Iodinated contrast agent precautions

Many X-ray imaging investigations, especially CT, use intravenous iodinated contrast

agents to obtain greater diagnostic information, for example, delineating the inner

structure of vessels and detecting path- ological processes including malignancy and

infection. In addition, the vascular supply to organs can be ascertained. The benefits of

using an iodinated contrast agent however must be weighed against the risk of its

potential adverse effects along with the risk of radiation exposure. In some

circumstances, an imaging study that does not use a contrast agent or radiation may

answer the question. The potential adverse effects of administering an iodinated contrast

agent can be divided into general, CIN and thyrotoxicosis.

General adverse reactions

Iodinated contrast agents may cause hypersensitivity reactions in sus- ceptible individuals, e.g. asthmatics, patients with other drug allergies, and patients who have suffered previous adverse reactions. The hyper- sensitivity reactions may manifest as:

• Immediate IgE-mediated hypersensitivity reaction – occurs within an hour of administration of the contrast agent and can range from urticaria to a major anaphylactoid reaction.• Delayed T-cell mediated hypersensitivity reaction – occurs later than one hour following administration of the contrast agent and usually causes erythematous skin rashes.

It is important to note that a patient with a previous delayed hypersen- sitivity reaction is not at increased risk of an immediate hypersensitiv- ity reaction, and vice versa, due to the different immunological processes.

Patients who develop adverse contrast agent hypersensitivity reac- tions should be managed according to the severity of the symptoms. Severe reactions must be treated as a medical emergency and may require immediate resuscitation with oxygen therapy, intravenous fluids and treatment with a bronchodilator, antihistamine and adrenaline.

Contrast-induced nephropathy (CIN)

CIN is defined as acute renal impairment that occurs within three days of administration of an intravascular contrast agent without any other identifiable cause. It is one of the commonest causes of hospital- acquired acute renal failure and is thought to be due to renal ischaemia and direct toxic effects on the renal tubular epithelium. Patients at highest risk are those with pre-existing renal impairment such as those with diabetes mellitus or taking nephrotoxic drugs. Preventive mea- sures should therefore be taken in patients with moderate or severe renal impairment, which is often based on their estimated glomerular filtration rate (eGFR):

Precautionary measures include:• Considering alternative investigations.• Withholding nephrotoxic drugs, e.g. metformin for 48 hours post- administration and rechecking the renal function before restarting.• Oral hydration (100 ml/hour for four hours) prior to administration and 24 hours post-administration is strongly recommended in patients with moderate renal impairment.• Intravenous hydration (100 ml/hour for 4 hours) prior and 24 hours post-administration is strongly recommended in patients with severe renal impairment. Hydration is thought to reduce the risk of renal ischaemia and dilute the contrast agent in the renal tubules.• Rechecking renal function 48–72 hours post-administration.

MR contrast agent precautions

The most commonly used contrast agent in MR scanning is gadolin- ium. Its safety is still

under assessment and several cases of nephro- genic systemic fibrosis (NSF) following

exposure to gadolinium have been reported in patients with pre-existing renal

impairment. NSF is a severe syndrome characterised by fibrosis of the skin, eyes, joints,

muscles, liver, lungs and heart. The use of gadolinium must therefore be used with

caution in patients with pre-existing renal impairment.