Commonly Missed Findings on CT Abdomen/Pelvis
- Jun 27th, 2016
- Emily Thompson
Author: Emily Thompson, MD (EM Resident Physician, North Shore University Hospital) // Edited by: Alex Koyfman, MD (@EMHighAK, EM Attending Physician, UTSW / Parkland Memorial Hospital) and Brit Long, MD (@long_brit)
Computed Tomography. The donut of truth. If you are like me (even if you aren’t an intern), you probably dread the patient with non-specific abdominal pain. I certainly breathe a little easier sending a patient home with a negative CT abdomen/pelvis. However, the power of x-ray vision doesn’t allow us to turn off our brains. Certain pathologies may have only subtle findings on CT, and others may lend themselves better to other imaging modalities, such as ultrasound. By being aware of these pathologies and how to identify them, we can better recognize patients at risk of a missed diagnosis.
A 45-year-old male is a restrained driver in a single car, head-on collision with a telephone pole. He presents to a level-one trauma center by BLS ambulance in a c-collar. His heart rate is 120, blood pressure is 100/60, respiratory rate is 24, and O2 saturation is 96% on 100% non-rebreather. His eyes are closed, and he is moaning and making gurgling sounds. He withdraws from pain, and he smells of alcohol. He has shortening of his left leg and a seatbelt sign. The patient is put on the monitor, 2 large-bore IVs are placed running normal saline, and the patient is intubated. The patient has bilateral breath sounds, a soft, obese abdomen, and intact distal pulses. FAST exam is negative and the patient’s blood pressure responds to 2L normal saline, so the pelvis is stabilized and the patient is transported to CT. The images are reviewed by the on-call radiology resident, as well as the ED resident and trauma surgeon, who found a left hip dislocation, an acetabular fracture, and two right lower rib fractures, with no other obvious injuries. On day 2 in the surgical ICU he develops a fever and peritonitis. Repeat CT shows free air and fluid.
Missed Injuries in Trauma
Trauma is the most common cause of small bowel perforations. These are the third most common traumatic perforations after liver and spleen and are usually associated with other injuries. The most common CT finding is free air, but the leaks are less dramatic than with gastroduodenal perforation. Other findings include a visible bowel wall lesion, “misty” mesentery, intestinal pneumatosis, free fluid, and extraluminal fecal matter. CT is more sensitive than plain x-rays and extremely specific (95.4%). However, free air may be as little as 50% sensitive for detecting bowel perforation.11
The best way to manage these patients is to be aware of the possibility of bowel injury, re-examine them frequently, and re-image them or even send them for ex-lap if their condition changes. One study reviewed trauma victims who underwent exploratory laparotomy after repeat CT. While the initial CT was only 30% sensitive for bowel perforations, the second was 82% sensitive.13 These patients likely developed new symptoms that led to their repeat scans, which introduces some bias into the study. However, it is important to remember that a single CT is only a “snapshot,” and patients with serious blunt trauma may have occult injuries that destabilize later.
Bladder injuries in trauma are relatively rare. They are most commonly caused by blunt trauma to the pelvis. They can be due to a direct blow to a full bladder, causing a dome rupture, or they can be associated with pelvic fractures, particularly disruption to the pelvic ring. Bladder rupture is usually not immediately life-threatening, and the more common extraperitoneal ruptures are managed expectantly with catheter drainage. Intraperitoneal ruptures, on the other hand, require surgical repair. Extravasation of urine into the peritoneal space can result in chemical peritonitis and sepsis.
While a CT may be able to directly visualize breaks in the bladder wall, findings are usually more non-specific: free fluid, ascites, or bladder wall enhancement. Suspicion should be high for GU injuries in pelvic trauma. Every patient with gross hematuria or inability to void requires evaluation for GU tract damage. Retrograde cystography is nearly 100% sensitive for bladder injury. CT cystography is being used more often as part of the standard trauma series and is 85-100% sensitive for bladder rupture.9
The diaphragm is in the shadowy borderlands between the thoracic and abdominal cavity and is caught at the top of the CT abdomen/pelvis. It can be damaged by penetrating trauma anywhere between the fourth rib or tip of the scapula and the inferior rib margin. It can also be ruptured by blunt trauma that increases intra-abdominal pressure. Injuries occur most commonly on the left side, as the liver shields the right. The diaphragm heals poorly and delay in diagnosis may result in expansion of the injury or herniation of abdominal organs into the chest cavity. These complications may present months to years after the injury and are very difficult to repair.1
CT diagnosis of a diaphragmatic injury is usually made by direct visualization of the irregularity or discontinuity in the diaphragm. Unfortunately, the diaphragm is thin, mobile, and difficult to image. CT has fairly poor sensitivity for diaphragmatic injuries. One study found CT was only 82% sensitive and 88% specific for a diaphragm injury.15 For those watching at home, that means that nearly a fifth of stable patients with no diaphragm injury identified by CT went on to have one diagnosed by laparoscopy at 48 hours. Another study, a retrospective review of blunt diaphragm injuries identified by laparotomy, found that only 57% of pre-op CT scans had signs of diaphragmatic damage.12
So how do we identify these small, slippery, and potentially serious lesions? Clinically correlate, of course. Be aware of injuries that have the potential to cross between the thoracic and abdominal cavities (for example, penetrating trauma as high as the fourth rib). Know that CT will be negative for a significant percentage of patients with diaphragmatic injury. If your suspicion is high enough, discuss laparoscopy with your friendly local trauma surgeon.1
A 76-year-old woman with a past medical history of hypertension, diabetes, hyperlipidemia, hypothyroidism, dementia, and atrial fibrillation presents to the emergency department with abdominal pain. She is mildly tachycardic, tachypneic, and normotensive, with an irregular heartbeat. She is moaning in pain and holding her abdomen, but is unable to characterize or localize the pain. Her abdomen is non-distended, soft, and diffusely tender with voluntary guarding. Her home health aid has her medication list, which includes an 81 mg aspirin, but is unsure who her primary doctor is. She has never had a colonoscopy. CT abdomen/pelvis shows a small amount of pelvic free fluid and thickening of the small bowel wall.
Non-traumatic Abdominal Pathology
The classic presentation of mesenteric ischemia is abdominal pain out-of-proportion to exam. Patients may lose blood supply to their bowels by a variety of mechanisms: generally embolic, thrombotic, or hypovolemic (dissection and vasculitis are less common causes).5 Each of these patients will have a different history, for example, atrial fibrillation in the patient with embolic ischemia.
While CT is generally the best imaging modality for mesenteric ischemia, CT findings are relatively non-specific. The most common finding is bowel wall thickening. This may be associated with the target sign, or an alternating high and low attenuation pattern due to submucosal hemorrhage or edema. Other findings include free fluid and bowel dilation. More specific signs include bowel pneumatosis and ischemia to other abdominal organs such as the liver and spleen (indicating that clots have been showered from a source like the heart). Finally, CT angiography may offer visualization of the clot itself, filling defects, or gas in the intestinal vessels.5,7
For further discussion, go here: http://www.emdocs.net/mesenteric-ischemia-power-review/
Torsion is the twisting of an organ around its blood supply. While virtually any organ can torse, the ones that will be missed by CT are ovaries and testicles. Ovarian torsion presents with sharp lower abdominal pain/tenderness and adnexal tenderness on bimanual exam. There may be a palpable mass either from the torsed ovary itself, the twisted vascular pedicle, or a mass that caused the torsion in the first place. CT may show displacement of the ovary toward the midline, enlargement, surrounding inflammatory changes, and uterine deviation toward the affected side.
Ultrasound is the imaging modality of choice. The most common finding is an enlarged ovary with heterogeneous echotexture. The follicles may also move peripherally giving the “string of pearls” sign. Unlike testicular ultrasound, Doppler plays virtually no role. The ovary has a dual blood supply, so a torsed ovary may have flow. A normal ovary can also have no flow, and flow asymmetry is common based on menstrual cycle phase. The whirlpool sign may be present, which is color Doppler of the actual blood vessels in the pedicle twisted around each other. See further discussion here: http://www.emdocs.net/ovarian-torsion-pearls-and-pitfalls/
Testicular torsion occurs when the testicle spins on its spermatic cord causing acute scrotal pain, pain with palpation, and a high-riding testicle in transverse lie. Unlike ovarian torsion, Doppler is key. The money is in the “buddy shot” of the testicles side-by-side showing asymmetric or absent flow to one. Grey scale abnormalities usually don’t appear until it is too late. Remember, a testicle is 100% salvageable at 6 hours, 20% at 12 hours, and approaches 0% at 24 hours, so don’t forget to check below the waist in a male patient with abdominal pain.2 See further discussion here: http://www.emdocs.net/testicular-torsion-pearls-and-pitfalls/
CT scan is only about 75% sensitive for gallstones. Gallstones often have the same density as bile, which makes them invisible on CT. Ultrasound is the best imaging modality for gallstones, with 96% accuracy, and should be used first to assess right upper quadrant pain.3
Contrary to popular belief, CT does fairly well identifying acute cholecystitis: 91.7% sensitive and 99.1% specific in one paper.8 Research directly comparing CT and ultrasound is limited, and most studies are small and showed their effectiveness to be similar. In 1981, a combination of major and minor criteria was found to be 100% sensitive and 96% specific for cholecystitis.14 However, in a 2012 study of over 5,000 patients, ultrasound is only 81% sensitive and 83% specific for cholecystitis.4
Ultrasound is still the preferred modality for diagnosing cholecystitis due to easy access, speed, and lack of radiation. The findings are the same for CT and ultrasound: hydrops, wall thickening or edema, and pericholecystic fluid. CT will likely not pick up a stone in the neck of the gall bladder. CT may be helpful for atypical presentations of acute cholecystitis, acalculous cholecystitis, and to identify complications such as gallbladder perforation. However most stable patients who are highly suspicious for biliary disease should undergo ultrasound and follow up with MRCP. For a fantastic review of pathologies and imaging modalities check out “Evaluating Patients with Right Upper Quadrant Pain” by Genevieve Bennett in the resources section.3
- Arora, Sanjay; Menchine, Mike. “Cracking the Chest: Paper Chase 2 – CT for Diaphragm Injury.” EM:RAP. 2016 Jan. Accessed 4 April 2016.
- Ayoob, Andres R; Lee, James T. “Imaging of Common Solid Organ and Bowel Torsion in the Emergency Department.” AJR. 2014 Nov;203:W470-81.
- Bennett, Genevieve L. “Evaluating Patients with Right Upper Quadrant Pain.” Radiol Clin N Am. 2015;53:1093-30.
- Cartwright, Sarah L; Knudson, Mark P. “Diagnostic Imaging of Acute Abdominal Pain in Adults.” American Family Physician. 2015 April 1;91(7): 452-60.
- Gray-Eurom, Kelly; Deitte, Lori. “Imaging the Adult Patient with Nontraumatic Abdominal Pain.” EB Medicine. Published: February 2007. Accessed: 2 April 2016. http://www.ebmedicine.net/topics.php?paction=showTopicSeg&topic_id=12&seg_id=84
- Fagenholz, Peter J, et al. “Computed Tomography is more sensitive than Ultrasound for the Diagnosis of Cholecystitis.” Surgical Infections. 2015, Oct 5;16(5): 509-12.
- Firetto, Maria Cristina; Lemos, Alessandro A. et al. “Acute bowel ischemia: analysis of diagnostic findings at MDCT angiography.” Emerg Radiol. 2013;20:139-147. DOI 10.1007/s10140-012-1078-4.
- Harvey, Robert T.; Miller, Wallace T. “Acute Biliary Disease: Initial CT and Follow up US versus Initial US and follow-up CT” Radiology. 1999 Dec;213(3).
- Hass, Christopher, et al. “Limitations of Routine Spiral Computerized Tomography in the evaluation of bladder trauma.” The Journal of Urology. 1999 July;162: 51-2
- Hefny, et al. “Usefulness of free intraperitoneal air detected by CT scan in diagnosing bowel perforation in blunt trauma: experience from a community-based hospital.” Injury. 2005 Jan;46(1):100-4
- Lo Re, Guiseppe; et al. “Small Bowel Perforations: What the Radiologist Needs to Know.” Semin Ultrasound CT MRI. 2006;37:23-30.
- Sprunt, Julie M, et al. “Computed Tomography to Diagnosed Blunt Diaphragm Injuries: Not Ready for Prime Time.” The American Surgeon. 2014 Nov 11;80(11): 1124-7.
- Walker, Mark L.; et al. “The Role of Repeat Computed Tomography in the Evaluation of Blunt Bowel Injury.” The American Surgeon. 2012 September;78(9): 979-85.
- Worthen, Nancy J, et al. “Cholecystitis: Prospective Evaluation of Sonography and 99mTc-HIDA Cholescintigraphy.” AJR. 1981;137:973-78.
- Yucel, Metin. “Evaluation of the diaphragm in penetrating left thoracoabdominal stab injuries: The role of multislice computed tomography.” 2015 Sep;46(9): 1734-7. doi:10.1016/j.injury.2015.06.022