A 75-year-old woman in Thailand told her care team that CT contrast dye had once made her neck swell, so she was given steroid pills before her next scan. The precaution did not prevent a repeat. Sixteen hours after the injection, painful swelling developed beneath both sides of her jaw.
The case, published Sept. 12 in the American Journal of Case Reports, comes from two radiologists at Ramathibodi Hospital, Mahidol University, in Bangkok. It describes a recurrence of iodinated contrast-induced sialadenitis, an inflammation of the salivary glands after exposure to iodine-based contrast agents, better known as "iodide mumps."
The authors call it a rare, underrecognized, delayed reaction that is not allergic, and their report shows it can return with repeat exposure.
A Second Reaction a Decade Later
The woman had stage 3a chronic kidney disease and was referred for a contrast-enhanced chest CT to evaluate long-standing bronchiectasis, a condition in which the airways become permanently widened.
About 10 years earlier, she said, her neck and the area under her jaw had swelled shortly after an intravenous contrast injection. Based on that history, she was prescribed the oral steroid prednisolone as premedication.
After receiving the iodinated contrast agent iopromide, she developed painful swelling on both sides beneath her jaw and returned to the hospital. Doctors found firm, mildly tender swelling of both submandibular glands, the salivary glands tucked under the jawline. There was no redness over the skin and no pus draining from the gland openings inside the mouth.
Importantly, doctors saw no swelling of the voice box and no airway spasm, findings that can signal a dangerous allergic reaction.
Ultrasound Pointed to the Contrast Dye
An ultrasound showed that both submandibular glands were diffusely enlarged, with an uneven texture, increased blood flow, and mild widening of the small ducts inside the glands. There was no salivary stone and no abscess, two common causes of gland swelling.
Based on those findings and the timing, the team diagnosed a recurrence of contrast-induced sialadenitis in both submandibular glands.
Treatment was simple. With warm compresses and oral pain relievers, her symptoms resolved completely within four days.
Recognizing the reaction's characteristic features, the authors say, can help clinicians avoid misdiagnosis as well as unnecessary contrast exposure, premedication, tests, and treatment.
A Reaction First Described in 1956
Iodide mumps is not new. It was first described in 1956 in the New England Journal of Medicine. A meta-analysis of published case reports later identified 77 cases across 65 reports. The median time to onset was 16 hours, and symptoms resolved in a median of three days. In that analysis, 35% of patients had impaired kidney function at baseline, and nonionic, low-osmolarity contrast agents were the most frequently reported.
How often it happens is uncertain. The meta-analysis noted that one study of adverse reactions in 337,647 patients did not record a single case, while other researchers have suggested the condition may be underdiagnosed, with a true incidence close to 1% to 2%.
Some settings may carry a higher rate. A study in the American Journal of Neuroradiology reviewed 780 patients who underwent therapeutic neuroendovascular procedures in 2022 and found an incidence of 4.2%. Female sex, larger contrast volumes, and the position of the guiding catheter were linked to higher risk.
The mechanism remains unclear. The swelling was once thought to be antibody-driven and has been mistaken for angioedema or anaphylaxis, according to the meta-analysis, which instead describes the reaction as pseudoallergic. Because a small share of contrast may leave the body through the salivary glands, some researchers propose that it builds up there and triggers local inflammation.
Does Steroid Premedication Help?
The Thai report adds to a small set of cases in which steroids given before a scan did not prevent the reaction. In an April case report in Cureus, Japanese physicians described a 60-year-old man whose neck swelled 14 hours after a CT scan and swelled again four months later, despite three doses of prednisolone and a switch to a different contrast agent.
Israeli allergists reported two similar cases in 2008 in which swelling appeared despite steroids. They wrote that no premedication is known to prevent iodide mumps and advised imaging without iodine-based contrast for such patients.
The evidence is not entirely one-sided. The Japanese authors cited a separate report in which steroids appeared to prevent a third episode in a man who had reacted twice, and they concluded that the preventive value of steroids remains uncertain because so few cases have been described.
Kidney function may also matter. Researchers have hypothesized that reduced kidney clearance could increase contrast buildup in the salivary glands, yet the meta-analysis found no difference in how quickly symptoms resolved between patients with impaired and normal kidney function. In a report on low-volume contrast published in 2023, a 40-year-old woman with normal kidney function developed painless swelling under her jaw within five hours of a brain CT that used just 40 milliliters of contrast.
Iodide mumps is generally self-limited, and a single case report cannot predict how any individual will respond to future contrast. Still, the Thai report highlights the value of a clear record of prior reactions.
People who have had facial, neck, or jaw swelling after a CT scan should tell their clinician and radiology team before any future imaging. Anyone with swelling accompanied by trouble breathing, throat tightness, or wheezing after contrast should seek emergency care immediately.
Key Questions Answered
What is iodide mumps?
It is swelling and inflammation of the salivary glands that can develop minutes to days after exposure to the iodine-based contrast dye used in some imaging tests.
Is it an allergic reaction?
The authors describe it as a delayed, non-allergic reaction. Its exact mechanism is unclear, but some researchers believe contrast accumulating in the salivary glands plays a role.
Why was this case notable?
The woman had a similar reaction 10 years earlier, and the swelling returned even though she took steroid premedication before her new scan.
Do steroids prevent it?
The evidence is limited and mixed. Several reports describe swelling despite steroids, while at least one describes apparent prevention.
What should patients with a past reaction do?
They should tell their clinician and imaging team before future scans so alternatives can be considered, and seek emergency care for any breathing difficulty.
