The administration of intravenous contrast media is one of the medical act of specific radiological competence.
Although the safety of these drugs is known,
it is good to know the possible adverse and hypersensitivity reactions which,
although rare,
may represent a life risk for the patient.
Intravenous contrast media are distinguished in radiographic contrast media (used in conventional radiology or in CT) and MR contrast media.
In the first group,
we find both water-soluble contrasts (administered by oral or by vascular route) and non-water-soluble contrasts (based on barium sulfate); the latter are not absorbed by the gastrointestinal mucosa and are toxic to the peritoneum: their use is strictly forbidden if there is even the slightest doubt of "continuous solution" of the intestinal wall.
The water-soluble contrast agents are used as "positive contrasts" in CT,
where they can be administered by vascular injection - for the purpose of studying vascular bed,
tissue vascularization,
the presence of any extra-vascular spillage - or oral,
in order to identify a continuous solution or extra-visceral distribution.
In MR contrast media are divided into diamagnetic (water,
etc.),
paramagnetic (gadolinium chelates) and superparamagnetic (iron oxide-based compounds).
Paramagnetic contrast agents - the most commonly used class in MRI - are administered by vascular injection and allow to evaluate vascular structures and organs vascularization (similarly to water-soluble iodates in CT); for some classes it is also possible to study biliary excretion thanks to the property of the molecules to be distributed in the bile duct system.
Iodinated contrast media
The iodinated contrast media used in CT and conventional radiology are molecular compounds possessing a benzene ring with bound Iodine atoms,
which provides the main contrast graphic features of the molecule:
- X-ray absorption
- Stable link
- Low systemic toxicity
Their main pharmacokinetic characteristic is represented by predominantly renal excretion.
From a molecular point of view,
the intravenous iodinated contrast media can be classified as shown in (Table 1).
Iodinated ionic contrast media are no longer used by injection due to their osmolality,
which is the main mechanism of many of their adverse reactions [1,
2].
Among non-ionic contrast agents,
Iodixanol has the property of being substantially iso-osmolal compared to plasma,
which makes it the first choice of contrast medium in patients with previous adverse or anaphylactoid reactions following contrast graphic examinations in many centers.
[1]
RM contrast media
The MR contrast agents,
represented by the gadolinium chelates,
interact with the tissues causing a reduction of the relaxation time T1 and make the hyperintense tissues in the sequences dependent on this weighing.
Based on the molecular composition,
the gadolinium chelates used in MRI are distinguishable in 4 groups according to whether ionic,
non-ionic,
linear or macrocyclic.
[3] (Table 2)
A recent review conducted by the European Medicines Agency (EMA) has demonstrated the accumulation of gadolinium in the brain tissues that seems to occur to a greater extent in the case of the use of gadolinium compounds in structure linear.
Although no harmful effects of this accumulation have yet been demonstrated,
the EMA and AIFA recently issued an information note that suspended the use of linear contrast media with the exception of Acido Gadoxetico and Gadobenico,
whose indication is however restricted to hepatic studies only.
[4]
Adverse reactions to contrast media
From a pathophysiological point of view,
adverse reactions to contrast agents can be classified - according to what reported in the literature by Feltrin and Collaborators [1] - in two large groups depending on the mechanism of development:
• Chemotoxic reactions: Contrast medium dose- and concentration- dependent; they are related to the intrinsic toxicity of the molecule and its physicochemical characteristics (osmolality,
viscosity,
hydrophilicity,
etc.).
Tissue damage is mediated by the interaction of these molecules with cell membranes and particularly with endothelial cells.
The most affected organs are kidney,
central nervous system,
and cardiovascular system.
• Anaphylactoid and hypersensitivity reactions: Non-dose-dependent reaction caused by non-Ig-E dependent release of biological mediators such as histamine,
prostaglandins,
leukotrienes,
adenosine,
and endothelin.
A mechanism of direct release of mediators by mast cells following a chemical/physical stress of their cell membrane by the contrast medium has been hypothesized.
On the other hand,
IgE-mediated anaphylactic reactions are considered exceptionally rare and generally occur with severe clinical conditions.
The classification considered to be clinically more useful is reported in Table 3.
Adverse reactions to an intravenous contrast medium are distinguished in renal and extrarenal,
in turn,
subdivided according to the temporal evolution in acute (within one hour),
late ( from one hour to 7 days),
and very late (after 7 days).
Immediate reactions are mainly caused by anaphylactoid mechanisms,
while those that occur later from the administration of contrast agents are mainly determined by chemotoxic events; in the latter case the clinical picture is different depending on the nature of the compound (iodate or chelate of Gadolinium).
[1]