Saturday, December 7, 2013

Emergency Radiology and Pregnancy

My apologies for the delay in posting something new, but this topic required a lot more reading than initially expected. There is a lot of information from very diverse sources, not just medical imaging literature but also from genetics, physics and the energy industry as well. Very interesting stuff!

The pregnant patient is, and will always be, a complex patient. The presentation of life threatening diseases is often atypical, symptoms that could be part of a normal pregnancy may also indicate serious pathology, the physiology of pregnancy is different from a non-pregnant female and to make things even more complicated, there is a little human being inside! No wonder why the sick pregnant patient in the ED scares the socks out of us. We all use our clinical acumen but that is often not enough to decide sick-or-no-sick, and we must rely on imaging technology to aid in the diagnosis. There is handful of non-pregnancy related diagnoses that create all the problems and split hairs, these include appendicitis, renal colic, ovarian torsion, hemorrhagic ovarian cysts, pulmonary embolism and trauma. It is well known that whenever possible, ultrasound is the modality of choice when it comes to imaging the pregnant patient; unfortunately, it is not always conclusive and CT scan becomes the better choice. That's when the "radiation talk" should take place. I have heard all kind of crazy things about this, mostly due to lack of information in patients and even providers. Therefore, I would like to cover some basic physics about radiation and its effects, just to put things in perspective.  It will not make you a physicist, but you can sound like one when talking to the radiologist, and that's very useful. Shall we...?

Fact number 1: Radiation comes from everywhere and everything; from natural sources like air, water, food, plants, ground and cosmic; as well as artificial sources like the electronics we use everyday, buildings, occupational exposures, nuclear and medical imaging, etc.

Fact number 2: There are several units to measure radiation. Depending on which system is used in the country you live in, there are Rads/Grays and Rems/Sieverts. 1 Rad = 1/100 Gray and 1 Rem = 1/100 Sievert. For the purpose of this review, it is worth remembering Grays and Sieverts. A Gray (Gy) is a measure of absorbed ionizing radiation which is equal to 1 Joule of energy release in 1 kg of mater. A Sievert (Sv) is the unit of the effective dose of radiation that has a biological effect on tissue, 1 Sv is equal to effect of 1 Gy over the exposed tissues multiplied by the specific weighting factor. If this sounds too complex (and believe me, IT IS) just remember that a Gray is the amount of radiation received by the tissues and a Sievert is the unit for the effects of 1 Gray of radiation in humans. Although technically are not the same, for practical purpose they can be thought as similar units.

Fact number 3: The average dose of background radiation a human accumulates just for being temporary habitant of this planet is somewhere between 2 and 7 mSv (mili-Sieverts) depending on location and altitude. There are high radiation locations in areas of nuclear disasters, natural occurring "leaks" from the earth, mines and high altitude.

Fact number 4: From the different types or radiation, ionizing radiation is the one used in medicine. Its ability to pass through tissues of different densities makes it ideal for imaging technology and treatment of cancers. The problem is that as it goes through to the tissues, it deposits enough energy to brake molecular bonds and displace electrons from atoms creating free ions (therefore the name ionizing); this results in damaged bonds in the DNA of living cells.

Fact number 5: In industrialized countries, the most common sources of artificial source of ionizing radiation is medical imaging with an average of 3 mSv per year per person (world's average is 0.6 mSv) and air travel with 2.1 mSv per year. Of note, heavy smoking (1 pack per day) results in radiation dose of ~160 mSv per year directly into the lungs! (If you smoke, you need to stop)

Well... I think that's enough physics for one day! Now let's apply these facts to the medical imaging in humans, including unborn babies.

We just learned about the effects of ionizing radiation in living cells and that we all are exposed to radiation from multiple sources at any given time. Then why aren't we all dropping death with cancers of all types? - Well, that is because there is an extremely sophisticated and highly specialized enzymatic complex system that detects, repairs or destroys damaged cells. This awesome system corrects billions of DNA mishaps a day and maintains cellular functions. Ionizing radiation in high doses, and specially after repetitive exposure, can eventually overwhelm this mechanism and lead to various types of malignancies. The fast mitotic fetal cells are particularly vulnerable to these effects from radiation, therefore is a good idea to limit fetal exposure whenever possible.

I found this table with the average dose of radiation from different studies, their equivalent to background radiation in years and its risk for malignancy

Too much is said about the fetal radiation risks for various types of imaging technologies derived from phantom models, animal and human observational studies; all thrown in the same bowl with extrapolated nuclear bomb survivors and nuclear disasters data. The result is an estimation of risk, but let's be clear about something... No one has solid, indisputable human information with a dosimeter next to a developing fetus to accurately measure radiation doses in-utero and its effects based on randomized studies (and we never will). All of the current available recommendations are predicated on estimated risks based on less-than-perfect data. Having said that, this is all we have and it seems to be enough to draw some conclusions.

The background dose of radiation for 9 months of pregnancy is estimated at 0.5 to 1 mGy, and the threshold for increased risk of fetal anomalies or pregnancy loss is 50 mGy (5 Rads) or 50 mSv (5 Rems). Standard radiological tests produce radiation doses far below the 50 mSv threshold. The aggregate risk for spontaneous miscarriage, major malformation, mental retardation and childhood malignancy in the general population is estimated to be about 28.6%. A dose of 50 mSv of ionizing radiation will increase this risk to approximately 28.8%. Specifically about childhood cancer, defined as any cancer with onset before age 15, the most common being leukemia, the average risk of leukemia in general pediatric population is about 0.036% (3.6 per 10,000), exposure to 50 mSv will increase this risk to approximately 0.06% (6 in 10,000).  From these statistical models we can conclude that although the risk of negative effects of the cut off of 50 mSv is not zero, it is indeed, very very small. The National Council on Radiation Protection and Measurements, and the American College of Obstetricians and Gynecologists have both agreed that the potential health risks to a fetus are not significantly increased from most standard medical tests. The American College of Radiology (ACR) has also come on record saying that "No single diagnostic procedure results in a radiation dose that threatens the well-being of the developing embryo and fetus" (Hall EJ. Scientific view of low level radiation risks. Radiographics. 1991;11:509)

This table shows the fetal dose of common radiologic tests. All these give less than 50 mSv, so it is safe to say that when we need to image a pregnant patient using ionizing radiation, we can proceed knowing that any of the studies we use in the ED represent low risk.

This next table summarizes the average dose of multiple radiologic studies and the number of studies needed to reach the aggregate dose of 5 Rads (50 mGy/mSv)

And this last table is from the ACR 2013 revision of its practice guideline on imaging the pregnant or potentially pregnant women using ionizing radiation, reaffirms what has been said by other organizations regarding the cut off of 50 mGv as safe level. 

What about contrast? - Well, I did find some useful information about that from the ACR 2013 manual on contras media. Basically it says that the water soluble iodinated low-osmolarity contrast media (the one use currently) does cross the placenta but there is no current evidence of mutagenic or teratogenic effects from it. As far as the effect on neonatal thyroid function, the document says that the amount of contrast in the fetal circulation is small and transient, and there are no reported cases of neonatal hypothyroidism in babies whose mothers received this type of contrast and the FDA has given it category B status. The ACR's recommendation about the Gadolinium-based contrast media (GBCM) used in nuclear medicine studies is not as strong. It says that although there have been no know adverse effects to human fetuses by the use of this agent, there is only one study of 26 pregnant patients who were exposed to Gadolinium during the first trimester, none had teratogenic nor mutagenic effects of the progeny. Therefore, the use of GBCM should only be used when the benefits justify the potential risk to the fetus. 

OK... are you with me so far? - Good! Let's now get practical and put all this theory where the rubber meets the road, at the bedside of the pregnant patient with a potentially serious diagnosis.

Let's start with the rule-out appendicitis case and the ultrasound comes back with something like this "Appendix not seen, acute appendicitis cannot be excluded, consider pelvic pathology... clinical correlation required". Now what? - Sure, you can try to put your pregnant patient in the MRI for 30 minutes and hope for a clear diagnosis; however, MRI is not as sensitive nor specific compared with CT, thus resulting in equivocal results, and most radiologist are far better diagnosing acute appendicitis on CT than MRI. CT with oral and IV contrast is the better choice, and the 25 mSv dose of radiation are still considered relatively safe by ACR and ACOG. Now the conversation with the patient should include the following... If this is acute appendicitis and we don't find out on time and it ruptures, there is between 6-37% chance for fetal loss, maternal morbidity and mortality range around the 5% and 1% respectively. and the risk of the radiation dose of the CT abd/pelvis for anomalies, fetal loss or childhood cancer is less than 1%. It seems clear that scanning is the best option. 

Urolithiasis with renal colic is the most common non-obstetric diagnosis requiring hospitalization, affects about 1 in 1500 pregnant patients and it is often confused with appendicitis, diverticulitis, ovarian pathology and placental abruption. Ultrasound is first line test to diagnose urolithiasis during pregnancy. When the stone is visualized that's great, but when all you see is hydronephrosis it is hard to tell if that is the hydronephrosis of pregnancy or due to a distal obstruction. The good news is that about 60-80% of stones will pass with conservative management, the bad news is that 20-40% will not and urologist use size of stone and location to determine treatment options. MRI is good to see hydronephrosis but not so much stones, so it doesn't really have significant advantage over ultrasound. Intravenous pyelogram has fallen out of favor because of the 50% higher radiation dose compared with CT scan and it only provides imaging of the urinary tract. CT scan again comes as top option for complicated cases of urolithiasis because of its high sensitivity and specificity, and ability to screen for other intra abdominal/pelvic pathology. 

The pelvic pathology including ovarian torsion, adnexal mass, hemorrhagic cyst and degenerating fibroid are best seen with ultrasound, so no surprises here. However, in late pregnancy the gravid uterus may obscure adequate view with the ultrasound. The MRI without contrast could be used in the stable patient. CT with IV contrast becomes the imaging modality of choice in the unstable patient with hemoperitoneum.

Pulmonary embolism is, on it self, a monster topic which becomes even more monstrous in the pregnant patient. With a mortality approaching 15% and significant morbidity of anticoagulation, we must get this right in a timely fashion. There are several protocols including trimester adjusted D-dimer + leg ultrasound in leu of pulmonary imaging. In patient with symptoms suggesting PE and (+) US for DVT, most will go ahead and treat; but when this approach is not diagnostic, pulmonary imaging becomes mandatory and the options are CT pulmonary angiography (CTPA) vs V/Q scan. The ACR rates both studies as adequate in the pregnant patient with radiation doses below the 50 mSv limit. The American Thoracic Society in its 2011 practice guidelines recommends plain chest x-ray as the initial radiation-associated step. If the CXR is normal, proceed with the perfusion phase of V/Q scan followed by the ventilation portion if abnormal. If CXR is abnormal, then CTPA is recommended. The algorithm looks like this...

The problem with this approach is that if the V/Q is inadequate or non-diagnostic (and many of them are), then you still have to proceed with CTPA. The advantages of CTPA is that it can also provide alternative diagnosis (i.e. pneumonia) and is more widely available compared with a V/Q scan. The fetal radiation doses of both studies is fairly comparable with an average of 0.2 mSv for CTPA, 0.12 for the perfusion-only V/Q and 0.2 for the ventilation portion of the V/Q scan. The remaining issue to discuss about CTPA is the radiation exposure to the hyperplastic breast tissue of the pregnant patient, which is said to increase life time risk for breast cancer in about 1%. Breast shields and timed beam techniques can significantly lessen this exposure.

The final diagnostic dilemma to review is trauma. This is the easy one because everyone agrees that when it comes to radiology studies, you just do it. Trauma is the number 1 reason for non-obstetric mortality during pregnancy, and unless you are ready to do a perimortal c-section, treating mom is the best way to treat baby. The pregnant trauma victim should be imaged just as the non-pregnant with x-rays, CT or angiography when required. Sure, you take a quick look with the US to check on the fetus and the placenta, but don't get hang on that screen while the mom is bleeding out, and remember that every pregnant trauma victim beyond 24 wks gestation once stable, should be placed in continuos cardio-tocographic, which is the most sensitive way to diagnose placental abruption. It is possible that mom may require multiple studies that may add up radiation doses of > 50 mSv and rarely > 150 mSv, and someone needs to keep track of what studies have been done and what studies are still needed. In such cases with high fetal exposure doses, therapeutic abortion should be discussed with the patient.

Wow... you are still reading! I am sorry this topic is too long, but I think it contains useful information that can be applied anytime when you pick up a chart saying "20 wk pregnant with abdominal pain". Now, it is time for final points.

Pregnancy test. The ACR says that for negligible risk examinations like CXR or extremity plain films, pregnancy test is unnecessary. Documentation; when higher risk examination is needed make sure to document clearly and completely. Tell the chart you have discussed alternative diagnostic options with the patient, mention the risks of doing and not doing the test, and what you feel is in the best interest of mother and baby. Finally, remember that avoiding radiologic tests in a potentially life threatening condition in order to avoid fetal exposure to ionizing radiation is not going to score you any points when you end up with a dead mother, so do what it right for your patients.


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