High-resolution optical coherence tomography (OCT) has been proven very useful for the diagnosis of MacTel type 2 and for the follow-up of affected patients. Even patients without any obvious funduscopic changes can be identified using OCT. In recent years, several features have been identified that characterize MacTel type 2 and rarely occur in other diseases (Charbel Issa et al. 2012; Gaudric et al. 2006).

The characteristic findings on OCT imaging may be present in various combinations. The changes are usually most prominent temporal to the foveal center. Larger longitudinal studies exploring the natural history on OCT imaging are still lacking. However, based on exemplary case studies, clinical experience, and the correlation with other imaging techniques and functional parameters, a time course of alterations may be suggested (Fig. 17.2).

The earliest subtle changes on OCT imaging may include temporal enlargement of the foveal pit (Fig. 17.2b), which then appears asymmetric with its thinnest sector temporal to the anatomic foveal center (Gillies et al. 2009; Charbel Issa 2016). This minor structural alteration appears to be due to changes in the outer nuclear/Henle’s fiber layer thickness. However, if capillary leakage occurs within the same area, this asymmetric thinning may disappear due to a slight thickening mainly within the inner retinal layers (Fig. 17.2c). Such vascular leakage may be associated with a slightly increased reflectivity of inner neurosensory layers.

Within the inner retina, hyporeflective cavities (Fig. 17.2d–f) may develop which are usually located in the foveal pit with a predilection for the temporal slope. These atrophic hyporeflective spaces may have the appearance on funduscopy of a lamellar macular hole (“pseudo-lamellar macular hole”). In contrast to other diseases such as diabetic maculopathy or Irvine-Gass syndrome, there is no angiographic leakage and pooling of fluorescein dye into the hyporeflective cavities in MacTel type 2 (Koizumi et al. 2006), and there is no similar neurosensory thickening. The hyporeflective cavities in MacTel type 2 were shown to have a reflectivity different from exudative macular cysts (Barthelmes et al. 2008). These differences suggest that atrophy, as may be found in retinal dystrophies, rather than exudation leads to the formation of these cavities in MacTel type 2. While outer retinal cavities appear to be due to tissue loss in the photoreceptor layer (see below), inner hyporeflective cavities might also result from loss of structural integrity with a pathophysiology similar to that seen in X-linked retinoschisis or retinitis pigmentosa. As well, detachment of the foveolar neurosensory retina from the inner limiting membrane may result in a foveal hyporeflective cavity and an apparent flattening of the foveal pit, especially in presence of additional thinning of the paracentral retina (Fig. 17.2f). Inner hyporeflective cavities may decrease with disease progression, possibly due to loss of supporting structures, surrounding atrophy and sometimes decreasing leakage.

Another sign that is considered characteristic of the disease is the disruption of the line now attributed to the ellipsoid zone. This sign may occur relatively early but is also observed in later disease stages. Discontinuity of this highly reflective line usually starts temporal to the foveola and is one of the most frequent findings in MacTel type 2 on OCT imaging. Modeling of 2-dimensional en face views (Fig. 17.3) at the level of the inner segment-outer segment border from densely recorded B-scans has been shown to reveal the extent of loss of this structural alteration (Sallo et al. 2012a, b). In a recent follow-up study, reappearance of a previously disrupted ellipsoid zone was shown in areas with preserved external limiting membrane. It was concluded that the latter might be a good prognostic factor (Wang et al. 2015).

In few eyes, accumulation of subretinal debris appears evident within the foveal area. The histopathological correlate was suggested to be degenerated photoreceptor elements that have lipofuscin-like autofluorescent properties, possibly explaining increased foveal autofluorescence and the funduscopic appearance of a yellowish foveal spot that is apparent in a small subset of eyes (Cherepanoff et al. 2012).

With disease progression, atrophy of the outer neurosensory retina, i.e., the photoreceptor layer, becomes increasingly prevalent (Fig. 17.2g–n). Possibly depending on the integrity of supporting structures, outer hyporeflective cavities may develop (Fig. 17.2g–h) and increase within the photoreceptor layer, then subsequently collapse leading to apposition of inner retinal layers onto the pigment epithelium (Fig. 17.2j–n). Such apposition often occurs without the intermediate stage of cavitation within the photoreceptor layer.

Hyperreflective intraretinal lesions (Fig. 17.2i) are usually associated with pigment plaques. They may first appear as slightly prominent hyperreflective elevation within areas of photoreceptor atrophy, progressing into the retinal tissue. Larger lesions may present as a flat hyperreflective structure in the inner retinal layer, masking features of the underlying tissue. Secondary neovascular complexes may present as a hyperreflective lesion on OCT images. In these cases the lesion typically is located in or external to the outer neurosensory retinal layers.

Quantitative analysis of OCT data showed decreased thickness of the central neurosensory retina in most studies, likely due to thinning mainly within the photoreceptor layer (Krivosic et al. 2009). Characteristic retinal thickness maps are shown in Fig. 17.4 that highlight asymmetry of the foveal depression, loss of retinal contour, and a general thinning of the macular area secondary to outer retinal atrophy. It has been pointed out that apparent normal or slightly increased macular thickness may occur due to the low-grade macular edema superimposed on the atrophic neurosensory retina (Charbel Issa et al. 2008a, b). Increased macular thickness measurements may be found in MacTel type 2 eyes with marked capillary leakage and only little atrophy, in eyes with foveal detachment, pronounced pigment proliferation (Gaudric et al. 2006) or neovascular membranes.

Choroidal thickness was also assessed in patients with MacTel type 2 using enhanced depth OCT imaging and is still controversial. Chhablani et al. found no different subfoveal choroidal thickness than in healthy controls (Chhablani et al. 2014), whereas a Nunes et al. (Nunes et al. 2015) suggested a thicker subfoveal choroid in patients with MacTel type 2.