Q91.0 Trisomy 18, nonmosaicism (meiotic nondisjunction)

Name of the Condition

• Trisomy 18, nonmosaicism (meiotic nondisjunction)

Summary

Trisomy 18, nonmosaicism (meiotic nondisjunction) is a chromosomal disorder characterized by the presence of an extra copy of chromosome 18 in all cells, resulting from an error in meiotic cell division. This condition leads to severe congenital anomalies and multisystem developmental abnormalities. The nonmosaicism form indicates the extra chromosome is present in every cell, distinguishing it from mosaic variants where only some cells carry the abnormality.

Causes

Trisomy 18, nonmosaicism is caused by meiotic nondisjunction, an error during the formation of reproductive cells (eggs or sperm) where chromosome 18 fails to separate properly. This results in a gamete with an extra chromosome, which, when fertilized, leads to a zygote with three copies of chromosome 18. The exact reasons for nondisjunction are not fully understood but may involve age-related factors in parental gamete production.

Risk Factors

• Advanced maternal age (increased risk with maternal age over 35).
• Paternal age (less significant but may contribute).
• Family history of chromosomal abnormalities (rare, as most cases are sporadic).

Symptoms

• Severe intellectual disability and developmental delays.
• Low birth weight and poor growth.
• Distinctive facial features (e.g., small head, low-set ears, small jaw).
• Cardiac defects (e.g., ventricular septal defects, patent ductus arteriosus).
• Skeletal abnormalities (e.g., clenched fists, rocker-bottom feet).
• Organ malformations (e.g., kidney, gastrointestinal, or lung defects).

Diagnosis

Prenatal diagnosis is typically achieved through chorionic villus sampling (CVS) or amniocentesis, which analyze fetal cells for chromosomal abnormalities. Noninvasive prenatal testing (NIPT) may also detect trisomy 18 via cell-free DNA analysis. Postnatal confirmation involves karyotyping (chromosomal analysis) of blood or tissue samples to identify the extra chromosome 18.

Treatment Options

Management focuses on supportive care, as trisomy 18 is not curable. Treatment addresses specific symptoms and complications, such as cardiac surgery for structural heart defects, physical therapy for motor delays, and nutritional support. Multidisciplinary care involving pediatricians, cardiologists, and geneticists is often required.

Prognosis and Follow-Up

Prognosis is poor, with most affected infants not surviving beyond the first year of life. Survivors often have severe developmental delays and require ongoing medical and developmental support. Follow-up care includes regular monitoring for complications (e.g., respiratory infections, feeding difficulties) and coordination with specialists to optimize quality of life.

Complications

• Respiratory failure due to underdeveloped lungs.
• Feeding difficulties and failure to thrive.
• Seizures or neurological impairment.
• Increased susceptibility to infections.
• Gastrointestinal issues (e.g., pyloric stenosis, esophageal atresia).

Lifestyle & Prevention

While trisomy 18 cannot be prevented, genetic counseling may be recommended for families with a history of chromosomal disorders. Prenatal screening and diagnostic testing can provide early detection, allowing for informed decision-making and preparation for care needs.

When to Seek Professional Help

Seek immediate medical attention if an infant exhibits symptoms of trisomy 18, such as poor feeding, weak cry, or abnormal physical features. Prenatal care providers should be consulted if screening tests suggest chromosomal abnormalities.

Tips for Medical Coders

Code Q91.0 is specific to trisomy 18 with nonmosaicism due to meiotic nondisjunction. Documentation should specify the chromosomal abnormality and mechanism (meiotic nondisjunction) to support accurate coding. Avoid using this code for mosaic trisomy 18 or other trisomy 18 variants, as they require distinct codes. Ensure clinical notes confirm the absence of mosaicism and the meiotic origin of the nondisjunction.