Ambiguity of pachychoroid
New findings will significantly change ideas about central serous chorioretinopathy and allied diseases.
Richard Spaide MD
This year’s EURETINA Lecture was delivered at the EURETINA 2020 Virtual Meeting by Richard Spaide MD, who took a hard look at the meaning of pachychoroid.
It turns out that pachychoroid is more complicated and confusing than we had previously thought, and Dr Spaide helped set the record straight.
What is pachychoroid, exactly? Dr Spaide, based in New York City, had the same question in mind when he decided to review all papers published between September 2013 and April 2020 and available on PubMed.
“Curiously, 17 of the 47 papers about pachychoroid contained no actual definition of pachychoroid. “Choroidal thickening” was the definition for seven papers.
Another 19 papers defined the amount of choroidal thickness in micrometers without any agreement about how thick the choroid should be to be considered pachychoroid,” he said. The thresholds were set at levels that were normal or even below normal levels for choroidal thickness.
Still other papers defined pachychoroid based simple on the appearance of the choroid, such as the presence of directly visible pachyvessels with or without choroidal vascular hyperpermeability.
“This resulted in the remaining 30 papers containing a total of 22 different definitions for pachychoroid. Many of these definitions would end up classifying normal eyes as having pachychoroid.”
One of the integral components of pachychoroid is pachyvessels, but there was a similar disagreement in papers regarding the definition of the latter.
“In a total of 25 papers on pachyvessels, five papers had no definition, eight papers had a simple definition and 12 papers contained seven complex definitions of the condition.” Dr Spaide also questioned the precision with which a subgroup of choroidal vessels, the choriocapillaris, could be measured.
Since the axial resolution of the most precise OCT devices is currently no smaller than 7 microns, it is not possible to see thinning of the choriocapillaris with the current generation of OCTs.
“What about other causes of a thick choroid?” he asked. “More than 20 distinct conditions, such as Vogt-Koyanagi-Harada syndrome, nanophthalmos, and dome-shaped macula in pathologic myopia, cause either generalised or focal thickening of the choroid, but are not considered to be pachychoroid syndrome.”
More recently, the definition of pachychoroid has been expanded. “In fact, the choroid does not even have to be thick to be considered pachychoroid.” So, what does it have to be? Dr Spaide suggested choroidal vascular hyperpermeability, but, he countered, “this occurs in more than 20 different, unrelated conditions that are not included in the pachychoroid spectrum – why not?”.
“So, I think you’ll agree that pachychoroid spectrum is incomplete, poorly defined and lacks thematic focus. Is it even a spectrum if most of the entities that could be included are not?”
Dr Spaide suggested that the term ‘pachychoroid’ might have utility as a term simply used to describe things, much like the term “white-dot syndrome”.
Dr Spaide then shifted his focus from choroidal thickness to the choroidal circulation itself, in which both the arterial supply and the venous drainage are segmental. Experimental occlusion of vortex veins shows poor communication between adjacent systems. In non-pathologic eyes, they are separated by watershed zones between the vortex veins where large vessels do not normally cross.
However, in central serous chorioretinopathy (CSC), large intervortex venous anastomoses are present. These are big vessels that cross the watershed zones and are concentrated in the macular region. In peripapillary pachychoroid syndrome, there are anastomotic vessels around the nerve.
In cases of CSC that have resulted in choroidal neovascular membranes, “giant” anastomotic vessels can be seen on indocyanine green angiography (ICG). Similar findings can be seen in cases in which CSC has progressed to polypoidal choroidal vasculopathy (PCV).
An online viewer asked, “Do these anastomoses form in a neovascular fashion?” Dr Spaide answered: “No, they seem to represent expansion of previously existing channels, and the pulsatile motion of the blood flow suggests that there might be a venous outflow problem in CSC.”
These new observations about choroidal vascular anatomy may lead to a new concept of pathophysiology of central serous chorioretinopathy and its allied disorders. Not only the large vessels are of interest, but also the tiny vessels of the choriocapillaris. Dr Spaide concluded that “choriocapillaris parameters might be more important than choroidal thickness itself”, he said.
Dr Spaide investigated the choriocapillaris in patients with normal choroidal thickness and those in the 95th percentile of choroidal thickness and found no difference in choriocapillaris parameters. However, the choriocapillaris in patients with CSC was really quite different, he said.
“However, looking at the choroid is still useful, because CSC-related diseases such as choroidal neovascularisation and PCV have been found to have choroidal vascular similarities not previously detected. We really need to look at the choriocapillaris to determine what is going on in these diseases.”
He concluded that pachychoroid is not that useful a term because of the diversity of definitions about every component, and because a thick choroid does not necessarily suggest pathology. The intervortex venous anastomoses are a new and important finding and have the potential to cause a paradigm shift in our understanding of disease.