Right iliac fossa pain at 24 weeks of pregnancy with a non-visualised appendix

The ultrasound is indeterminate, not negative. Failure to visualise the appendix in a pregnant patient is common - the caecum is displaced cephalad by the gravid uterus and bowel gas and the fetus limit the acoustic window. The mild right hydronephrosis is a physiological finding of pregnancy in most cases and does not explain guarding and fever.

Next step: MRI of the abdomen and pelvis without gadolinium.

• MRI has sensitivity and specificity above 90% for appendicitis in pregnancy and is the recommended second-line test when ultrasound is non-diagnostic (ACR Appropriateness Criteria; ACOG and ACR guidance).
• It avoids ionising radiation entirely and also identifies alternative diagnoses - ureteric calculus, ovarian torsion, degenerating fibroid, preterm labour.
• It should be done urgently, not electively: the value falls if surgery is delayed pending a 9 am slot.

If MRI is genuinely unavailable within a few hours and the clinical picture is evolving, a low-dose CT is acceptable, because the fetal dose from an abdominal CT is on the order of a few tens of milligrays - well below the threshold for deterministic effects, with only a very small stochastic risk. Untreated perforated appendicitis is far more dangerous to mother and fetus than the scan.

Protocol (1.5 T preferred, no oral or intravenous contrast, no bowel preparation):

• Coronal and axial T2-weighted single-shot fast spin echo (HASTE/SSFSE) of the abdomen and pelvis - fluid-sensitive, motion-robust, no breath-hold needed.
• Axial and coronal T1-weighted gradient echo (in- and out-of-phase if available), which shows the appendix as a blind-ending tubular structure and highlights haemorrhage or proteinaceous fluid.
• Axial fat-suppressed T2 (or STIR) to look for periappendiceal fat stranding and free fluid.
• Diffusion-weighted imaging (b = 0, 400, 800) with ADC - restricted diffusion in an inflamed appendix increases confidence and is quick to add.
• Limit the field to the region of interest; keep the examination under about 30 minutes and use feed-and-warm/positioning rather than sedation.

Gadolinium: avoid unless the diagnosis cannot be made without it. Gadolinium crosses the placenta, and although no teratogenic effect has been shown in humans, the safety data are limited; ACR guidance is to use it in pregnancy only when it will materially change management. A non-contrast study is almost always sufficient for appendicitis. If a abscess or alternative diagnosis genuinely requires contrast, discuss with the obstetric team and document the decision.

This is now a clinical, not a radiological, emergency. Increasing pain with rebound tenderness in a pregnant patient with fever and leucocytosis is a surgical indication irrespective of imaging. The differential includes perforated appendicitis, which carries high fetal loss and maternal morbidity if left.

Options, in order of preference:

1. Proceed to diagnostic laparoscopy without further imaging, with the surgical and obstetric teams agreeing pre-operatively. Laparoscopy is safe in pregnancy at 24 weeks with open (Hasson) entry and low insufflation pressure; the risk of a negative appendicectomy is outweighed by the risk of perforation.
2. Low-dose CT abdomen/pelvis if the surgical team is genuinely undecided and a scan can be obtained faster than theatre. Justify it explicitly: fetal dose is small, the stochastic risk is negligible compared with untreated perforation, and CT also excludes calculous disease and abscess.
3. Transfer to a centre with 24-hour MRI only if the patient is stable and transfer time is short; a 10-hour wait is not acceptable.

Do not simply observe and repeat the ultrasound. Document the multidisciplinary discussion, including the radiation and surgical risk counselling given to the patient.