Glossary


Amniocentesis

This is a procedure in which fluid is aspirated, usually under ultrasound control, from the amniotic cavity which surrounds the fetus in the uterus.  Fetal cells, which are shed into the amniotic fluid, can be cultured for chromosomal analysis.

Anechoic

Structures and areas which have no internal reflectors or from which no echoes are returned are said to be anechoic and they produce the black areas of the image.

Aneuploidy

An abnormality in the chromosomes.  Either duplication, deletion or transposition of parts of all of each of the chromosomes may occur.  The results of these alterations in the DNA sequence are extremely varied.

Cyst

In the usual ultrasound definition, a cyst is a thin-walled, circumscribed, round, fluid filled, anechoic (q.v.) structure producing distal enhancement (q.v.).  Such a definition was used initially in reference to the benign serous cysts of the visceral  parenchyma and this type of cyst can be present in many organs.  Different pathological events can precipitate the formation of a fluid filled structure which may meet the classic criteria, such as a pseudocyst of the pancreas, or a porencephalic cyst of the brain.  Conversely benign serous cysts may have appearances that do not meet the classical criteria due to internal haemorrhage, pressure from adjacent organs, or the apparent merging with other cysts into multiloculated structures.  

Echogenic

In most contexts, interchangeable with hyperechoic (q.v.) which is the more correct term.

Echopenic

In most contexts, interchangeable with hypoechoic (q.v.) which is the more correct term.

Enhancement

Areas which have lower attenuation relative to those for which the Time Gain Compensation (TGC) curve has been set will produce incongruous echo amplification for the regions below (deep to) them.  This results in a brighter area in tissues which would otherwise be isoechoic (q.v.), such as behind fluid-filled structures like the gall-bladder or a cyst in the liver.

Euploidy

The normal status of the chromosomes is two pairs of 23 types of autosomal chromosomes and one pair of sex chromosomes.  46XX is female euploidy, 46XY is male euploidy.

Hyperechoic

Structures are said to be hyperechoic when strong echoes are reflected from their internal contents.  In some contexts, hyperechoic may to refer to areas which produce stronger reflections than might be expected for that type of tissue. They produce the more white or bright parts of an image.

Hypoechoic

Structures are said to be hypoechoic when only low-level echoes are reflected from their internal contents, producing the darker grey areas of the image. In some contexts, hypoechoic may refer to areas which produce weaker reflections than might be expected for that type of tissue.  Areas in shadow  (q.v.) could be described as hypoechoic. 

Isoechoic

Areas which have similar echogenicity are said to be isoechoic to each other.

Shadow

Structures which have high attenuation by either absorption, reflection or refraction can produce a sonographic shadow  behind (deep to) them.  By affecting the incident and /or reflected beam these structures, such as the fetal skull vault, produce hypoechoic (q.v.) areas which would otherwise be isoechoic (q.v.) with the adjacent tissue. 

 

Triploidy

All 23 pairs of the autosomes have an extra chromosome, with the result being 69 chromosomes.

Trisomy

A third chromosome is attached to one of the 23 pairs of normal autosomes in trisomy.  If a third chromosome attaches to the 18 chromosome pair, this becomes Trisomy 18.

 

Trisomy 18 (Edward’s Syndrome, T18)

Edward’s Syndrome was first described in 1960, where infants with an extra 18 chromosome were noted to have a pattern of defects primarily consisting of;  clenched hands with camptodactyly, short sternum, abnormal fingerprints, single umbilical artery (SUA),  intra-uterine growth retardation (IUGR) and ventricular septal defect (VSD).  Over 130 different abnormalities have been reported in association with T18.  Survival prospects are limited, as only 10% remain alive after a year, with severe mental retardation and poor quality of life.  The incidence of T18 is variously quoted as being 1/7000, 1/5000,  0.3/1000 (1/3333) or  1/6600.  These figures are quite skewed as they do not necessarily include the very great fetal losses due to miscarriage or stillbirths.  Pregnancy loss rates are quoted as 70%.  Several large series of amniocenteses have found T18 to be the most prevalent in-utero chromosomal abnormality. 

 

According to the Perinatal Data Collection Unit of the Consultative Council on Obstetric and Paediatric Mortality and Morbidity in Victoria, chromosomal abnormalities form a significant part of the congenital malformations and birth defects notified in Victoria.  In 1991, there were 260 chromosomal malformations reported, out of a total of 2462 malformation, a rate of 10.5%.  These figures include those induced after diagnosis, still births, neonatal deaths, infant and childhood deaths and those still alive. 

 

 T18 is the principal genetic malformation associated with CPC, and in 1990 and 1991 there were 23 and 34 cases reported respectively.  Over the two years, this constitutes a rate of 57/131,733 or 1/2311 (0.433%).  The higher incidence of these Victorian figures reflects the reporting of stillbirths and pregnancies terminated after prenatal diagnosis of T18 was made, and therefore more closely represents the incidence of this defect in the fetal population.  +

Trisomy 21 (Down Syndrome, T21)

Down Syndrome was classified in 1866 when it was noted by Down that a “large number of congenital idiots are typical Mongols.”  The following abnormalities constitute this typically; hypotonia with a tendency to keep the mouth open and protrude the tongue, small stature and awkward gait, brachycephaly with flat occiput, low nasal bridge, inner epicanthal folds, hypoplasia of the middle phalanx of the fifth digit, wide gap between first and second toes, ventricular septal defect, thickened posterior neck and a variable degree of mental deficiency.  The incidence of Down syndrome has been quoted as 1/660 newborns, making it, according to Smith, the most common pattern of malformations seen in humans.