Chapter 3:          What is a Choroid Plexus Cyst?

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3.1           Anatomical Definition

These fluid filled structures seen within the choroid plexus, which when they reach a certain size can now be seen on ultrasound, originate from entrapment of CSF by the out-folding fronds of choroid, and were further described by Shuangshoti and Netski^{172, 174}.  They defined cysts and tubules with the following observation:

 

... a tubule is a neuroepithelial-lined duct formed by the folding of epithelium on the surface into the stroma of the choroid plexus.  The ducts often branch and, hence may be simple or compound.  A neuroepithelial cyst is a rounded structure formed by folding of the neuroepithelium into the stroma or of the matrix into the cerebral ventricle.  We arbitrarily designated cysts as having a diameter 5 times larger than the average tubule.”¹⁷⁴

 

An anatomical dissection of ultrasound-revealed cysts was performed by Farhood⁷⁵, in which the “cysts were composed of a space surrounded by the loose stroma of the choroid plexus.”  Similarly, one of Fitzsimmons’⁷⁹ autopsy cases had microscopic sections cut which showed “a simple cyst lined with cuboidal epithelium, similar to findings reported by Shuangshoti et al.” 

 

These findings are crucial in that they confirm that the cysts are part of normal choroid development and not some independent anomaly seen only in malformed fetuses.

3.2           Ultrasound Definitions

Several definitions of the ultrasound appearance of CPC have been suggested, but a true consensus on what constitutes a “cyst” may never be reached.  The first attempt at a description was provided by Chitkara⁴⁷, who described them as:

 

 ... echolucent structures within the choroid, [which] have sharply circumscribed margins.”

 

DeRoo⁶² was more specific when she described a cyst as:

 

... a discrete round, anechoic structure greater than 2mm in diameter completely within the substance of the choroid plexus ... reproducible in two orthogonal planes.”

 

However, many “cystic” spaces do not fall into such a tight category and to some it might have seemed rather arbitrary to exclude such appearances without any scientific justification.  Hertzberg¹⁰² provided two categories, simple and complex:

  

Lesions classified as simple cysts satisfied all of the following criteria: anechoic, round or ovoid shape, sharply defined, without septations or nodules.  The abnormalities classified as complex failed to satisfy one or more of these characteristics.”

 

Fig 4.  Example of a typical “simple,” oval CPC.

 

 

We have noted several “complex” CPC, which by other’s definitions may have been termed multiple or multilocular.  An example is given in Fig 5.

 

Fig 5.  Example of “complex”, loculated CPC.

 

 

Gabrielli⁸³ also noted the presence of “thick, hyperechogenic walls” on cysts, a finding which has not been corroborated by other authors.  This appearance may represent enhancement caused by transmission through the cyst, or by an artifact or “optical illusion” whereby the the normal echogenicity of the choroid plexus surrounding the cyst appears relatively bright when seen against the echopenic developing brain in coronal-section images, a phenomenon known to produce edge enhancement (Mackie lines).  In our experience, cysts may appear to have bright walls in one plane of section but not when viewed in other planes, and therefore represent artifact.

 

Bronshtein³⁰ noted a “spongy appearance of the plexus in the 14th week [that] was shown to develop into a simple cyst” in one case.  We have also noted many cysts which were complex (by Hertzberg’s classification) or spongy (by Bronshtein’s description) with a cork-screw or serpiginous shape within the choroid – we described them as tubular.  An example is given in Fig 6.  At first we thought these structures were dilated veins in the choroid (being presumed to be a predominantly vascular structure), but evaluation with pulse and colour Doppler failed to produce a frequency shift.  We believe these correspond to the larger tubules, the incipient cysts, described by Shuangshoti et al^{172, 173, 174}.

 

Fig 6.  Example of a compound or tubular CPC.

 

3.3           Choroid Plexus “Pseudocyst”

Nelson¹²⁶ described the appearance of a choroid plexus “pseudocyst”, which was the result of a partial volume artifact.  In his “pseudocyst”, the corpus striatum is projected within the choroid plexus on certain planes of section.  Scanning at right angles is said to remove this artifact and to reveal the true nature of the structure as being outside the choroid plexus.  This artifact is very easy to obtain.  Images of cysts reproduced is in other articles do not appear to be “pseudocysts”.  Similarly, we are confident that none of the CPC in the present study are “pseudocysts”.

 

3.4           Conclusion

As these CPC are not formed in the same manner as abdominal or pelvic serous cysts, it seems inappropriate to restrict the definition to fluid collections that have the classic ultrasound characteristics (round, anechoic, etc.).  As these serpiginous, spongy or complex tubules constitute part of the spectrum of abnormal ultrasound appearances in the choroid plexus, it seems difficult to exclude them from discussions of the significance of CPCs.  Sanders¹⁶⁷ offers a less restrictive definition when he states:

Echopenic areas that form in the choroid plexus visible between 15 and 25 weeks are known as choroid plexus cysts.