Uses, advantages and disadvantages of radiocontrast media

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Signs and symptoms of anaphylaxisThe radiocontrast media use is one of the daily practices for every radiologist and radiographer and though every medical/paramedical personnel working in the field of radiology and medical imaging is experienced in the indications and different types of radiocontrast agents available in the market, many surveys conducted in a lot of countries have shown that less than half of the radiologists are aware of the potential complications of radiocontrast agents, the risk factors that make their patients more susceptible to develop such complications, and most importantly, the measures to guard against the development of such complications and how to deal with them in a quick and efficient way whenever they appear. In this article we will try to shed some light on the possible complications of different radiocontrast agents and how to deal with them in a simple manner.

The intra-venous radiocontrast agents or the I.V dyes as they are more commonly called are used in a lot of diagnostic radiological procedures either to visualise the blood vessels as in the angiography studies (coronary angiography, pulmonary angiography,...etc), or to improve visualization of tissues as in many CT scan studies of different anatomical sites, and sometimes to assess the walls of hollow organs as in barium swallow, barium meal, barium enema, and the intravenous pyelography studies of the bladder.

The most commonly used radiocontrast dyes are manufactured from barium (in GIT imaging), and iodinated compounds in the case of intravenous dyes.

The contrast media used in radiological studies can be broadly classified into two basic types (and this classification is of great importance when discussing the possible complications):

1. The ionic high-osmolality contrast media: more commonly used nowadays, less expensive and readily available, but with a worse side effects and complications profile.

2. The non-ionic low-osmolality contrast media: more preferred due to less risk of complications associated with its use, but being expensive limits its use on a wide scale.

Both types of I.V radiocontrasts contain iodine and most of these agents are derivatives of tri-iodobenzoic acid. The idea of using iodine dyes is the simple fact that iodine molecule absorbs x-rays effectively in the energy range at which most clinical imaging systems operate, which enhances the obtained image greatly allowing better visualisation and a more accurate diagnosis.

There are many types of radiocontrast dyes available in the market like the "Optison", the "omnipaque" used in the angiocardiography and aortography, CT enhanced studies of the head and neck, as well as many other uses. There is also the non-ionic "Vasipaque" which is used in peripheral arteriography, cerebral arteriography, as well as the angiocardiographic studies.

The complications of radiocontrast dyes include both allergic reactions to the dye material, and the common serious complication known as Radiocontrast induced nephropathy, which is a variant of acute renal failure that is related to the high-osmolality contrast agents. The allergic reactions associated with radiocontrast dyes are not true allergies, which means that it is not a typical immune hypersensitivity reaction with antibody production or T-cell activation which is the case in the hypersensitivity reactions or true allergies, but it is a direct effect of the iodinated dye material that acts directly on mast cells releasing histamine and other biochemical mediators causing the clinical picture of an allergic reaction, and that is why the possibility of development of allergic reaction and its severity is directly proportional to the concentration of iodine in the specific dye used, as well as on the dose of the intravenous dye that is injected.

The allergic reactions associated with the radiocontrast agents range from mild allergic symptoms to severe life threatening anaphylaxis, and affects about 5-8 % of patients exposed to radiocontrast agents. The mild symptoms of allergic reactions include a feeling of warmth, nausea, and vomiting that develops shortly after injecting the dye, and requires no treatment other than observing the patient. The moderate reaction includes severe persistent vomiting, hives and swelling, and it requires treatment as well as close observation. The treatment involves I.V anti-emetic agents as well as the H1 antagonist agent Diphenhydramine in a dose of 50 mgs injected intravenously or intramuscularly, and sometimes a short acting steroid (hydrocortisone) in a dose of 100 mgs I.V injection may be used. The severe reaction include the life threatening anaphylactic shock, presenting as severe bronchospasm with cyanosis and a circulatory shock, and sometimes a cardiovascular arrest.

The severe reactions affect about 1% of the cases and death is reported in one case in each 75 000 cases, and the risk for developing severe reaction appear to be higher in patients with conditions like food allergies, iodine skin allergy, bronchial asthma, kidney and thyroid diseases, as well as patients on metformin or beta blocker therapy, so special attention should be given to this patient population prior to applying a radiocontrast dye.

The treatment of severe anaphylactic shock must be initiated immediately while calling for the code blue team of intensivists according to each hospital protocol, and it implies immediate infusion of intravenous fluids to support the circulation, deep intramuscular injection of 0.5 mgs of epinephrine , to be repeated every 2-5 minutes if there is no response ( I.V injection would be more appropriate in a setting of severe shock with compromised perfusion for a more optimum effect ), 200 mgs I.V injection of hydrocortisone, 50 mgs of diphenhydramine as well as respiratory support with oxygen therapy and nebulised salbutamol or even intubation and mechanical ventilation according to the severity of respiratory symptoms. It is worth noting that patients with a higher risk for severe radiocontrast allergic reactions could benefit from prednisolone oral administration several hours prior to the procedure as well as diphenhydramine one hour before applying the dye.

Other than the allergic complications, there is the common serious form of kidney injury (acute renal failure) known as radiocontrast induced nephropathy.

Contrast induced nephropathy is defined as impairment of the renal functions measured as a 25% increase in serum creatinine level, or 0.5 mgs absolute increase in serum creatinine level within 48-72 hours of administration of radiocontrast dye. Contrast-induced nephropathy is associated with a higher morbidity and mortality which necessitates proper identification of patients at a high risk to develop this condition, and taking the proper precautions that includes good preparation of the patient and preferably using a non-ionic low-osmolality agent in this patient subgroup to minimise the risk of this form of acute kidney injury.

The iodine compounds are toxic to the kidneys, but an important etiological factor is the hyperosmolarity of the dye material injected, and that is why the development of this complication is more common with the ionic hyperosmolar dye preparations and for a better understanding of this pathology, we will give a brief idea about the concept of osmolality.

The osmolality of a solution is the measurement of the number of molecules and particles in a solution per kilo-gram water. In other words, osmolality is simply the measurement of the number of molecules that can replace water molecules in a solution. Having understood the concept of osmolality, we can now understand the meaning of a hyperosmolar dye material, which means that the osmolality of the radiocontrast dye is higher than the osmolality of the plasma, which besides the nephrotoxic effect of iodine causes injury to the renal tubules and leads to the development of a specific variant of acute renal failure named contrast induced nephropathy. It appears that the presence of pre-existing impairment of renal functions, Diabetes Mellitus, as well intravascular volume depletion greatly increases the risk of contrast induced nephropathy, and that is why it is very important for a radiologist to make sure that a document of renal functions test, random blood glucose test, as well as a brief history and vital data are sent side by side with the radiological study request. In the high risk patient subgroup, there are several preventive measures that could be done to minimize this risk (beside using a non-ionic low-osmolality agent with a minimal dose).

The preventive measures include:

1. Infusion of an isotonic sodium bicarbonate in 5% Dextrose solution 1 hour before, and six hours after the procedure, or infusion of normal saline instead, which is not as effective as sodium bicarbonate, but good hydration is still an important and effective preventive measure.

2. N-acetyl cysteine 600 mgs orally every 12 hours before and after the procedure. N- Acetyl cysteine is a very potent antioxidant that can protect the kidney tissues against the toxic injury by the iodinated dye compounds.

These preventive measure minimize the risk for developing this complication, but they do not prevent it completely, so it is always preferable to admit the high risk patients in an intensive care unit or a high dependency unit for observation after the procedure for 72 hours, as with good supportive care and timely intervention, the prognosis of contrast induced nephropathy is very good, and complete recovery of renal functions is possible with most patients who survive the acute episode of renal failure. The treatment of acute kidney injury is beyond the scope of medical radiology, but generally it depends on supporting urine output and correction of acidosis and electrolytes disturbance associated with renal failure, and waiting for recovery of the renal functions.

At last, it is very important to emphasize that a better understanding of the complications associated with procedures or pharmacological agents radiologists / radiographers use would ensure a better and a more safe practise to optimize the quality of our diagnostic and therapeutic efforts, while standing on a solid ground of knowledge.

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