Discussion

CM is a relatively rare anatomical abnormality, which can be confused with parasitic infections or even vascular fundus tumours. To date this series is the most sizeable collection of CM cases reported, with this entity mainly described in case reports in older adults. We observed a patient outside the commonly reported range, aged 27 years, and therefore an early onset or congenital nature of CM cannot be excluded. The youngest affected individual in the literature is a case reported by Turgut and Kobat in a 12-year-old with retinitis pigmentosa.11 In agreement with previously reported cases, CM was found in females more frequently.2 Caucasian and African ethnicities were found, but ethnicity was unrecorded for a number of patients. The participant numbers are too small to know whether this is an important factor or it is just easier to discern a CM in a pale fundus.

CM is considered an isolated ocular finding, since no confirmed systemic association has been found. The systemic conditions previously reported in the literature include: hypothyroidism in 2 patients,2 8 hyperlipidaemia in 2 patients,2 hypertension in 1 patient,2 migraine in 1 patient,8 chronic pancreatitis in 1 patient,8 urinary incontinence in 1 patient,8 skin basal cell carcinoma in 1 patient.2 The most frequently observed systemic comorbidities in our cohort were: hypertension in 5 patients (39%), diabetes mellitus in 3 patients (23%), hypothyroidism in 1 patient (8%), peripheral vascular disease in 1 patient (8%), heart disease in 1 patient (8%), hyperlipidaemia in 1 patient (8%), benign prostate hypertrophy in 1 patient (8%), vertigo in 1 patient (8%) and breast cancer in 1 patient (8%).

CM has seldom been found with other ocular disorders in the past, occurring with dry AMD in only three patients8–10 and macular drusen in one patient.5 In contrast, our cohort had a wider spectrum of ocular conditions, including glaucoma, AMD, non-proliferative diabetic retinopathy, retinal detachment, central retinal vein occlusion, branch retinal vein occlusion. It is possible that an ocular comorbidity simply makes it more likely that an asymptomatic CM is detected, rather than representing a true association.

Previous reports found only unilateral CM. In our study, we observed bilateral CMs in 23% of cases. CMs reported previously were all located within the macula, temporal to the fovea with extension towards the equator. Instead, in our cohort we observed CMs at the nasal edge of the fovea and some were extramacular. We also found CMs with an oblique and a vertical orientation. All these observations are novel, and it would be interesting to see if larger studies corroborate these.

Choudhry and Rao,7 Mori et al 12 and Dalvin et al 2 each reported 1 symptomatic case: the first two having metamorphopsia and dry OCT, and the second with decreased vision from SRF and ellipsoid zone alteration/disruption. In our cohort, despite observing ellipsoid zone alteration/disruption in 10 eyes, only the 2 eyes with subfoveal SRF were symptomatic. The exact pathogenic mechanism of SRF is unknown; a possible explanation is that compression exerted by the CM on the choriocapillaris leads in some cases to failure of RPE pump function. One patient was affected by concomitant nAMD, likely to contribute more to symptoms than the CM, though Inoue et al hypothesised that a CM may hasten AMD from relative ischaemia due to compression of the choriocapillaris or direct mechanical action on the RPE cells.10 Other OCT features included irregular RPE in 50% and choroid thickening in 25% of eyes. Using the OCT calliper function, we observed an average vertical diameter (233 µm) of the CM similar to that previously reported (250 µm) and a slightly shorter average length (7.3 mm vs 8.6 mm).2

FAF was normal in 50% of eyes with CM but other patterns were a hyperautofluorescent/hypoautofluorescent track or linear hyperautofluorescence. These findings may represent different stages of the abnormality and its local effects on the overlying RPE. Previous data about FAF are limited and included a variable hyperfluorescence over the lesion.4 5 10

The visualisation of choroidal vascularity on OCT-A is not always straightforward owing to the presence of imaging artefacts16–18: the deep choroidal vessels can be difficult to visualise due to projection artefacts from the vessels of the more superficial layers. In the first report of OCT-A in CM,9 it was described as a relatively low flow vessel in the choriocapillaris. On OCT-A our 2 CMs, identified at the level of both choriocapillaris and choroid, the internal flow was outside the detectable threshold, and this signal fading could relate to a too fast flow creating a fringe washout artefact.17 This finding, together with the early filling on ICGA suggests an arterial nature and hence a high flow of CM.

ICGA is an important tool to understand the vascular nature of the CM and helps to rule out choroidal vasculitis and other alternative diagnoses, such as a choroidal haemangioma. FFA showed non-specific findings, in line with those previously reported,1 2 4 5 7 12 and is of limited value for the diagnosis of CM but it helps in differentiating it from inflammatory causes.

Parasitic infestations are a frequent differential diagnosis of CM as the pigmented chorioretinal atrophic spots may be confused with residual scars from an inflammatory process. Nematode infections, especially Oedemagena Tarandi and Alaria Americanus, cause a serpiginous cicatricial track at the level of the RPE generally accompanied by exudation.1 Only one patient had a history of tropical travel, but intraocular inflammation was absent in all our cases. Diffuse unilateral subacute neuroretinitis has been included in the differential diagnosis of CM,1 but it does not produce subretinal tracks and OCT-A allows the visualisation of the worm as an avascular hyper-reflective vermiform lesion.19 20

Some cases are referred with a possible choroidal neoplasm, particularly circumscribed choroidal haemangioma. However, circumscribed choroidal haemangioma has different features: on OCT it has a dome-shaped configuration and circumscribed dilated choroidal vascular channels, and on ICGA has an early filling with early maximal fluorescence and a late wash-out.21 22 This pattern and the shape of the lesion is different to the CM.

Other vascular lesions also need to be ruled out. Vortex veins ampullae are venous in nature while CMs appear to be arterial. CMs differ from retinochoroidal anastomoses that shunt retinal venous drainage into the choroid, since they do not communicate with the retinal circulation. Moreover, none of our cases had a history of previous ocular trauma, inflammation and perifoveal teleangectasis that could have caused abnormal shunts.23 24 Anomalous posterior ciliary vessels provide the blood supply for the anterior uvea, unlike the CM, which extends posteriorly. Congenital retinal macrovessels can be distinguished by retinal location on OCT and delineation on FFA showing a prolonged transit time with possible ischaemia and leakage.1 7

The limitations of our study include the retrospective nature and the small sample cohort, hence the repeatability of our fundoscopic classification needs to be verified. There is likely to be an ascertainment bias as CMs in the posterior pole, rather than in the periphery, are more likely to be detected. We acknowledge that the measurement of the length or calibre of an irregular curved structure in three dimensions is prone to bias. OCT-A was performed in only 2 eyes, so it is unclear how far our findings would generalise to CM. No data on axial length are available. Longitudinal studies including larger numbers of patients will help overcome these barriers to better understand the epidemiology, pathogenesis, natural history and prognosis of CM. Multimodality imaging could help make more easily the diagnosis of this entity, and therefore, will become better recognised.