POCUS in Abdominal Trauma: Old Gadget, New Insights

Abstract

Abdominal trauma is difficult to identify, especially in a patient with multiple injuries. Mechanism of injury can guide us to the likely organs injured, but the extent and location cannot be accurately pinpointed in most cases. Owing to the multitude of structures located in the abdomen, timely identification and appropriate intervention are crucial to ensure the good patient outcomes. Focused assessment with sonography in trauma (FAST) and its extended version (eFAST) has become the standard care as per ATLS guidelines in patient evaluation. The main goal is to identify hemoperitoneum, hemothorax, and/or pneumothorax. However, sonography can be applied to detect varying injuries to abdominal viscera, beyond the elementary eFAST examination. This includes assessment of solid organs, hollow viscus, vascular structures, and even soft tissues. Sonography, when wielded with necessary knowledge and practice, can be an incredible asset at the bedside. This chapter aims to explore these possible applications of point of care ultrasonography (POCUS) in abdominal trauma.

1. Introduction

Abdominal trauma has varied clinical presentations, most of which are subtle, and patients may appear stable at presentation [1]. The injuries sustained are not easily identified by surface appearance or examination. Abdominal trauma is usually not found as an isolated injury, but rather as part of polytrauma cases [2]. A patient with intoxication, altered sensorium due to head injury, severe injuries over the limbs or thorax may have sustained trauma to the abdomen, which may go undetected due to the distracting nature of the other symptoms. A high index of suspicion is required in such cases. Any trauma sustained between the nipple line to the pelvis warrants a search for intraperitoneal organ injury. Point of care ultrasonography (POCUS) is a powerful tool that can aid in the search.

1.1 Relevant anatomy

Abdomen holds complex structures along with a large potential space. Knowledge of anatomy is key to identifying the possible organs involved. In general, the abdomen is divided into nine regions (Figure 1) for purposes of evaluation. However, this division studies the abdomen only from the anterior aspect. A more inclusive division of abdominal regions for trauma cases would be into four regions–lower chest, anterior abdomen, and flank and posterior abdomen. Based on the mechanism and location of the injury, the likely structures to be affected can be ascertained based on this division (Table 1). Retroperitoneum and pelvic structures can also be injured, however, are not well imaged by ultrasonography. POCUS can then be utilized effectively by screening these suspected structures with care, instead of performing an extensive abdominal scan that would delay the time and outcome for the patient.

Figure 1.

Surface anatomy and regions of abdomen [3].

Abdominal region	Extent	Suspected organs injured
Lower chest	Mid to lower thoracic cage	Liver, spleen, diaphragm, stomach
Anterior	Between anterior axillary lines	Liver, spleen, bladder, stomach, pancreas, transverse colon, ileum, jejunum
Flank	Between anterior and posterior axillary lines	Kidney, ureter, ascending and descending colon
Posterior	Between posterior axillary lines	Great vessels, duodenum, pancreas, spinal cord

Table 1.

Four abdominal regions.

The ultrasound machine is usually equipped with three basic probes - curvilinear, linear, and phased array probes (Figure 2). A curvilinear probe is of low frequency, allowing deeper imaging with a wide-angle view. A linear probe is of high frequency and visualizes superficial structures. By convention, the ultrasound probes when used in a sagittal or coronal plane have the pointer/marker toward the head end of the patient. In the transverse plane, the pointer is to be facing the right of the patient.

Figure 2.

Ultrasound probes – Linear (a) and curvilinear (b) [4].

1.2 Focused assessment with sonography in trauma (FAST)

Advanced trauma life support (ATLS) guidelines have incorporated bedside sonography by non-radiologists in the evaluation of trauma patients. It is now standard of care to perform extended focused assessment with sonography in trauma (eFAST) in all cases as part of the primary survey and its adjuncts [5, 6]. eFAST serves to locate any obvious evidence of bleeding in the potential spaces of the abdominal and thorax. It can also help detect pneumothorax (Table 2). The abdomen is scanned in four areas (or views) – subxiphoid, right hypochondrium, left hypochondrium, and suprapubic (Figure 3). Both pleural cavities just above the diaphragm and pleural line movement on each side are also checked. FAST has even been utilized to grade the amount of intraperitoneal hemorrhage and help decide the need for laparotomy [8, 9]. Apt use of FAST has been shown to reduce time to intervention, the need for computed tomography (CT) scans, and reduce hospital stay as well as costs [10, 11, 12].

Area examined	Organs visualized
Subxiphoid	Heart
Right hypochondrium	Liver, right kidney, diaphragm
Left hypochondrium	Spleen, left kidney, diaphragm
Suprapubic	Bladder, uterus/prostate
Hemithorax (each side)	Pleural line, diaphragm

Table 2.

eFAST examination.

Figure 3.

FAST examination views – (from left to right) subxiphoid, right upper quadrant, left upper quadrant and suprapubic [7].

1.3 Beyond eFAST

With the advent of bedside ultrasonography, POCUS has become revolutionary in patient care. Its use in trauma settings no longer has to be limited to eFAST. Rather, this gadget can be handy in detecting several injuries within the abdomen - solid organ, hollow viscus, and even blood vessels. Timing of ultrasonography is critical so as to ensure that performing POCUS does not hinder or delay patient care. In any unstable patient with a suspected abdominal injury, the first objective is stabilization. This can include any and all measures, such as securing airway, chest drain placement, fluid resuscitation, blood transfusion, splinting, and suturing. These life-saving interventions take time, which provides the window to examine the abdomen by POCUS simultaneously without causing undue delay. As per ATLS guidelines, unstable patients who do not respond to resuscitation require transfer to the operating room (OR). POCUS-assisted identification of injuries at the bedside would guide the operating surgeons when exploratory laparotomy is underway.

In stable patients, the timing of POCUS is more complex. Patients with suspected abdominal trauma who are hemodynamically stable may be evaluated with contrast-enhanced computed tomography (CT) scans. POCUS is useful in those cases where CT scans are deferred, such as in pregnancy, intravenous contrast allergy, centers without CT capacity, and those who refuse or are uncooperative with the scan. In penetrating trauma, POCUS can be used in stable patients without the urgent need for operative intervention.

In this chapter, we will explore POCUS in abdominal trauma, which can be performed by even trained non-radiologists at the bedside.

2. Solid organ injury

POCUS for abdominal examination is usually done with the low-frequency curvilinear probe. A phased array probe may be utilized when imaging between rib spaces is required.

2.1 Liver and spleen

Liver is the most commonly injured abdominal organ in both blunt and penetrating trauma. The transducer is first positioned in the sagittal plane, pointer toward the head end of the patient and then in transverse position, and pointer toward the right of the patient. The organ to be examined is to be scanned from medial to lateral and cephalad to caudal. The liver is scanned from subxiphoid region to the left and right. Spleen is located by scanning above the left costal margin.

Hypoechoic areas within the liver parenchyma, which have no flow on color Doppler are likely lacerations or hematoma in the setting of trauma. These lesions can be measured and their extent delineated on ultrasonography. The perimeter of the organ should be scanned for anechoic collection, including sub-diaphragmatic area (Figure 4).

Figure 4.

Spleen injury of subcapsular hematoma as seen on CT (left) and USG (right) [13].

2.2 Kidney

Kidney is a retroperitoneal organ well visualized by ultrasonography. For visualizing the right kidney, the transducer is placed in the mid axillary line at the right costal margin and moved caudally till the kidney comes into view. The maximum length is identified by turning the probe obliquely and this plane marks the longitudinal axis of the kidney. The parenchyma is scanned by fanning the transducer in anterior and posterior directions. The transducer is then rotated 90 degrees to obtain a transverse view. The probe is tilted superiorly and inferiorly to examine the upper pole, hilum, and lower pole. The longitudinal view of the left kidney is visualized by placing the transducer in the posterior axillary line at the left costal margin and then moving caudally. The remaining views are obtained the same way as for the right kidney.

Major renal injury markers on ultrasonography are subcapsular hematoma, perinephric hematoma, and calyceal dilatation with internal echogenicity. Mixed echogenic material with disorganized renal architecture can be seen high-grade renal injury, such as fractured kidney with retroperitoneal hematoma (Figure 5). Fresh parenchymal hematoma on POCUS will be isoechoic and difficult to identify. Over time, it will become hypoechoic and easier to locate. The additional role of bedside ultrasonography includes follow-up of patients with these findings for resolution. It helps avoid repetitive radiation exposures and high costs.

Figure 5.

High grade renal injury on USG with completely disrupted architecture [14].

Renal vascular injuries are identified by carefully scanning the hilum for abnormal Doppler flow. Segmental infarcts of the cortex are confirmed by the absence of perfusion on scanning.

2.3 Pancreas

Semierect patient position is preferred if possible to avoid interference from bowel gas. The left lobe of liver and spleen provide an acoustic window to visualize the pancreas. The transducer is placed in sagittal and transverse planes along the vascular landmarks. In the transverse plane, the pancreas is seen beneath the left lobe of the liver and crossing above the abdominal aorta and inferior vena cava and the splenic artery acts landmark as it runs along its posterior surface. In the sagittal plane, tthe tail of the pancreas is usually seen with a coronal view in a right posterior oblique position. The pancreatic duct is seen within the body of the pancreas seen as a tubular structure with reflective walls and a maximum diameter of 2 mm. Normal echotexture of the pancreas is similar to or more echoic than the adjacent liver.

When edema develops in the pancreas, the echogenicity reduces. Traumatic pancreatitis may be identified when it becomes a heterogeneous mass with an ill-defined border. There may be hypoechoic or anechoic collection around the pancreas due to bleeding or exudation. Rarely, pancreatic duct disruption can be detected (Figure 6).

Figure 6.

Pancreatic injury on USG seen as nonhomogeneous echotexture [15].

2.4 Newer modalities/techniques

2.4.1 Contrast-enhanced ultrasonography

Solid organ injury can be detected by POCUS. Imaging can be augmented by contrast-enhanced ultrasonography. Second-generation contrast agents have been introduced that contain perflutren microbubbles that can cross the pulmonary capillaries and enter into the systemic circulation. The microbubbles vibrate with the high-frequency waves generated by ultrasonography probes, which in turn makes them more reflective than normal tissue (Figure 7).

Figure 7.

Liver injury on traditional sonography (A) as hypoechoic lesion on right lobe (arrow), on contrast-enhanced USG (B) as extended liver rupture (arrows) and on CT (C), which confirmed the rupture (white arrow) [16].

In injuries to liver, spleen, and kidney, contrast-enhanced sonography images have been studied to allow better detection of injury extent compared to traditional sonography [16, 17].

2.4.2 Color Doppler in renal ultrasonography

Renal blood flow is evaluated by color Doppler to measure renal artery resistive index. It is a measure of tissue resistance to perfusion caused by vasoconstriction. This has been studied as a predictor of occult hemorrhagic shock in polytrauma patients with normal hemodynamic status in presentation. A value greater than 0.7 was studied to be predictive [18].

3. Hollow viscus injury

3.1 Pneumoperitoneum

Air is usually cited to be the enemy of ultrasonography as it obscures the view and affects image acquisition. However, this can be used to the advantage of the observer when examining pneumoperitoneum, that is, free air in the peritoneal cavity. The patient is usually examined in a supine position with the bed elevated by 30 degrees at the head end or semilateral position with the elevation of the right flank. This is to allow the collection of free air in the peritoneal space anterior to the liver surface. Linear array probe is used in the sagittal plane with a pointer toward head end with right intercostal scanning.

The probe is placed gently over the right upper quadrant and scanned for air. Free air is visualized as a hyperechoic line with reverberation artifacts similar to that of the pleural line of the lung (Figure 8). When the caudal end of the probe is pressed gently over the area, the air can be displaced and the hyperechoic line will disappear and with release of pressure, the reverberation artifact will reappear. In some cases, the pleural line above the diaphragm is seen along with the pneumoperitoneum over the liver producing the pleuro-peritoneal step-off sign.

Figure 8.

Free air of pneumoperitoneum overlying liver surface with enhanced peritoneal stripe (black arrows) and reverberation artifacts (white arrows) [19].

3.2 Small and large bowel

In patients with abdominal trauma, the bowel wall can be examined. However, the focused examination would be more effective than screening the entire bowel at the bedside. A high-frequency linear array probe is used to scan the bowel. It can even distinguish between the bowel wall layers. The small bowel can be differentiated from the large bowel by the size, location, and absence of haustrae. When scanning the large bowel, gentle pressure is applied with the transducer to displace bowel gas and small bowel loops from the view.

Studies have shown POCUS to be superior to plain radiography in identifying bowel obstruction. Features suggestive of obstruction include dilatation of bowel with fluid content proximal to the obstruction and collapsed distal bowel (Figure 9). In the case of ileus, there would be no transition point from dilated to collapsed bowel. As time progresses, the bowel wall thickens, and peritoneal free fluid develops. Peristaltic activity in the bowel can be seen as a to-and-fro movement of spot echoes within the bowel loop. Bowel strangulation can also be identified with POCUS – dilated aperistaltic proximal bowel loop with peristalsis seen in further proximal bowel, asymmetric bowel wall thickening with an accumulation of intraperitoneal fluid.

Figure 9.

Dilated bowel loops in intestinal obstruction as seen on USG [20].

3.3 Genito-urinary tract

3.3.1 Urinary bladder

Urinary bladder is scanned by placing the transducer in the suprapubic region in sagittal and transverse planes. A moderately filled bladder is ideal for visualization, which cannot be guaranteed in a trauma patient. If not visualized, then the probe is tilted inferiorly toward the pelvis to obtain a view. In some cases, normal saline may be introduced into the bladder via a per-urethral catheter to allow better imaging of the structures.

Bladder ultrasound can help guide suprapubic catheter placement in patients with urethral injury having urinary retention.

3.3.2 Ureter

Normal ureters are not visualized by ultrasonography. However, in case of obstruction to urine outflow, distended ureters can be seen with POCUS. The cause of obstruction in trauma can be varied, such as hematoma and foreign body (Figure 10).

Figure 10.

Dilated proximal ureter with hydronephrosis [21].

4. Vascular injury

4.1 Abdominal aorta

Ideally, the abdominal aorta is to be scanned along its full length from the diaphragm to its bifurcation in both transverse and longitudinal planes with the patient in the supine position. Doppler imaging is not necessary unless to differentiate from other surrounding structures. Bowel gas and obesity may impede imaging, which can be reduced by applying firm pressure with the transducer probe.

Aorta is scanned starting at the epigastrium in the midline and proceeds caudally along its length. In the transverse plane, the aorta is identified lying anterior and to the left of the hyperechoic line of the spine as a pulsatile and round to oval structure. Tracing caudally, branches of the aorta, including superior mesenteric artery, renal artery, and then its bifurcation, can be identified. Similarly, a longitudinal view in the midline and coronal view from the right side along the anterior axillary line can be used to visualize the aorta from different angles.

Contained rupture of the aorta may be seen as a hypoechoic mixed density area surrounding the aorta. Traumatic aortic dissection can also occur, which can be seen as a dissection flap within the lumen of the aorta (Figure 11).

Figure 11.

Aortic dissection flap seen on USG in transverse view (A) and longitudinal view (B) [22].

4.2 Inferior vena cava

Inferior vena cava (IVC) is identified by its termination into the right atrium of the heart by placing the curvilinear transducer in the longitudinal plane at the epigastrium with the pointer facing the head end of the patient. It is seen as a tubular structure with thin walls, varying size with respiration, and can be compressed with pressure from the transducer (Figure 12).

Figure 12.

Inferior vena cava on longitudinal view with M-mode applied for measurement of diameter and its variation with respiration [23].

Injuries to the IVC are uncommon. However, it has utility in trauma settings to assess the volume status of the patient. A cause of hypotension or any hemodynamic instability in a patient with trauma is due to blood loss unless proven otherwise. This is the reason ATLS guidelines stipulate every patient to receive one liter of pre-warmed lactated Ringer’s solution and arrange blood products for transfusion.

IVC diameter and degree of collapse correlate with volume status in the setting of hemorrhagic shock. Diameter is less than 1 cm in hypovolemia and more than 50% collapsing. In patients with obstructive shock secondary to cardiac tamponade or tension pneumothorax, the IVC may be dilated more than 2 cm with less than 50% collapsing nature. Repeat measurements show the patient response to resuscitation.

In pediatric population, IVC/aorta ratio is used to allow assessment independent of patient size. IVC/aorta ratio less than or equal to 0.8 correlates well with hypovolemia.

4.3 Retroperitoneal hemorrhage

Retroperitoneal region is divided into three compartments – anterior, middle, and posterior. The anterior compartment houses the bowel, pancreas, and great vessels, middle is occupied by the kidneys, and posterior compartment contains muscles, such as psoas and quadratus lumborum. Bleeding into these compartments, due to trauma or vessel rupture, can sometimes be seen with POCUS.

5. Special populations

5.1 Pediatric

20–30% of pediatric trauma cases involve injury to the abdomen. CT is the most common modality of imaging utilized in pediatric trauma. Though sensitive to detecting injuries, children are more susceptible than adults to the adverse effects of exposure to ionizing radiation. Therefore, POCUS provides a safe, repeatable and quick alternative. In addition to screening for free fluid, POCUS can also detect injuries similar to those in adults. So far, observational studies have expounded on the utility of ultrasound imaging in pediatric trauma. Definitive trials are indicated to establish POCUS as the standard of care [24].

5.2 Pregnancy

Trauma in pregnancy endangers both mother and fetus, more often causing fetal than maternal mortality. However, the priority of care is given to the mother in order to ensure good outcomes for both. Ultrasonography is advantageous in pregnant trauma patients due to the lack of ionizing radiation and contrast exposure, which can be done at the bedside quickly. POCUS can detect free fluid in the abdomen, which can indicate either blood or amniotic fluid secondary to uterine rupture. Furthermore, fetal cardiac activity and gestational age can also be evaluated. POCUS can sometimes detect uterine rupture and placental abruption, but a negative scan cannot be used to rule out these diagnoses.

6. Discussion

The American College of Surgeons have detailed the use of ultrasonography for FAST and extended FAST (eFAST) examination as an adjunct to a primary survey of all trauma patients [5]. eFAST application was then well studied over the years. Netherton et al. conducted a systematic review and meta-analysis on the diagnostic accuracy of eFAST in trauma, which included seventy-five studies with a total of more than 24,000 patients [6]. Pooled specificity for detection of pneumothorax, pericardial effusion, and intra-abdominal free fluid were 99%, 94%, and 98%, respectively; whereas the sensitivity ranged between 69 and 91%. They concluded that eFAST was capable of ruling in the above diagnoses, but not adequate to rule out the same when negative. Their subgroup analysis showed that eFAST was more specific in detecting intra-abdominal fluid in normotensive than in hypotensive patients.

A similar meta-analysis on the application of FAST in the pediatric age group was conducted by Liang et al., including eight studies and an aggregate of more than 2000 patients [24]. FAST had a pooled specificity of 96%, but a poor sensitivity of 35%.

Practical implications of FAST in terms of predicting the need for laparotomy, cost-effectiveness, and reducing time to operative intervention have also been studied. A study by Lane BH showed that FAST examination is clearly cost-effective in unstable patients [12]. Moylan et al. noted that even in normotensive patients with blunt trauma, the positive FAST examination had a strong association with therapeutic laparotomy, or 116 [8]. In the pediatric population of unstable blunt abdominal trauma, Long et al. observed that positive FAST examination at 2 hours after ED arrival had 100% specificity and positive predictive value for early surgical intervention [11].

Applications of ultrasonography beyond eFAST then began to be explored. Solid organ injury is easily identified by POCUS. Richards et al. studied over 2000 patients with blunt abdominal trauma who underwent POCUS for identifying the splenic injury [13]. They found that ultrasonography had an overall sensitivity of 69% for splenic injury, and this increased to 89% for grade 3 or higher injuries. Similarly, McGahan et al. noted that acute renal trauma can be detected by ultrasonography, especially with higher-grade injuries [14].

A newer technique was introduced that allowed better delineation of solid organ injury - contrast-enhanced ultrasonography (CEUS). Valentino et al. studied 133 patients with blunt abdominal trauma by performing standard ultrasonography and CEUS [16]. These were compared with injuries detected on CT scans. CEUS had a sensitivity and specificity of 96.4% and 98%, respectively, and was thus nearly as accurate as CT scans.

Other than solid organ trauma, hollow viscus and vascular structures can also be scanned with POCUS. Moriwaki et al. performed ultrasonography on 484 patients with severe abdominal pain with or without blunt trauma for detecting intra-peritoneal free air. It had a sensitivity of 85.7% and a specificity of 99.6% [25]. Ultrasound-guided suprapubic catheter placement into the urinary bladder has been studied to be safe as per Muhammad AS et al. even in resource-poor settings without incidence of any major complication [26]. Scanning of large blood vessels of the abdomen, such as aorta and inferior vena cava, is challenging due to the overlying bowel with gas content. Sefidbakht S et al. measured the diameter of inferior vena cava (IVC) and its variation with respiration in 88 patients of trauma, with or without hemodynamic instability. The average diameter of the vessel was smaller in those with shock (p < 0.0001) and the collapsibility index of IVC was significantly higher in the unstable trauma patients (p < 0.001). Thus, IVC diameter can be a reliable indicator of shock [23].

This chapter has described the possible applications of POCUS in abdominal trauma, but it is not an exhaustive list. Research and development in this field are ever-growing and further utility of ultrasonography remains to be seen.

7. Conclusion

POCUS is a great tool in the armamentarium of a physician in the evaluation and monitoring of a patient with abdominal trauma. Trained non-radiologists can perform POCUS reliably and identify injuries at the bedside.

Diagnostic utility of POCUS at the bedside has been well studied. It is a noninvasive, easily repeatable, and cost-effective modality. It is crucial to the time when POCUS is performed so as to prevent any delay in treatment of patients with abdominal trauma. This can be achieved by using the time of resuscitation in the emergency department. A focused approach based on the type and location of injury can be used to closely scan the likely affected organs. Trauma to solid organs, that is, liver, spleen, and kidney are best picked up on ultrasound.

Newer techniques, such as Doppler mode and contrasted-enhanced ultrasonography, allow better visualization and/or identification of injuries without any increase in risk to the patient. Thus, POCUS is a growing field and its applications are vast, beyond the standard eFAST.

Conflict of interest

The authors declare no conflict of interest.