Chromosomal Abnormalities

Chromosomal abnormalities are a broad category of genetic conditions resulting from numerical or structural abnormalities of chromosomes.

They include aneuploidy, an abnormal chromosome number. This includes trisomy, the presence of an additional copy of a specific chromosome, as in Down syndrome, the commonest autosomal trisomy compatible with prolonged survival. They also include structural abnormalities: deletions, duplications, translocations and inversions.

This entry provides the shared conceptual and diagnostic framework underpinning the individual conditions. The category is broad and diverse, and each condition has its own distinct clinical presentation, natural history and management considerations.

Classification and pathophysiology

• Numerical chromosomal abnormalities (aneuploidy) reflect an abnormal total chromosome number. They most commonly arise from non-disjunction (failure of normal chromosome separation) during meiotic cell division in the formation of the egg or sperm.
• The resulting risk increases substantially with advancing maternal age for the autosomal trisomies and for certain sex chromosome aneuploidies.
• The autosomal trisomies include trisomy 21 (Down syndrome), together with the generally more severe trisomy 18 and trisomy 13. Both of the latter are associated with substantially higher rates of major structural anomaly and significantly reduced life expectancy compared with trisomy 21.
• Sex chromosome aneuploidies include Turner syndrome (45,X), a single X chromosome with absence of the second sex chromosome. It is associated with characteristic short stature, ovarian insufficiency, and specific cardiac and renal anomalies.
• They also include Klinefelter syndrome (47,XXY), an additional X chromosome in a phenotypically male individual. It is associated with hypogonadism, infertility and, frequently, mild learning difficulty.
• These and other, less common sex chromosome variations form a distinct category, generally associated with a more variable, and frequently milder, overall phenotype than the autosomal trisomies.
• Structural chromosomal abnormalities arise from chromosomal breakage and abnormal rearrangement. The principal categories are deletions and translocations.
• Deletions are loss of a chromosomal segment, including 22q11.2 deletion syndrome, associated with a characteristic constellation of cardiac, immunological and craniofacial features.
• Translocations are exchange of chromosomal material between two chromosomes. A balanced translocation has no net loss or gain of genetic material and generally no phenotypic abnormality in the carrier, though it has important implications for reproductive risk in future pregnancies.
• An unbalanced translocation has net loss or gain of genetic material and a correspondingly abnormal phenotype.
• Mosaicism is the presence of two or more genetically distinct cell lines within the same individual, reflecting a post-zygotic error occurring after normal fertilisation. It is an important additional consideration across this category.
• The proportion and tissue distribution of the abnormal cell line directly influence the phenotype, generally producing a milder, more variable clinical presentation than the equivalent non-mosaic condition.

• Clinical presentation varies enormously according to the particular chromosomal abnormality. It ranges from profound, multisystem structural and functional abnormality incompatible with prolonged survival, through to subtle, sometimes barely apparent phenotypic variation. No single, unified clinical description captures the full breadth of this category.
• Common, recurring themes across many chromosomal abnormalities include characteristic dysmorphic facial features. The pattern varies by condition, though it frequently provides an important initial clue prompting further genetic assessment.
• Congenital structural anomalies are also common, particularly cardiac and renal, reflecting the sensitivity of these developing organ systems to genetic dosage abnormality.
• Growth abnormality (either growth restriction or, less commonly, overgrowth) and variable degrees of developmental delay or intellectual disability also recur. The presence, pattern and severity of these features differ substantially between conditions.
• Antenatal identification has become increasingly common and central to the diagnostic pathway for many of the commoner chromosomal abnormalities. It allows proactive parental counselling and pregnancy management planning.
• Antenatal routes are combined first-trimester screening (nuchal translucency ultrasound measurement together with maternal serum biochemical markers), non-invasive prenatal testing (analysing cell-free fetal DNA circulating within maternal blood), and, where indicated, invasive diagnostic testing (chorionic villus sampling or amniocentesis).
• Postnatal presentation, for conditions not identified antenatally, may occur through recognition of characteristic dysmorphic features on newborn examination, or identification of an associated structural anomaly (including a congenital heart defect).
• For milder conditions, presentation may come later with growth abnormality, developmental delay, or specific associated features becoming apparent over the course of childhood.

• Karyotype analysis, examining chromosome number and gross structure under microscopy, is the traditional, foundational diagnostic approach for identifying numerical abnormalities and larger structural rearrangements. Its resolution is limited for smaller, submicroscopic structural abnormalities.
• Chromosomal microarray analysis offers substantially higher resolution than standard karyotype analysis. It is the current first line diagnostic investigation for the majority of clinical indications and can identify smaller deletions and duplications not detectable through standard karyotyping.
• Its important limitation is that it cannot detect balanced structural rearrangements, because there is no net loss or gain of genetic material, which is what microarray technology detects.
• Karyotype analysis therefore retains an ongoing role in certain clinical contexts, including assessment for balanced parental translocations relevant to reproductive risk counselling.
• Fluorescence in situ hybridisation uses specific, targeted fluorescent probes to identify a particular, suspected chromosomal region of interest. It provides a rapid, targeted diagnostic approach where a specific chromosomal abnormality is clinically suspected from the presenting phenotype, and complements the broader, genome-wide approaches.
• Full clinical assessment is an essential, integral component of the diagnostic and management approach. It includes detailed examination for characteristic dysmorphic features and associated structural anomalies, together with imaging (including echocardiography and renal ultrasound, given the frequent association between chromosomal abnormality and structural anomaly at these sites).
• Genetic counselling is essential, both at the time of initial diagnosis and for future reproductive planning, given the potential implications for recurrence risk in subsequent pregnancies.
• Recurrence risk varies substantially according to the mechanism of the identified abnormality, including whether it reflects a sporadic, de novo event or an inherited, parental structural rearrangement.

• Management is directed entirely according to the specific chromosomal abnormality identified and its clinical manifestations. It follows the detailed, condition-specific principles (Down syndrome specifically, together with the broader principles applicable to other, related conditions within this category), rather than any single, unified approach.
• The standard, essential approach is full, coordinated multidisciplinary management. It addresses the structural anomalies, developmental needs and associated medical complications of the particular condition, delivered through specialist paediatric and, where relevant, clinical genetics service involvement.
• Genetic counselling for the family addresses both the implications of the identified condition for the affected individual and the recurrence risk for future pregnancies.
• Appropriate, sensitive, ongoing psychosocial support for the family throughout the diagnostic and subsequent management process underpins compassionate, family-centred care across this category.
• Early, proactive engagement of clinical genetics services is an essential, foundational component of coordinated management from the point of initial diagnosis.