Faecal impaction is a common cause of abdominal discomfort and constipation in children, often leading to emergency presentations. Traditional diagnostic methods rely on clinical examination and abdominal X-rays, which can be subjective and expose young patients to unnecessary radiation. Increasingly, ultrasound, particularly rectal diameter measurement - is being recognised as a safe, accurate, and non-invasive method to support the diagnosis of faecal impaction in paediatric patients.

Understanding Faecal Impaction in Children

Faecal impaction refers to the accumulation of hard stool in the rectum, making it difficult or impossible for a child to pass naturally. It can result from chronic constipation, low dietary fibre, inadequate fluid intake, or behavioural factors such as stool withholding.

Symptoms may include:

• Abdominal pain and bloating

• Reduced appetite

• Soiling or overflow diarrhoea

• Painful bowel movements

• Palpable abdominal mass

While these symptoms are suggestive, clinical examination alone cannot reliably distinguish faecal impaction from other causes of constipation.

Why Ultrasound Is a Preferred Diagnostic Tool

Ultrasound offers several advantages:

• Radiation-free: particularly important for children.

• Immediate results: allows same-day assessment and management.

• Non-invasive and well-tolerated: most children cooperate well with minimal discomfort.

• Quantifiable findings: rectal diameter measurement provides an objective parameter for diagnosis.
  
  

Rectal Diameter Measurement: Technique and Thresholds

The transabdominal ultrasound technique is typically performed with the child in a supine position. A curvilinear or linear probe is placed in the lower abdomen, just above the pubic symphysis. The rectum is visualised posterior to the bladder.

The transverse diameter of the rectum is measured in millimetres at its widest point, ideally when the bladder is moderately filled to aid acoustic window clarity.

Interpretation:

• A rectal diameter of more than 3 cm is generally considered abnormal and strongly suggests faecal impaction.

• The rectum appears dilated with hyperechoic contents showing posterior acoustic shadowing, representing impacted stool.

• In contrast, a normal rectum appears empty or contains minimal fluid with no acoustic shadowing.
  
  

Supporting Evidence

Multiple paediatric studies have demonstrated a strong correlation between rectal diameter >3 cm and clinical faecal impaction. In some cases, ultrasound diagnosis has outperformed abdominal radiography in accuracy and speed.

Furthermore, ultrasound helps exclude other causes of abdominal pain such as intussusception, urinary tract obstruction, or appendicitis, providing a comprehensive evaluation in one session.

Clinical Application

Rectal diameter measurement is particularly useful in:

• Emergency settings: when rapid diagnosis is required.

• Recurrent constipation: to monitor treatment effectiveness.

• Follow-up visits: to assess rectal emptying after disimpaction therapy.

By integrating this technique into paediatric care, clinicians can reduce radiation exposure and enhance diagnostic confidence.

Accurate, Child-Friendly Ultrasound Diagnosis for Faecal Impaction

Ultrasound measurement of rectal diameter is a simple, non-invasive, and reliable method for diagnosing faecal impaction in children. It complements clinical assessment and can be easily performed in outpatient or emergency settings.

At Ultrasound Insight, our sonographers are trained in the latest paediatric imaging techniques, ensuring gentle, accurate assessments tailored to young patients.

Book a paediatric ultrasound today and experience compassionate, expert care for your child’s digestive health.