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Radiology
Introduction to Aneuploidy and Other Cytogenetic Abnormalities
Built from Obstetric Imaging
What’s inside
8 sections · 81 slides
Overview
- What this topic covers
- The one-line version
Chromosomes and what a normal count means
The counting rules everything else depends on
- The normal human chromosome count
- How one chromosome is built
- Why the copied chromosome matters
- What aneuploidy actually means
- Two sizes of the same problem
- When only a piece is wrong
- Polyploidy: whole extra sets
- Where triploidy comes from and how it ends
How common aneuploidy is
The number moves with maternal age and with how early in pregnancy you look
- Two things change the frequency
- 1:1065
- Why an early test finds more than the birth rate
- Loss between 12 weeks and term
- Loss rates in trisomy 21 pregnancies
- The same woman, two different numbers
Screening and diagnosis
What raises suspicion, and what settles the question
- How screening has changed
- What cell-free DNA screening is
- Screening is not the same as diagnosis
- Every positive screen needs confirming
- From risk to answer
- How an amniocentesis sample is taken
- What each sample gives you
- A structural anomaly changes the odds
Why the count goes wrong
Cell division, and the moment it fails
- Where the fault happens
- Meiosis: making an egg or a sperm
- The four steps of meiosis
- What separates in each division
- Disjunction and its failure
- From a faulty gamete to a trisomy
- Nondisjunction in meiosis leading to trisomy 21
- Errors after fertilisation
- Which chromosome fails where
- How much happens after fertilisation
- What drives nondisjunction
- 1:1339
The conditions themselves
Which ones survive to birth, and what they look like
- Missing an autosome is almost always fatal
- Turner syndrome, the one survivable monosomy
- A karyotype with only one X chromosome
- What Turner syndrome shows on the scan
- Not every trisomy is fatal before birth
- How often each one occurs
- Survival with trisomy 13 and trisomy 18
- An extra copy of chromosome 18
- Trisomy 18 on ultrasound: the hands
- Trisomy 18: the rest of the picture
- Trisomy 13 is rare but shows itself
- Why the trisomy 13 face and brain go together
- The prechordal mesoderm story
- Trisomy 13 findings from head to abdomen
- Trisomy 21, the one seen most often
- Three copies of chromosome 21
- Trisomy 21 on ultrasound: structural findings
- Trisomy 21 on ultrasound: the softer markers
- Why ultrasound alone is not enough
- Klinefelter syndrome
- Triple X and the extra Y
- Sex chromosome conditions that reach adulthood
Imaging and the tests that follow
From a suspicious scan to a named genetic cause
- What the scan can and cannot do
- Where MRI fits
- When the scan finds a structural abnormality
- The limit of the karyotype
- Why chasing the exact cause is worth it
- Chromosomal microarray
- What microarray adds in practice
- Testing in tiers, coarse to fine
- What to take from the table that follows
- Most Frequent Microduplication or Microdeletion Syndromes With Structural Ultrasound Findings
- Microdeletion syndromes as printed in the book
- When karyotype and microarray are both normal
- Using sequencing responsibly
- The whole pathway in one picture
Key points
What to carry away
- Key points: the condition
- Key points: the testing
- References
- References (continued)
- References (continued)
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- References (continued)
- Suggested reading
- Obstetric Imaging: Fetal Diagnosis and Care, 2nd Edition