Acute enlargement of the blind spot without disc swelling was described in observational studies on MCP (Khorram et al. 1991; Callanan and Gass 1992). The etiology of the blind spot enlargement is unknown. Some investigators explained that it was because the nasal retina is more involved through the medial posterior ciliary artery, as MCP develops hematogenously (Brown and Folk 1999; Hayreh 1975). Others suggested that preferential involvement of rod photoreceptors in MCP may induce a field defect around the blind spot (Curcio et al. 1990; Reddy et al. 1996).

Electroretinography (ERG) findings in MCP show diffuse involvement depending on the disease severity associated with chorioretinal involvement. Normal to borderline ERGs were noted in 41% of examined MCP eyes, while moderately to severely depressed ERGs were observed in 38% (Dreyer and Gass 1984). Abnormal ERG responses in MCP were described as rod dysfunction, prolonged cone b-wave implicit times, and poor oscillatory potentials (Reddy et al. 1996). Multifocal ERG revealed a diffuse and irreversible depression in first-order responses in MCP patients, most of which did not correspond to the scotomas on the Goldman visual field (Curcio et al. 1990).

MCP is a chronic disease that may persist for many years and typically has multiple recurrences in one or both eyes (Reddy et al. 1996; Fung et al. 2014). Reactivation of inflammation includes cellular infiltrates of the anterior chamber and vitreous, enlargement of previous lesions, and the appearance of new lesions (Reddy et al. 1996). The mean final visual acuity was 20/42 when 65 eyes with MCP were followed-up for an average of 92 months (Fung et al. 2014). The reported incidence of cystoid macular edema (CME) in MCP varies, ranging from 14% (Dreyer and Gass 1984) to more than 40% (Kedhar et al. 2007). CNV, responsible for 45% of irreversible visual loss to 20/200 or worse (Thorne et al. 2006), has been reported in 32–46% of patients with MCP (Dreyer and Gass 1984; Reddy et al. 1996; Fung et al. 2014). Epiretinal membrane (ERM), with a reported incidence of about 5% in eyes with MCP (Kedhar et al. 2007), accounted for 28% of vision loss to less than 20/50 in one study (Thorne et al. 2006).

Diagnosis

The diagnosis of MCP is usually a clinical one based on the characteristic fundus findings, and ancillary tests only help establish the diagnosis of MCP.

Fluorescein Angiography

In the acute phase, the active lesions of MCP may appear hypofluorescent on the fluorescein angiogram (FA) with staining and leakage in the late phase. The punched-out atrophic scars in the healed phase show early hyperfluorescence with fading out in the late phase as window defects (Fig. 4.5). If CNV is present, characteristic early hyperfluorescence with late leakage appears in the peripapillary or macular areas.

Indocyanine Green Angiography

Indocyanine green (ICG) angiography provides additional information not detected by fundus examination and FA alone, which helps to understand the pathophysiology of MCP, to differentiate from other inflammatory chorioretinopathies and to identify the progression and recovery of MCP. Multiple hypofluorescent spots of variable size can be found on the posterior pole on ICG angiography that are not apparent on FA or clinical examination (Slakter et al. 1997) (Fig. 4.6). These hypofluorescent lesions on ICG angiography generally represent acute or subacute disease. They are not constant, resolving spontaneously as the acute process subsides or with anti-inflammatory therapy. Visual field defects and corresponding areas of hypofluorescence on the ICG study were found in the same study group (Slakter et al. 1997). In seven eyes showing enlarged blind spots on visual field testing, ICG angiography showed confluent hypofluorescence surrounding the optic nerve (Slakter et al. 1997). The authors described that the hypofluorescent lesions in MCP may be the result of focal collections of inflammatory cells or debris at the level of the choriocapillaris producing “blocked fluorescence” or in the middle layer of the choroid causing a “space occupying effect.” Underlying perfusion abnormalities of the choroid and choriocapillaris have been suggested as another potential pathophysiology of the hypofluorescent lesions seen on ICG angiography.

Fundus Autofluorescence

Two major types of hypoautofluorescence have been identified. Chorioretinal hypoautofluorescent spots greater than 125 μm were usually observed corresponding to the area of a punched-out scar visible by color fundus photography (Haen and Spaide 2008) (Fig. 4.7). Autofluorescence imaging also demonstrates hypoautofluorescent lesions measuring less than 125 μm that were typically not visible by color fundus photography (Haen and Spaide 2008) (Fig. 4.8). Subsequently, in another study of 16 MCP eyes, 12 eyes predominantly presented with punctate hypoautofluorescent spots corresponding to multiple areas of chorioretinal atrophy (Yeh et al. 2010). Macular hyperautofluorescence may be observed in the area of active chorioretinitis (Fig. 4.8), which also correlated with complete resolution with immunosuppressive therapy.

Optical Coherence Tomography

Optical coherence tomography (OCT) is useful not only in the evaluation of chorioretinal lesions in the macula but also in the detection of CME and CNV lesions. Thinning of the retina with disruption of the photoreceptor inner and outer segment junction was described mainly in the macular lesions and in the peripapillary atrophic scars of MCP by several authors (Spaide et al. 2008; Yasuno et al. 2009). Drusen-like accumulation at the sub-RPE layer and inflammatory cells in the posterior vitreous can also be found in the acute lesions of MCP (Vance et al. 2011) (Fig. 4.9). The sub-RPE lesion may sometimes have a vertically elevated “volcano eruption-like” appearance on OCT, which can decrease in size in response to anti-inflammatory therapy (Fig. 4.10). OCT also allows visualization of inflammatory cells in the posterior vitreous. The choroidal pathologies were visualized by the development of high-penetration OCT. Localized thinning of the choroid, occlusion of the choroidal vessels, and localized hyper-reflectivity were noted in MCP lesions, accounting for hyper-pigmentation of the choroid (Yasuno et al. 2009) (Fig. 4.11). It can be difficult to distinguish the acute lesions of chorioretinitis from inflammatory CNV based on clinical examination and FA. However, CNV demonstrates characteristic OCT features, such as a sub-RPE component infiltrating the outer retinal layer and associated fluid exudation, not so commonly seen with inflammatory lesions (Amer et al. 2015)