle around the ONH, but provides a tridimensional cube of peripapillary data; the 3.4 mm circular scan is subsequently
extracted once the centre of the disc is automatically identified, so that the circle is automatically centred. By doing so
the operator reduces the measurement variability typical of TD-OCT, where scan repeatability is strongly influenced by improper
location of the circle around the ONH.6
It is important to highlight that the instruments manufactured by the different companies provide different, non-interchangeable
measurements. Although only a few studies have directly compared some of these instruments,7,8 the published values clearly show that the algorithms developed by each manufacturer to identify and quantify the RNFL thickness
are different. The 360° peripapillary RNFL thickness ranges between 89.8 and 113 microns.1,7–16 None of these measurements are interchangeable with those obtained by TD-OCT.
Circular peripapillary RNFL thickness measurements by SD-OCT are still affected by the same issues that influence TD-OCT:
1) optic disc size influences RNFL measurements, which are higher in larger discs due to the reduced distance between the
ONH edge and the scan.11,17 and 2) axial length should be accounted for, as longer eyes show falsely low RNFL thickness values and shorter eyes falsely
high values, due to the camera magnification.11,18–20 Unfortunately, current normative databases do not take into account these important factors, so that individuals with small
or large discs, or with short or long axial length may be misdiagnosed.
Figure 1: Retinal nerve fibre layer thickness and ganglion cell analysis in a patient with preperimetric glaucoma progressing
in OU from 2009 to 2012.
Compared to TD-OCT, one the main advancements of SD-OCT is represented by the RNFL thickness deviation map, which provides
additional spatial and morphologic information about RNFL damage and improves the diagnostic sensitivity for glaucoma detection.
Recent studies have demonstrated that the above-mentioned map has significantly higher value for diagnostic purposes compared
to the standard circular scan.21,22
Last generation SD-OCT devices also offer the advantage of specifically designed software to assess glaucoma progression.