How macular OCT scanning affects glaucoma evaluation

December 22, 2016

By Ashley Speilburg, OD, FAAO
Michael Chaglasian, OD, FAAO

Article

Let’s examine what this specific (and separate) scan pattern can offer diagnostically

One aspect of optical coherence tomography (OCT) scanning that is often overlooked or misunderstood in glaucoma suspects is the macular or “ganglion cell” scan.

Let’s examine what this specific (and separate) scan pattern can offer diagnostically.

Glaucoma and macula

Glaucomatous optic neuropathy results from damaged retinal ganglion cells and their axons.¹

Retinal ganglion cells (RGC) are most heavily populated in the macular area. At least 30 percent of RGCs are found in the central ±8 degrees of the macula,² and together, the ganglion cell layer (GCL) and retinal nerve fiber layer (RNFL) contribute 30 to 35 percent of the total thickness of the retina in the parafoveal region.³

While initial OCT scan patterns for glaucoma concentrated on the circumpapillary RNFL (axons of the RGCs), several early studies investigated a macular relationship to glaucomatous damage.

They found significantly less total macular volume in eyes with advanced glaucoma compared to normal eyes using time-domain OCT.⁴ In addition, total macular thickness was correlated to visual field mean deviation.⁵

Spectral-domain (SD) OCT offers improved imaging resolution over older time-domain systems, allowing reliable and refined segmentation of individual retinal layers. With SD-OCT scanning in the macula, there is the opportunity to identify the GCL thickness. Currently, all commercially available SD-OCTs offer some form of a “specialized” macular scan for use in glaucoma evaluation.

OCT and macular integrity

Early glaucomatous damage can affect the macula. The concept of scanning the macula in glaucoma is based on research showing that the traditional circumpapillary RNFL scans and central 24-2 visual fields may miss this early macular damage.

The 24-2 and 30-2 test pattern are the most common visual field test patterns for diagnosing and monitoring glaucoma. In these patterns, test points are spaced 6 degrees apart, and only four test locations (not including the central foveal point) correspond to the central macula.

Hood and colleagues have shown that considerable damage can occur between these tests points and be missed or show up only as a faint depression on the 24-2 pattern, but expand into clear abnormalities when tested on the finer 10-2 pattern.²

Additionally, these researchers have shown that large sections of the inferior macula (termed the macular vulnerability zone) course into the inferior quadrant of the optic disc, a location known to be more damaged in glaucoma.²

Wang et al compared temporal quadrant and clock hour summary measures of circumpapillary OCT scans to GCL+IPL (inner plexiform layer) scans to determine the sensitivity of the circumpapillary RNFL scans to detect macular damage. Their results showed that circumpapillary RNFL scans will often miss macular damage in glaucoma.⁶

From this evidence on a limitation of functional assessment for glaucoma, it becomes clear how important it is to use an SD-OCT to “structurally” assess the integrity of the macular region.

Macular scan for glaucoma

Macular scans for glaucoma differ from retinal macular scans, and protocols vary based upon the instrument used.

Macular scans in glaucoma are sensitive to picking up macular changes from glaucoma that may be missed on traditional circumpapillary RNFL scans and 24-2 visual fields but will not pick-up areas of RNFL loss beyond the macular region. Furthermore, macular scans have not been found to diagnose glaucoma earlier than circumpapillary RNFL scans.⁷ Thus, these two different scans are best used together in a complimentary fashion rather than in isolation.

Four instruments, different approaches

Optovue iVue SD-OCT performs glaucoma macular analysis by segmentation of the inner three layers of the retina from the outer retinal layers. Together, these three layers include the RNFL, GCL, and inner plexiform layer (IPL) and are collectively referred to as the ganglion cell complex (GCC).

Imaging the GCC has been shown to improve diagnostic accuracy in glaucoma as compared to total macular thickness and performs very well as compared to RNFL scanning.⁸

A color-coded thickness map is produced, and acquired GCC values are compared to a normative database of age-matched controls and displayed as a significance map. In addition, a summary table provides color-coded thickness values for average, superior, and inferior GCC thickness.

Cirrus HD-OCT (Carl Zeiss Meditec) uses a ganglion cell analysis (GCA) scan segmentation protocol separating the GCL and IPL from the remaining layers but omits the RNFL. The GCA thickness values are displayed on a color-coded thickness map and compared to age-matched normals for display on a deviation and sector map.

Additionally, mean and minimum GCA thickness values are displayed in a color-coded data table. Studies have shown thickness deviation map showed good diagnostic ability in detecting preperimetric and early glaucoma and was comparable with the RNFL thickness deviation map.^9,10

Heidelberg’s Spectralis SD-OCT system measures total retinal macular thickness corresponding to the central 20 degrees of the visual field. Thickness values are displayed on a grid, and intra-eye symmetry analysis compares superior and inferior hemifields, which are plotted out on a color-coded gray scale.

An inter-eye symmetry evaluation is also performed comparing the two eyes. The large area of macular thickness measured includes more peripheral areas that may also be damaged in glaucoma.¹¹

Topcon Maestro 3D OCT-1 is most recent to the U.S. market with the 2016 FDA approval of its reference database. It offers both a GCC scan (GCL+IPL+RNFL) and GCL+IPL scan.

Related: Managing glaucoma in women

Using macular scan for glaucoma

Keep these tips in mind when using a macular scan for glaucoma.

• The ganglion cell scan is best utilized in conjunction with the RNFL scan and other diagnostic tests. Agreement of abnormality among several tests is a much stronger statement than a single test being abnormal. Look for the RNFL scan to anatomical “match” the defect on the macular scan.

• A lot of artifact identified by macular scans is not related to glaucoma damage. This is the so-called “red disease” area flagged on the report in red color, but it is not due to glaucoma. Macular pathologies that can affect the scan include:

• Epiretinal membranes

• Macular edema

• Vitreal traction

• High myopia

Bottom line: You also have to look at the macular ophthalmoscopically.

• If the 24-2 visual field test is normal but the ganglion cell scan is abnormal, consider running the 10-2 pattern to identify a small scotoma.

OCT and glaucoma

Macular OCT analysis is a welcomed addition to our armamentarium of diagnostic tools for glaucoma. It provides additional information about ganglion cell loss in glaucoma and compliments both traditional circumpapillary RNFL OCT imaging and visual field testing.

References

1. Nickells RW. The cell and molecular biology of glaucoma: mechanisms of retinal Ganglion cell death. Invest Ophthalmol Vis Sci. 2012 May 4;53(5):2476-81.

2. Hood DC. Macular Damage: The Diagnostic Missing Link. Rev Ophthalmology. 9 Dec 2013. Available at: https://www.reviewofophthalmology.com/article/macular-damage-the--diagnostic-missing-link. Accessed 12/22/16.

3. Zeimer R, Asrani S, Zou S, Quigley H, Jampel H. Quantitative detection of glaucomatous damage at the posterior pole by retinal thickness mapping. A pilot study. Ophthalmology. 1998 Feb;105(2):224-31.

4. Lederer DE, Shuman JA, Hertzmark E, Heltzer J, Velazques LJ, Fujimoto JG, Mattox C. Analysis of macular volume in normal and glaucomatous eyes using optical coherence tomography. Am J Ophthalmol. 2003 Jun;135(6):838-43.

5. Greenfield DS, Bagga H, Knighton RW. Macular thickness changes in glaucomatous optic neuropathy detected using optical coherence tomography. Arch Ophthalmol. 2003 Jan;121(1):44-46.

6. Wang DL, Raza AS, de Moraes CG Chen M, Alhadeff P, Jarukatsetphorn R, Ritch R, Hood DC, . Central glaucomatous damage of the macula can be overlooked by conventional OCT retinal nerve fiber layer thickness analyses. Trans Vis Sci Technol. 2015 Nov;4(6):4.

7. Na JH, Sung KR, Baek S, Sun JH, Lee Y. Macular and retinal nerve fiber layer thickness: which is more helpful in the diagnosis of glaucoma? Invest Ophthalmol Vis Sci. 2011 Oct;52:8094-8101.

8. Kim NR, Lee ES, Seong GJ, Kim JH, An HG, Kim CY. Structure-function relationship and diagnostic value of macular ganglion cell complex measurement using Fourier-domain OCT in glaucoma. Invest Ophthalmol Vis Sci. 2010 Sep;51(9):4646-4651.

9. Sung MS, Yoon JH, Park SW. Diagnostic validity of macular ganglion cell-inner plexiform layer thickness deviation map algorithm using cirrus HD-OCT in preperimetric and early glaucoma. J Glaucoma. 2014 Oct-Nov;23(8):e144-151.

10. Mwanza JC, Durbin MK, Budenz DL, Sayyad FE, Chang RT, Neelakantan A, Carter R, Crandall AS. Glaucoma diagnostic accuracy of ganglion cell-inner plexiform layer thickness: comparison with nerve fiber layer and optic nerve head. Ophthalmology. 2012;119(6):1151-1158.

11. Um TW, Sung KR, Wollstein G, Yun SC, Na JH, Schuman JS. Asymmetry in hemifield macular thickness as an early indicator of glaucomatous change. Invest Ophthalmol Vis Sci. 2012 Mar;53:1139-1144.