Fear of radiation can lead to harm: A rational approach to diagnostic imaging following trauma in pregnancy
Imagine this patient presents to your emergency department:
A 25 year old female is brought in by paramedics with a history of being involved in a motor vehicle collision. She was an unrestrained passenger and reported abdominal pain on scene. Her initial vitals are: BP 110/70 mm Hg, HR 98 BPM, respiratory rate of 18 per minute, Sats of 98% on RA and GCS 15/15. She has a singleton pregnancy at 21 weeks gestation.
Your team begins to assess her. She appears to be haemodynamically stable currently, but has significant lower abdominal tenderness on palpation. An eFAST is negative, and the fetal heart beat is present.
She also has significant tenderness in the lower abdomen at any attempt on moving her hips. You suspect a pelvic fracture and order a pelvic X-ray. (If you have any concern about ordering a pelvic x-ray in this scenario, read on)
The pelvic X-ray shows: bilateral superior and inferior pubic rami fractures are present. The fetal rib cage is visible overlying the right iliac crest.
How would you manage this patient further?
You need to consider the pubic rami fractures and their potential associated organ injuries.
You also need to consider if further imaging is necessary? If so, what?
Pubic rami fractures are associated with bladder, urethra and vaginal injuries. In pregnancy, the bladder is displaced as the uterus enlarges and is at further risk of injury. A careful examination of the perineum needs to be done. A urethral catheter can be placed provided that no signs of urethral trauma are present. Urethral trauma in females is rare. Signs of urethral trauma include: blood at the urethral meatus and bruising of the perineum. Macroscopic haematuria indicates that bladder and upper urinary tract injuries may be present and further imaging, such as a CT cystogram, needs to be done.
It’s important to remember that a tremendous force is required to cause a pelvic fracture. A pelvic fracture in pregnancy should always make you think of significant pelvic haemorrhage, as the pelvic veins are engorged. The pelvic cavity can also “hide” a significant amount of blood.
A CT of the abdomen and pelvis may be indicated in this instance as the risk of missed injury outweighs the risk to the fetus.
Diagnostic imaging in Pregnancy – is it safe?
Diagnostic imaging in the pregnant patient is commonly thought to be dangerous, even amongst medical professionals. The myth that doing an X-ray in pregnancy is prohibited is surprisingly pervasive. Research from radiation disasters such as the Hiroshima and Nagasaki demonstrated that that there are significantly increased risks of fetal malformations, spontaneous abortions and cancer following extreme radiation exposure. So, should we be worried that we will subject our patients and their babies to the same risk by performing an X-ray or CT scan?
Absolutely not. The fear of radiation has been blown out of proportion. The dose of radiation required in diagnostic imaging falls far below the dose required to induce fetal malformations and miscarriage, and falls exceedingly far below the levels present in radiation disasters such as Hiroshima. If a critically injured pregnant patient warrants a CT scan, even a “panscan”, she should get it. This represents a paradigm shift in many people’s minds, so let’s look at the evidence.
The dose of radiation absorbed is measured in milliGrays (mGy) in most parts of the world, apart from North America where it’s measured in millirad (mRad). With doses of more than 100 mGy there is a small increase in risk of spontaneous abortion, organ malformation and mental retardation.
Below 50 mGy there is no risk for any of these adverse events. All guidelines on diagnostic imaging in pregnancy state that it safe to perform imaging if the cumulative dose falls below this cut-off.
This is stated in international radiology, gynaecology and trauma guidelines. There is no recognisable harm to the fetus or pregnancy below this 50 mGy cut-ff. Imaging should not be avoided if it is indicated to help guide management of the patient.
Of course, one should always practise ALARA, (as low as reasonably possible) and consider other diagnostic imaging such as ultrasound or MRI if the same amount of information can be gained from these tests. However, bear in mind that the sensitivity for the FAST in pregnancy is less than in the non-pregnant patient, and MRI is not a useful modality if your patient is critically injured, as it takes a long time to acquire.
Let’s look at what diagnostic imaging falls below 50 mGy:
A CT of the brain, cervical spine and chest all fall well below 50 mGy as the fetus is out of the field of view. The dose of radiation in these cases is minimal and barely adds to the baseline radiation that the fetus is already exposed to.
A CT of the abdomen and pelvis also falls well below 50mGy, provided that the scan is planned well. For example, instead of doing a pre, post and delayed contrast scan, do a single phase with IV contrast.Further dose attenuation can be done radiographers. It’s important to decide on an imaging protocol for pregnant patients in advance with your radiology department at your hospital.
Between 50-100 mGy the risk for adverse effects on the fetus is small, and uncertain. At present, 50 mGy is considered by most the as the maximum acceptable total dose for diagnostic imaging during pregnancy. There may be times when this dose may be reached, such as when CT cystogram needs to be done to visualise a bladder injury. This risk should be discussed with the patient, and the risk-benefit ratio for imaging and treatment considered by the relevant medical specialties involved.
What about the risk of childhood cancer?
The risk of childhood cancer appears to have a different threshold of risk compared to congenital malformations. The baseline risk for an individual getting a childhood cancer is 0.2 – 0.3%. A 10 mGy fetal dose would increase this incidence by 0.05% to approximately 0.35%. This is a small but appreciable risk so we should limit excessive diagnostic imaging where possible, but without compromising the mother. Shielding the abdomen has not been shown to reduce radiation absorbed by the fetus, as it is the internal scattering of photons that the fetus is exposed to.
Estimation of harm caused by imaging in pregnancy has been grossly exaggerated.
Diagnostic imaging should not be withheld from the pregnant trauma patient where the mechanism of injury is worrying. X-rays and CT of the brain, cervical spine and chest should be done when clinically indicated.
The ALARA principle should be followed when a CT of the abdomen and pelvis is ordered to minimize radiation where the fetus is directly exposed. This can be done by radiation reducing techniques if the CT is important to reduce maternal morbidity from missed diagnosis.
Click on the title below to read the other posts in this three part series: