Ultrasound: Looks can be deceiving

Understanding sonographic differences in pediatric patients.

An appropriately restrained infant presents to your community hospital by ambulance directly from the scene of an MVC. The EMS crew notes that the parents are likely to arrive a few minutes later as the front doors of the vehicle are intruded. The infant is brought in still restrained in the car seat, awake and crying.

Pediatric trauma is rare at your facility, but your team quickly assembles in the trauma bay. You carefully transfer the patient to a stretcher and proceed with your primary survey noting no obvious abnormalities.  The patient has a heart rate of 145, blood pressure of 82/46, Sa02 of 98% and a respiratory rate of 22.

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You decide to use your ultrasound machine to perform a FAST exam and quickly wish that you hadn’t. In every view, you see what appears as free fluid implying injury worse than you had suspected based on the patient’s appearance.

In the subxiphoid and parasternal cardiac views, you see a small hypoechoic area that suggests pericardial effusion. (Figure 1). This mostly disappears as the heart fills, but some persists. Is this blood?

In the right upper quadrant view, there appears to be a hypoechoic space between the liver and diaphragm. You are concerned that this may represent intraperitoneal fluid (Figure 2).

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However, there is no fluid in Morison’s pouch or around the liver tip. What is this hypoechoic stripe? Is this just an artifact?

Adjacent to the bladder are small, but clearly visible pockets of free fluid. (Figure 3). It is a small quantity, but could it be pathologic?

You ask for your nurse to draw a set of labs and you call for a chest and pelvis x-ray. These x-rays appear normal and are later read as such by the radiologist.

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The parents arrive, anxious but happy to see their baby who reaches for them enthusiastically. They ask if their baby has any serious injuries and if they can take their baby home.

The nearest pediatric trauma center is a three-hour trip away and you decide to call them for advice. They suggest that your FAST exam may be difficult to interpret given the baby’s age, but are happy to accept the transfer. Since the patient is stable, they suggest that you obtain CT imaging of the patient to clarify the extent of the injuries. Shortly after the CT imaging, you discuss the study with the radiologist who sees no sign of injury including no pericardial or peritoneal fluid.

After further consultation with the family and the pediatric surgeon, and another reassuring physical exam of the patient, you admit them to your hospital’s general pediatric ward for observation. The patient does well overnight and is discharged in the morning after an uneventful period of observation.

The next day, you are relieved to learn that the child did well, but add these topics to your reading list.

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Key Teaching points:

1. Common pediatric ultrasound applications like the FAST exam may demonstrate findings different from those found in adults. This is thought to be due to structures being more easily visible in young people due to less subcutaneous fat and shallower depth to the visualized targets for example. This includes the pediatric diaphragm which appears as a hypoechoic stripe and can be confused as pleural or peritoneal fluid. Clinical correlation with mechanism of injury as well as the presence or absence of free fluid in other locations such as Morrison’s pouch and near the liver tip can help decide how to interpret this finding. [3]
2. On its own, pediatric FAST appears to be less sensitive overall than in adults. Combining the FAST exam with reassuring lab work (transaminase, lipase) and a reassuring  physical exam improves sensitivity. [8,9]
3. The presence of trace intraperitoneal fluid in healthy children is well documented. It is most often seen in the pelvis and is present as often as 7% of the time in asymptomatic children and more often in those with abdominal pain with or without a diagnosis localizing to the abdomen (ie. gastroenteritis). If free fluid is seen in and outside of the pelvis, this should raise concern for pathologic free fluid. [1,3]
4. Some healthy children have trace pericardial fluid without known clinical significance. Children may also have an epicardial fat pad like adults. Both of these circumstances may cause confusion when performing the FAST exam. An epicardial fat pad is almost always over the RV (often seen best in the subxiphoid view) but does not interfere with diastolic filling of the heart and does not surround the heart. Increasing the gain causes the epicardial fat to have texture or marbling that is seen with fat in other areas of the body. Trace pericardial fluid might be easily seen, but should obliterate with diastole and is barely perceptible in most cases. [4,5,6]
5. Confounders seen in adult patients when looking for a pericardial effusion also apply to children and should not be forgotten. In addition to epicardial fat, also consider a large left pleural effusion and ascites that can be mistaken for pericardial fluid in certain views of the heart. [3,4]
6. Did you just upgrade machines or get a higher frequency probe? Increased resolution and image processing power in newer machines demonstrate potential spaces and structures that were not visible in older machines. This is only compounded in children where more detail is often visible regardless of the machine and where you may be using a higher frequency probe than you would be using for the same application in an adult. [3]
7. Though fat is usually seen as relatively bright or hyperechoic, there are some locations in which it can be dark, thus confusing the operator and suggesting fluid. Known locations of occasionally hypoechoic fat include the perinephric, anterior abdominal wall and pericardial spaces.[3]

Pearls and Pitfalls:

1. Small amounts of fluid in anatomic spaces including the pericardium or retrovesicular space in the pelvis are often seen on ultrasound in children and can be normal. Familiarity with the appearance of these findings and correlation with the clinical context is necessary to make sense of them. If you are uncertain of whether a finding is pathologic and if the clinical context is concerning, then definitive imaging and/or expert consultation is warranted.
2. Consider using ultrasound in known normal children and children with completed radiology ultrasounds to become more familiar with the appearance of pathology and normal variants.
3. Often obtaining multiple sonographic views of the same space can help clarify confusing findings and usually requires only minimal time. For example, if you suspect pericardial fluid in subxiphoid views, correlate with a parasternal long view. Trace pelvic fluid is not as likely pathologic if the right upper quadrant is normal.
4. Clinical algorithms designed in the context of previous generation ultrasound platforms and adult populations (ie. FAST) are complicated in the pediatric context due to age-related differences. As such, they should be applied with caution. The addition of a transaminase < 120 and a lipase < 100 as well as a reassuring physical exam and repeat ultrasound examinations all improve test performance of the FAST exam in pediatric patients.

References:

1. Berona, Kristin, Tarina Kang, and Emily Rose. “Pelvic free fluid in asymptomatic pediatric blunt abdominal trauma patients: a case series and review of the literature.” The Journal of emergency medicine 50.5 (2016): 753-758.
2. Wang, Crystal C., Kadine L. Linden, and Hansel J. Otero. “Sonographic evaluation of fractures in children.” Journal of Diagnostic Medical Sonography 33.3 (2017): 200-207.
3. Baer Ellington, Aimee, Walter Kuhn, and Matthew Lyon. “A Potential Pitfall of Using Focused Assessment With Sonography for Trauma in Pediatric Trauma.” Journal of Ultrasound in Medicine 38.6 (2019): 1637-1642.
4. Blaivas, Michael, Daniel DeBehnke, and Mary Beth Phelan. “Potential errors in the diagnosis of pericardial effusion on trauma ultrasound for penetrating injuries.” Academic Emergency Medicine 7.11 (2000): 1261-1266.
5. Reddy, Surendranath, et al. “Prevalence of pericardial effusions in children with large atrial or ventricular septal defect.” The American journal of cardiology 103.2 (2009): 271-272.
6. Mohammad Q. Najib, Jhansi L. Ganji, Amol Raizada, Prasad M. Panse, Hari P. Chaliki, Epicardial fat can mimic pericardial effusion on transoesophageal echocardiogram, European Journal of Echocardiography, Volume 12, Issue 10, October 2011, Page 804
7. Fox JC, Boysen M, Gharahbaghian L, Cusick S, Ahmed SS, Anderson CL, Lekawa M, Langdorf MI. Test characteristics of focused assessment of sonography for trauma for clinically significant abdominal free fluid in pediatric blunt abdominal trauma. Acad Emerg Med. 2011 May;18(5):477-82. doi: 10.1111/j.1553-2712.2011.01071.x. PMID: 21569167.
8. Retzlaff T, Hirsch W, Till H, et al. Is sonography reliable for the diagnosis of pediatric blunt abdominal trauma? J Pediatr Surg. 2010; 45: 912–915.
9. Shihabuddin, Bashar. “Clinical findings Increase the Specificity of the FAST Exam: A Strategy to Guide Imaging in Blunt Pediatric Trauma.” (2018): 119-119.