Nonidiosyncratic Reactions

Nonidiosyncratic reactions are a diverse group (2,14), ranging from mild symptoms that are typically considered a physiologic effect of contrast media rather than a reaction to severe, even life-threatening, states. Nonidiosyncratic contrast reactions are for the most part dose dependent: they are less likely to occur and, if they do occur, are less likely to be severe when smaller doses of contrast material are employed. In general, nonidiosyncratic reactions occur less often and with less severity when LOCM is administered than when HOCM is administered.

There are a number of commonly encountered nonidiosyncratic effects of RICM. Intravascular injections of RICM normally produce peripheral vessel dilatation, which the patient perceives as a sensation of heat or warmth (15). Nausea and vomiting are probably the result of a transient effect of the contrast material on the central nervous system. Vasovagal reactions are acute nonidiosyncratic hypotensive reactions that typically include nausea, diaphoresis, anxiety, and bradycardia.

Less commonly, RICM produce adverse toxic effects on other systems. RICM, on contact with the central nervous system, lowers the seizure threshold. Seizures can occur from intravascular injection of contrast media, especially if a disease process has resulted in the breakdown of the blood-brain barrier. Contrast media injection results in a temporary decrease in cardiac contractility and may induce arrhythmias. When contrast medium reaches the pulmonary vasculature, it causes increased pulmonary vascular resistance as well as bronchospasm (typically subclinical; thus, when symptomatic bronchospasm occurs, it is not always clear if this represents a nonidiosyncratic or idiosyncratic reaction). Because HOCM and, to a lesser degree, LOCM have osmolality higher than that of human blood, water is drawn into the vascular system, with the consequent expansion of intravascular volume that may not be tolerated well by patients with tenuous cardiac status (15). Rarely, contrast media induce noncardiogenic pulmonary edema, a phenomenon that is poorly understood but is suspected to be due to a toxic effect leading to increased pulmonary capillary permeability.

Contrast-induced nephrotoxicity is another nonidiosyncratic reaction. It will be covered in more detail in a separate section entitled "Contrast-Induced Nephrotoxicity."

Patients at Risk

A variety of diseases can be aggravated by contrast media (2,14,15). In patients with cardiac disease (and rarely in patients without), contrast media administration can induce arrhythmias or precipitate an episode of angina. The osmotic increase in blood volume can result in a worsening of congestive heart failure. HOCM administered to patients who have a pheochromocytoma can result in release of vasoactive substances leading to a hypertensive crisis; however, in comparison, it appears that the risk of this occurrence following the administration of LOCM is very low (16). In patients with myasthenia gravis, acute myasthenic crisis has been reported following injection of HOCM (14) but not LOCM; the acute worsening of the respiratory symptoms of myasthenia usually presents the greatest danger to the patient. Thyroid storm has been reported following contrast media administration to patients with uncontrolled hyperthyroidism. In patients who suffer from sickle cell anemia (but not sickle trait), contrast medium administration may precipitate a sickle cell crisis. As previously mentioned, intravascular RICM administration can lead to seizure activity. However, this is a rare occurrence, with patients with brain lesions that have resulted in the breakdown of the blood-brain barrier at higher risk.

Reducing Risk

Premedication

For some, but not all, of the nonidiosyncratic reactions, it may be possible to reduce risk via premedication. For example, patients who have a pheochromocytoma can be premedicated with phenoxybenzamine to produce alpha blockade (16). This prevents the most clinically significant adverse effects of the release of vasoactive substances if that should occur following contrast media administration but is rarely done when LOCM is administered. Risk can be reduced in patients with hyperthyroidism if their disease can be controlled prior to contrast media administration.

Choice of Contrast Medium

In general, LOCM produces fewer and less severe nonidiosyncratic reactions than HOCM. Regarding iso-osmolality contrast media, there is insufficient evidence to draw conclusions about its use in patients at risk for nonidiosyncratic reactions, though common sense would suggest that it might reduce the risk of worsening active congestive heart failure because it should have a smaller volume expansion effect.

Because nonidiosyncratic reactions in general are dose dependent, the risk can be reduced by lowering the amount of RICM administered to the minimum necessary to answer the clinical question for which the examination is being done. In some cases, this might involve choosing a different study that does not require RICM.

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