Suspected Aortic Dissection in a Man With Prior Contrast Reaction and New Renal Impairment
A 61-year-old man with type 2 diabetes and hypertension presents at 02:10 with 6 hours of sudden tearing interscapular pain. Blood pressure is 168/94 mmHg in the right arm and 151/88 mmHg in the left; he is haemodynamically stable and pain is controlled with IV morphine. Bedside transthoracic echocardiography by the emergency physician suggests a possible intimal flap in the descending aorta. D-dimer is 4,200 ng/mL. Admission creatinine is 168 micromol/L with an eGFR of 34 mL/min/1.73 m2; a value from 5 weeks ago was 118 micromol/L. He takes metformin 1 g twice daily and has been nil by mouth since arrival. Six years ago he developed urticaria and periorbital swelling within 10 minutes of an iohexol-enhanced CT, treated with antihistamine only, with no wheeze, hypotension or airway compromise. The vascular surgeon requests a CT aortogram immediately and asks whether the prior reaction and the renal function mean the study must wait until morning.
1. Do you delay CT angiography for corticosteroid premedication? Give a specific regimen and state what benefit it can realistically be expected to provide.
Read the worked answer
Do not delay.
Reasoning:
- The index reaction was mild: urticaria and periorbital oedema, no bronchospasm, no hypotension, no airway involvement. That predicts a low likelihood of a severe breakthrough reaction.
- The clinical question is a life-threatening one. Untreated type A dissection carries roughly 1-2% mortality per hour in the first 48 hours; a 13-hour oral premedication regimen is incompatible with the diagnosis.
- Premedication reduces but does not abolish breakthrough reactions (large series report breakthrough rates of roughly 0.5-1% even after adequate steroids), so it must never be presented as a guarantee.
Practical approach:
- Use a non-ionic, low-osmolar agent (iohexol 350, iodixanol 320) and keep the volume as low as technically possible.
- If more than about 4 hours can be bought, use prednisone 50 mg orally at 13, 7 and 1 hour before, plus diphenhydramine 50 mg orally or IV 1 hour before. If time is shorter, give IV methylprednisolone 40 mg (or hydrocortisone 200 mg) plus an H1 blocker immediately and proceed.
- Have an anaphylaxis kit and anaesthetic support present, keep the patient for 30 minutes of observation in a monitored area, and document the discussion and consent.
- Note that a prior reaction increases the risk of a repeat reaction roughly three- to five-fold overall; it does not convert an emergency into an elective study.
2. How would you reduce the risk of contrast-induced acute kidney injury, and how do you handle the metformin?
Read the worked answer
The dissection question overrides the renal question: the study is done, but it should be done as gently as possible.
Contrast strategy:
- Use a non-ionic low- or iso-osmolar agent; there is no convincing evidence that one low-osmolar agent is superior to another, but high-osmolar agents should be avoided.
- Minimise volume. For a CT aortogram, a targeted chest-abdomen protocol with a weight-based injection of roughly 60-80 mL is usually achievable; avoid a 'full-body' run-off study that is not clinically required tonight.
- Expand the intravascular volume with isotonic saline: 1 mL/kg/h for 6-12 hours before and after, or 3 mL/kg/h for 1 hour before followed by 1 mL/kg/h. Avoid dextrose, mannitol and forced diuresis.
- Stop nephrotoxins (NSAIDs, aminoglycosides, diuretics where possible), avoid repeat contrast within 48 hours, and recheck creatinine at 48-72 hours.
- Note that the creatinine has risen from a baseline of 118 micromol/L, so this may be AKI on CKD rather than stable CKD; that raises risk further and strengthens the case for volume expansion and minimal volume.
Metformin:
- Standard advice is to withhold metformin at the time of contrast and for 48 hours in patients with eGFR below 30, established AKI, or when contrast is given intra-arterially.
- With an eGFR of 34 and an acute rise in creatinine, the safest course is to hold metformin now, monitor for nausea, malaise or tachypnoea, recheck renal function at 48 hours, and restart only if function is stable.
3. The CTA is reported as showing no dissection. The patient becomes hypotensive an hour later with a pulse deficit in the left arm. What are the options now, including non-contrast and non-ionising alternatives?
Read the worked answer
A negative CTA does not end the episode; a new pulse deficit and hypotension re-open the question and suggest either a false-negative study or extension/progression.
Immediate steps:
- Re-examine for a newly audible aortic regurgitation murmur, neurological deficit, or malperfusion (lactate, urine output, limb ischaemia).
- Review the CTA rather than simply repeating it: was the true lumen and the entire arch to the iliac bifurcation adequately opacified? Motion, poor bolus timing and unopacified false lumen are the usual causes of a false-negative.
- Activate the aortic team and the operating theatre rather than the scanner if the patient is unstable.
Alternatives:
- Transoesophageal echocardiography - no nephrotoxicity, no ionising radiation, portable, excellent for the ascending aorta and aortic valve (sensitivity around 98% for type A), but blind to much of the arch and descending aorta and dependent on operator skill.
- Non-contrast CT - can demonstrate a high-attenuation intramural haematoma, displaced intimal calcification and pericardial effusion, but cannot show a patent false lumen.
- MR angiography - excellent accuracy and no ionising radiation, but impractical in an unstable patient and gadolinium in an acutely injured kidney is best avoided.
- Repeat catheter angiography - rarely needed now, but still used when a percutaneous or surgical decision must be made at the same sitting.
A normal chest radiograph would be of no value; a widened mediastinum is neither sensitive nor specific.