|
|
|
|
|
|
|
|
|
|
|
|
|
|
Chapter 7: The Paradigm is Established
Not unreasonably, those reporting on the appearances of CPC
have sought to provide a set of objective criteria which would discriminate
between the cyst which is a normal variant and the cyst which is associated
with a chromosomal abnormality. Their
readers were seeking guidance. Several
physical appearances of the cyst seemed worthy of comment by these authors,
but from very small numbers generalisations were made which, upon evaluation,
now appear hasty. However, these initial
suggestions are still in print and read by many ultrasound practitioners in
training. They constitute the paradigm
by which many sonographers still filter their experience of CPC.
7.1 Early Protocols used the Size, Persistence, Bilaterality Paradigm
The early authors offered the first protocols for dealing
with CPC, and these soon became the established way of rationalising further
investigation. Several authors who also
published smaller series or case studies on this topic were at a loss to
produce any novel criteria and preferred rather to reiterate the suggestions of
earlier authors. Because of the
paradigm, they were unable to challenge or contradict the established opinion,
despite not having the actual data to support it. These decisions were probably based more on
caution than on any logical evaluation of the arguments initially proposed or
on any new scientific evidence. This has
had the unfortunate effect of reinforcing those early suggestions and the
anecdotal support for the criteria became entrenched scientific opinion. Subsequently and inevitably, contradictions
arose in further articles as the number of reported cases increased, but not
all of these contradictions and inconsistencies appear to have been noted by
the obstetrical community, typical treatment for facts which do not fit a
predominating paradigm.
Initially, no comment was made from the first reports
regarding associations and protocols.
Ricketts159 first suggested that CPC were a normal
variant. Ostlere144
considered that further studies were needed, as did Clark50, DeRoo62
and Chan45. They soon came, but
they did not settle the issue.
The first protocol suggested was that of amniocentesis for
CPC which were large, bilateral, persistent or associated with other
abnormalities. This was initially
suggested by Chitkara47, and based on the data from Furness81,
Nicolaides131 and Ricketts159. It was supported in the late eighties and
early nineties, either in full or in part, by Donnenfield69,
Hertzberg102, Ostlere144, 145, Khouzam111,
Camurri40, Twining189 and Walkinshaw193. As other studies began to show that
bilaterality and persistence had less of an association, these criteria began
to lose ground. Large cyst size however
remained a frequent recommendation for amniocentesis. There seemed to be move away from these
criteria from about 1992 (see Table 7.)
|
Karyotyping Suggested |
Routinely |
Only If: Large |
Bilateral |
Persistent |
other Anomalies |
Repeat Scan |
|
Before
1992 |
6 |
|
4 |
3 |
14 |
4 |
|
After 1992 |
8 |
1 |
1 |
1 |
7 |
3 |
Table 7: Recommendations for karyotyping. Some authors suggested several
criteria.
Repeat scans were frequently recommended. This of course was a necessity when
persistence was being evaluated, but it was also suggested to check for
enlarging or diminishing size, or for normal brain development. Another reason to rescan was to obtain the
better anatomical detail achievable later in pregnancy. A recent article125 suggested
rescanning every 2 - 3 wks to check for development of IUGR, a common finding
in T18.
The very high association of T18 with structural defects
became obvious as better equipment was being used, and this was certainly a
less contentious qualification, except when small cardiac defects and other
subtle findings were being missed152. The pick-up of fetal malformations with
ultrasound has not been impressive.
Authors who claimed to have done detailed scans appeared to be missing
many potentially detectable structural malformations, which were subsequently
found on autopsy. A harsher protocol was
suggested.
7.2 Routine Amniocentesis
Because of their disappointment with the criteria several
authors independently suggested that amniocentesis should be offered routinely
in the presence of CPC at the time of diagnosis. Hershey100 was concerned about
delaying diagnosis until beyond the legal age of termination in the
7.3 Anomaly Scan
All authors agree that the presence of concomitant
abnormalities warrants amniocentesis.
Six authors were of the opinion that good detailed scanning was suitably
accurate to detect most cases of T18 on an extended anomaly scan. These latter authors – Nadel123,
Howard106, Oettinger142, Gross91 and Buttery36
– were concerned that the cyst criteria advocated earlier may be of dubious
value. They felt that the rate of
diagnosis by detailed scanning was sufficient to indicate those at a level risk
higher than that the loss rate of amniocentesis, so that normal pregnancies
would not be lost unnecessarily.
7.4 Australian Protocols
Two articles about CPC have been published by Australian
authors. The first was by Furness81
in 1986, which seems to have started much of the controversy. She found 30 CPC, three of which were T18 and
one of these was “extensive”, replacing the choroid. Furness describes her current protocol (late
1993) as “fence-sitting”†
. She obtains the karyotype in CPC
larger than 8mm, and obtains the AFP in those 7 - 8mm. Smaller cysts are disregarded after a
thorough anatomical survey (particularly of the hands and heart) shows no
abnormality.
The second article was more recent, by Burrows34. Her late 1993 article was a case report,
which came to the conclusion that the diagnosis of any CPC should be
followed-up with amniocentesis. This
conclusion prompted several responses in the Australian journal.
One correspondent258
suggested amniocentesis if there were concurrent abnormalities and a repeat
scan to check for persistence if there was not. Another respondent detected a “slight lack
of logic,” and calculated a risk of 1:454 for isolated choroid plexus cysts,
therefore making amniocentesis too risky.
A final response came from a group of high profile
obstetrician/sonologists including the late Beresford Buttery, who concluded
that:
“The correct path is to recommend a thorough morphological assessment by a skilled operator with appropriate equipment. Other markers or abnormalities would then be identified and, in their presence, a stronger case for amniocentesis would be sustained.” (Buttery et al36)
7.4 Publishing Bias in Response to the Paradigm.
Many of the smaller studies, and particularly the case
studies, may have been subject to a publishing bias on the part of the authors
and editors. As the paradigm of cyst
criteria became popular, the meaning of CPC changed. They took on a new significance. They became established structures, not just
unexplained irregularities in the choroid plexus, often totally ignored. They now had a name, and they had size,
number, bilaterality, complexity and the potential to persist. These were the criteria which had been
described in the literature and now which needed to be evaluated. As the paradigm became established, only
those cysts to which these characteristics could be ascribed were
noticed.
Some sonographers have attempted to rationalise their
response to irregularities in a patient’s choroid plexus by saying they are not
“cysts”, because they are smaller than some arbitrary size, or irregular in
shape or outline. As discussed earlier,
the problems of a universal definition might have affected the figures in the
published articles, particularly in relation to size and shape.
If a CPC was large and bilateral in a trisomic fetus, the
instinct to write up the case would be much higher than if it was in a normal
fetus. The editor would be more likely
to publish papers which conformed to the preconceptions. In a series where contradictory facts are
evident to unbiased assessment, emphasis would be placed on those “facts” which
fitted the paradigm. Evidence of this is
seen in the articles by Furness81 and Chitakara47 which
are discussed further in the following sections. In articles where no conclusion should be
drawn, the paradigm is confirmed with little if any comment. Evidence of this is seen in the articles and
letters by Khouzam111, Hertzberg102, Hershey100,101,
Lodeiro120, Burrows34 and Carmody238. These biases towards the paradigm are
discussed in the later chapters of this research.
7.5 Conclusion
The paradigm thus became
established. In late 1992, the following
statement was made by Hershey101:
“The experience of other physicians has suggested that the risk of trisomy 18 may be a concern only in that subgroup whose cysts fulfil three criteria: large size, bilaterality, and persistence”. (Emphasis mine)
Note that he uses the word “experience” rather than
“scientific analysis”, hinting at the
fact that anecdotes (as cases histories) still play a particularly strong role
in guiding treatment in modern medicine.
These three criteria have in fact being espoused less and less in the
literature since 1992. This suggests
that they are of no value in the day-to-day examination of the low-risk patient
and that there are no characteristics of the CPC itself which will differentiate
chromosomally normal from aneuploid fetuses.
As recent larger studies use more careful, reasonable and conservative
evaluation techniques, the paradigm is in the process of being broken down.
In the following chapters, the publishing history of those
articles which support these criteria will be reviewed and the arguments used
to support them will be critically assessed.
In the light of all the reported cases available to my research, their
significance will be re-evaluated.