Acute right lower quadrant pain in a young woman: ultrasound, CT, or MRI?

Initial strategy: transabdominal and transvaginal ultrasound first, with CT if ultrasound is non-diagnostic and the clinical picture remains concerning.

Reasoning:

1. Ultrasound is the first-line test in a young woman with suspected appendicitis (ACR Appropriateness Criteria: usually appropriate). It avoids ionising radiation, is readily available, and can assess the appendix, adnexa, and uterus. Graded compression technique with a high-frequency linear probe is used; a non-compressible blind-ending tubular structure >6 mm with surrounding fat stranding supports appendicitis.
2. Ultrasound has limitations: it is operator-dependent, may be non-diagnostic in obese patients or those with a retrocaecal appendix, and has a reported sensitivity of ~75–90% in adults. A negative or inconclusive ultrasound does not exclude appendicitis.
3. CT is the next step if ultrasound is non-diagnostic and clinical suspicion remains. It has high sensitivity and specificity (~95–98%) and can identify alternative diagnoses. In a young woman, a low-dose appendicitis protocol can reduce radiation dose.
4. MRI is an alternative in pregnancy or when CT is contraindicated, but it is not available after hours here and is not necessary as a first test in a non-pregnant patient.
5. Breastfeeding is not a contraindication to CT with iodinated contrast. The amount of contrast excreted in breast milk is negligible, and current guidance (ACR, ACOG) does not require interruption of breastfeeding. The patient should be reassured.
6. Communication: explain to the registrar that ultrasound first is appropriate, that a negative ultrasound will not close the case, and that CT will follow if needed. Document the discussion.

Proceed to CT abdomen and pelvis with IV contrast (low-dose appendicitis protocol if available).

Reasoning:

1. A non-visualised appendix on ultrasound is a non-diagnostic result, not a negative result. In a patient with a high clinical probability (migratory pain, rebound tenderness, neutrophilia, CRP 48), the post-test probability of appendicitis remains substantial.
2. CT is the most appropriate next test because it has high diagnostic accuracy, is available after hours, and can identify alternative diagnoses (e.g. gynaecological pathology, ureteric calculus, inflammatory bowel disease, mesenteric adenitis).
3. Low-dose CT should be used where available (e.g. reduced tube voltage, iterative reconstruction) to minimise radiation, particularly in a young woman.
4. MRI is an alternative if CT is contraindicated or if the patient strongly prefers to avoid radiation, but it is not available after hours and may not change immediate management. If the patient is stable, MRI could be arranged the following morning, but this delays diagnosis and risks progression to perforation.

Risks of each option:

• CT: radiation-induced cancer risk is small but non-zero, particularly in a 24-year-old woman (lifetime attributable risk of the order of 0.1–0.5% for a standard abdominopelvic CT; lower with low-dose protocols). Contrast risks: allergic reaction (rare), contrast-induced nephropathy (low with normal renal function).
• MRI: no ionising radiation, but longer scan time, claustrophobia, availability, and potential need for contrast (gadolinium) - though non-contrast MRI appendicitis protocols exist.
• Observation without imaging: risk of perforation, abscess, sepsis, and increased morbidity; not appropriate in a patient with a high clinical probability.
• Diagnostic laparoscopy: invasive, with surgical risks, and may be negative; imaging should precede surgery where possible.

Decision: proceed to CT with contrast, after explaining the rationale and obtaining informed consent. Document the discussion and the low-dose protocol used.

Diagnosis: acute uncomplicated appendicitis (12 mm appendix, fat stranding, appendicolith, no abscess, no free gas).

Management options:

1. Laparoscopic appendectomy is the standard of care for uncomplicated appendicitis in a fit young adult. It is definitive, removes the risk of recurrence, and allows histological confirmation.
2. Non-operative management with antibiotics is an option in selected patients (e.g. those who wish to avoid surgery, or with significant comorbidity), but it is associated with a higher rate of recurrence (up to 20–30% within 1–2 years) and the need for interval appendectomy in some protocols. It requires close follow-up and clear counselling.
3. Interval appendectomy is generally reserved for patients managed non-operatively with a persistent appendicolith or recurrent symptoms, or for those with a phlegmon/abscess initially treated with antibiotics and drainage.
4. Percutaneous drainage is indicated for a large abscess (>3–4 cm) - not applicable here.

Role of imaging in follow-up:

1. Routine imaging after uncomplicated appendectomy is not required if the patient recovers uneventfully.
2. If managed non-operatively, follow-up imaging (CT or MRI) may be considered at 6–12 weeks to confirm resolution and to assess for an appendicolith or persistent changes, though practice varies; clinical follow-up is the mainstay.
3. If the patient deteriorates (fever, increasing pain, sepsis), repeat imaging (CT) is indicated to look for abscess, perforation, or an alternative diagnosis.
4. Histology should be reviewed; if an unexpected tumour (e.g. neuroendocrine tumour, adenocarcinoma) is found, further imaging and MDT discussion are needed.
5. Counselling: explain the diagnosis, the options, the risks of recurrence with antibiotics, and the follow-up plan; document the discussion and the patient's choice.