The macular grid is based upon the grid employed by the Early Treatment of Diabetic Retinopathy Study (ETDRS) to measure area and proximity of macular edema to the anatomic center of the macula, also called the fovea. This grid was developed as an overlay for use with 32mm film color transparencies and fluorescein angiograms in the seminal trials of laser photocoagulation for the treatment of diabetic retinopathy. Subsequently, this grid has been in common use at reading centers since the 1970s, has been incorporated into ophthalmic camera digital software, and has been employed in grading other macular disease in addition to diabetic retinopathy. This grid was slightly modified for use in Time Domain OPT models developed in the 1990s and early 2000s in that the dimensions of the grid were sized to accommodate a 6 mm diameter sampling area of the macula.
The grid for macular OPT is bounded by circular area with a diameter of 6 mm. The center point of the grid is the center of the circle. The grid is divided into 9 standard subfields. The center subfield is a circle with a diameter of 1 mm. The grid is divided into 4 inner and 4 outer subfields by a circle concentric to the center with a diameter of 3 mm. The inner and outer subfields are each divided by 4 radial lines extending from the center circle to the outermost circle, at 45, 135, 225, and 315 degrees, transecting the 3 mm circle in four places. Each of the 4 inner and 4 outer subfields is labeled by its orientation with regard to position relative to the center of the macula – superior, nasal, inferior, and temporal. For instance, the superior inner subfield is the region bounded by the center circle and the 3 mm circle the 315 degree radial line, and the 45 degree radial line. The nasal subfields are those oriented toward the midline of the patient's face, nearest to the optic nerve head. The grids for the left and right eyes are reversed with respect to the positions of the nasal and temporal subfields – in viewing the grid for the left eye along the antero-posterior (Z) axis, the nasal subfields are on the left side and in the right eye the nasal subfields are on the right side (nasal as determined by the location of the subfield closest to the nose).
The OPT macula thickness report consists of the thickness at the center point of the grid, and the mean retinal thickness calculated for each of the 9 subfields of the grid. In the context of the macular disease considered for the diagnosis, and qualitative interpretation of morphology from examination and OPT and/or other modalities, the clinician uses the macula thickness report to determine if the center and the grid subfield averages fall outside the normative range. Monitoring of macular disease by serial grid measurements allows assessment of disease progression and response to intervention. Serial measurements are assessed by comparing OPT thickness or volume reports, provided that the grids are appropriately centered upon the same location in the macula for each visit.
Figure UU.5-1 Macular Grid Thickness Report Display Example
The center point of the grid should be aligned with the anatomic center of the macula, the fovea. This can be approximated by having the patient fixate upon a target coincident with the center of the grid. However, erroneous retinal thickness measurements are obtained when the center of the grid is not aligned with the center of the macula. This may occur in patients with low vision that cannot fixate upon the target, or in patients that blink or move fixation during the study. To determine the expected accuracy of intervisit comparisons, clinicians would benefit from knowing the alignment accuracy of the OPT data from the two visits. Ophthalmologists may also want to customize locations on the fundus to be monitored at each visit.
The following figure illustrates how the content items of the Macular Grid Thickness and Volume Report are related to the ETDRS Grid. Figure shown is not drawn to scale.
Figure UU.5.2 – ETDRS GRID Layout