Introduction

Some patients have unremarkable CT findings despite mental deterioration after trauma. Although the most likely cause may be diffuse axonal injury, other aetiologies should be ruled out. Diffusion-weighted MRI (DWI) is a new MR technique that detects the microscopic random movement of water molecules in tissues. Recent studies have shown that DWI can detect and localise acute ischaemic stroke with remarkable sensitivity.1 2 Shortly after the onset of ischaemic stroke, DWI shows infarction as a homogeneous lesion of high-signal intensity resulting from diffusion changes caused by cytotoxic edema.1 DWI of the entire brain can be accomplished in 10 seconds because of the new technology of echo-planar imaging.2 However, DWI is not yet widely available in most emergency departments (EDs).

In this paper, we report our experience using DWI to evaluate unconscious trauma patients with negative brain CT in the ED.

Case reports

Case 1: diffuse axonal injury

A 49-year-old man was brought to the ED following a motor vehicle accident. The accident occurred 30 minutes prior to the patient's admission to the ED. On initial physical examination, his Glasgow Coma Scale (GCS) score was 6 (E1M4V1), his blood pressure was 180/100 mmHg, his heart rate was 74/min and his respiratory rate was 30/min. An initial plain radiograph showed traumatic haemothorax and a left tibial fracture. A CT scan of the brain was normal (figure 1A). The DWI performed 12 hours later revealed typical lesions (figure 1B, C). He was diagnosed with diffuse axonal injury and admitted to the intensive care unit for conservative management. On the 22nd hospital day, due to his improved GCS score of 14 (E4M6V4), he was transferred to the local hospital for further conservative care.

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Figure 1

A 49-year-old man admitted to the emergency department after a motor vehicle accident. A CT scan of the brain was normal (A). Diffusion-weighted imaging revealed a high-signal intensity in the splenium of the corpus callosum (B) and the cerebral peduncle of the midbrain (C).

Case 2: cerebral fat embolism

A 25-year-old man involved in a motorcycle accident was transferred to the ED. The patient sustained a segmental fracture of his right femur and a left tibiofibular fracture. On initial examination he was alert, his blood pressure was 120/80 mmHg, heart rate was 120/min and oxygen saturation was 99%. Three hours later, his level of consciousness had deteriorated to drowsy. The patient suddenly developed dyspnoea and had a general, tonic clonic-type seizure. A CT scan of the brain showed normal findings (figure 2A). The laboratory findings as follows: haemoglobin was 8.7 g/dL, platelet count was 77 000/dL and D-dimer was 7.9 ng/mL (reference 0–1). He had petechiae on the lower abdomen and right axilla. DWI was performed 40 h after presentation, revealing multiple lesions on both deep cerebral white matter (figure 2B). The patient was diagnosed with a cerebral fat embolism and admitted to the intensive care unit for conservative management. A follow-up DWI 1 month later showed that the previous lesions had disappeared. The patient was discharged with minimal memory impairment.

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Figure 2

A 25-year-old man admitted to the emergency department after a motorcycle accident. The brain CT findings were normal (A). Diffusion-weighted imaging revealed multiple lesions with high-signal intensity in the deep white matter bilaterally (B).

Discussion

The pathological triad for a diffuse axonal injury includes diffuse damage to axons and focal lesions in the corpus callosum and the brain stem adjacent to the superior cerebellar peduncles.3 About 30% of patients with negative brain CT have radiographic evidence of diffuse axonal injury on conventional MRI.4 However, DWI can detect early stages of injury even within 1 h, which shows high-signal intensity in the typical sites as seen in case 1.5 6 Although the early detection of axonal injury does not alter the treatment options, DWI can provide prognostic information.7 8 Early identification of patients at high risk for poor outcome may assist in aggressive clinical management of traumatic brain injury patients.8

DWI also allows differential diagnosis of cerebral fat embolism, a potentially life-threatening complication of long bone fractures, with the classic triad of respiratory insufficiency, cerebral decompensation, and skin petechiae. Cerebral fat embolism may be read as normal on brain CT despite mental status deterioration and focal neurological deficits. As demonstrated in case 2, DWI has a typical starfield pattern of scattered bright spots on a dark background.9 Cerebral fat embolism should be suspected clinically in any patient who develops neurological deterioration after traumatic long bone fracture. DWI should always be included in the initial workup of these patients. DWI does not provide prognostic information in fat embolism syndrome. However, early detection allows optimal support of pulmonary and cardiac functions, which benefits prognosis.10

However, despite its advantages, routine use of DWI in the ED has the limitations of limited availability and high cost. Additionally, close monitoring of the critically ill patient is difficult when they are under the magnet.

Conclusions

DWI can be used to determine the cause of altered mental status in trauma patients with normal CT scans and can distinguish between cerebral fat embolism and diffuse axonal injury. Emergency physicians should consider using DWI in unconscious trauma patients when their brain CT scans are unremarkable.