Cranial computed tomography in trauma

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1 Hong Kong Journal of Emergency Medicine Cranial computed tomography in trauma TH Tan, KL Ong Introduction Head trauma accounts for a substantial proportion of morbidity and mortality in all age groups of patients. Currently over a million people die each year from brain injuries, and a similar number are disabled. All too often these injuries have profound effects on the quality of their daily lives. The British Society of Rehabilitation Medicine has defined the scale of head injuries as mild (Glasgow coma score 13-15), moderate (9-12) and severe (<9). The society reveals some interesting facts that after moderate head injuries 63% of patients remain disabled one year after their accident and after severe injuries the figure rises to 85%. Of those who sustained minor head injuries, 79% suffered severe headaches, 59% suffered memory problems and 34% unemployed three months after the injury. Only 45% of patients who have suffered a minor head injury fully recovered a year later. Generally, there is no difficulty in deciding which patients should be admitted or transferred to a neuro-surgical unit when they sustained severe head injuries. With less severe injuries, patients may be admitted when history or examination demonstrate impaired consciousness at the time of assessment or a history of loss of consciousness for more than 5 minutes, persisting neurological symptoms or Correspondence to: Tan Thuan Heng, Lawrence, FRCR, FHKCR, FHKAM(Radiology) Pamela Youde Nethersole Eastern Hospital, Department of Radiology, 3 Lok Man Road, Chaiwan, Hong Kong tanth@ismart.net Prince of Wales Hospital, Accident & Emergency Department Ong Kim Lian, FRCSEd, FHKCEM, FHKAM(Emergency Medicine) signs. Patients who are difficult to assess (intoxicated) and patients with concomitant diseases or medications that pose increased risk (coagulopathies and anticoagulants) are also candidates for admission. However, any patient discharged should be advised to return if they develop any complications such as vomiting, severe headache, visual disturbance which are listed in the "head injury" pamphlet. Care should also be taken to ensure that they would be in the care of a competent adult. Anatomy The scalp consists of 5 different anatomical layers that include the skin, subcutaneous tissue, galea aponeurotica, loose areolar tissue and the periosteum of the skull bone. The subcutaneous layer contains an abundant communicating network of vessels that can result in a significant blood loss when the scalp is lacerated. The galea aponeurotica is poorly fixed to the underlying periosteum of the skull by the loose areolar tissue and provides little resistance to shear injuries that may result in large scalp flaps. This loose areolar tissue layer also provides little resistance to haematoma or abscess formation. The bones of the skull consist of 3 distinct layers in the adult, the hard internal and external tables and the cancellous middle layer or diploë. Although the average thickness is approximately 5 mm, there is variation in thickness, the thickest region being the occipital bone and thinnest, the temporal bone. The calvarium is covered by periosteum on both the outer and inner surface while the inner surface fuses with the dura to become outer layer of the dura.

2 Tan et al./cranial computed tomography in trauma 177 Diagnostic imaging Accurate, rapid, noninvasive assessment of victims sustaining cranial injuries is often required for appropriate triage and management. The findings on imaging often demonstrate distinct patterns relating to specific mechanisms of injury. X-rays or even CT scan. An apparent normal skull radiograph series or CT cannot exclude a base of skull fracture. Basal skull fractures are most frequently diagnosed by clinical findings, making clinical assessment skills critical. The clinical evidence of skull base fracture includes cerebrospinal fluid rhinorrhoea or haemotypanum. Skull radiograph In general, although skull radiograph may help in the management of the acutely injured patient, there is still much debate regarding their role. However, if a CT is indicated, skull radiographs are often not necessary and insisting on obtaining them may cause delay. The commonest difficulty interpreting the skull radiograph is deciding whether a radiolucent line is a fracture or a normal structure, such as a suture or vascular marking. Fractures are generally straighter or bend sharply with parallel edges, more radiolucent and cross suture lines. Depressed skull fractures, unlike linear fractures appear as an area of double density, representing overlapping bone. Sutures occur in typical location and are usually symmetrical and tends to have a more zig-zag appearance. Normal variant such as the frontal (or metopic) suture divides the frontal bone in two and usually fuses by about 3 years of age, but it may persist in adults and should not be mistaken for a fracture. Vascular markings are less lucent and sharply demarcated and usually taper as they run distally. It is prudent to remember that skull fractures can occur without associated loss of consciousness and conversely that intracranial injuries can be severe even in the absence of a skull fracture. Limitations of imaging Basal skull fractures are often not detectable with skull Computed tomography Computed tomography (CT) scan is the diagnostic procedure of choice in the evaluation of the acutely injured patient or patient with acute neurological deficit. It can be done quickly even for uncooperative patients and allowing a firm diagnosis to be made, as well as in excluding alternative diagnoses or the sequelae of other pathology. In addition, CT has been valuable in identifying and localising intracranial bony/metallic fragments. Unlike MRI, patient monitoring is simple and safe, currently, patient's stabilisation and monitoring devices pose no difficulty with CT imaging. Most hospitals now have 24-hour access to computed tomography scanning, and in some cases availability of electronic transfer of scans allow viewing by neurosurgeons at distant hospitals. This allows early decision regarding appropriate management to be made. Magnetic resonance imaging Currently, magnetic resonance imaging (MRI) is of great value in the evaluation of subacute and chronic changes, and identification of subtle abnormalities such as shearing injuries and cortical contusions. In acute situations, possible presence of sharp magnetic foreign bodies, interference from patient's motion and patient's monitoring devices and the relatively long imaging time are few of the reasons why MRI is not as widely utilised as CT. In the near future with faster machines and sequences, as well as MRI units that are more open and comfortable for unstable or acutely injured patients, we can expect MRI to play a greater role in the imaging of the acutely injured patients.

3 178 Hong Kong j. emerg. med.! Vol. 8(3)! Jul 2001 Epidural haematoma The dura mater is composed of the visceral or meningeal layer, which lines the intracranial space, and the parietal layer which functions as the periosteum of the calvarium. Epidural haematoma almost always occur from a fall or blow to the head producing a linear skull fracture that crosses an arterial branch and tearing it. Bleeding then occurs into the closed space between the calvarium and outer dural surface. This often occurs over the temporo-parietal bone where it tears the middle meningeal artery. The dura attachment is quite strong and thus give rise to its characteristic biconcave shape, with a well defined margin. CT appearance is a characteristic homogeneous highdensity lens-shaped or bi-convex blood collection against the calvarium. As the haematoma enlarges, due to the tightly adhered dura to the suture, haematoma almost never crosses the sutures. This may lead to significant mass effect with compression of the ipsilateral ventricle and possible midline shift that may require neuro-surgical intervention. Figure 1. Acute extradural haematoma: Acute right frontal extradural haematoma appear as a bi-convex homogenous hyperdense area and exhibiting mass effect on the adjacent cerebral cortex. There is no midline shift as the falx cerebri is central in location. As the dura splits to encase the major venous sinuses and form the falx cerebri and tentorium cerebelli, inward displacement of the superior sagittal or transverse sinus indicates an epidural process. Although often arterial in origin, venous epidural haematomas can occur when the fracture disrupts one of the sinuses and occur more commonly in the posterior fossa. On occasion, the haematoma may appear to have a heterogenous density in the case of a very acute bleed as blood clot does not have enough time to form. A spiral lucency ("swirl sign") is seen with the hyperdense clot suggesting an ongoing active bleed. Figure 2. Acute extradural haematoma with active bleeding: Right parietal bi-convex heterogenous density representing presence of active bleeding - "swirl sign". The extradural collection causes mass effect with compression of the ipsilateral right trigone of the lateral ventricle and displacement of the midline structures. This constitute an neuro-surgical emergency as the extradural haematoma is expanding from the active bleed.

4 Tan et al./cranial computed tomography in trauma 179 Subdural haematoma The subdural space is bound externally by the meningeal layer of the dura and internally by the arachnoid mater. Subdural haematoma (SDH) occurs more often than epidural haematoma. It may be equally severe due to the mass effect it produces and thus requiring urgent neurosurgical intervention. These arise between the dura and arachnoid, often resulting from tear of the bridging cerebral veins or dural sinusoid crossing this potential space. Occasionally the source of a SDH is a cortical artery. The elderly patients are predisposed to these haemorrhages because the presence of cerebral atrophy increases the tension applied to these veins. Based on a temporal sequence, SDH can be subdivided into acute (less than 3 days), subacute (3 days to 3 weeks) and chronic (after 3 weeks). SDH are bilateral in 10% of patients. On CT, their appearance in the acute stage is a crescent shape high-density blood collection. The haematoma is confined by the reflected dura of the tentorium and falx. However, the dural sutures do not limit the haematoma in its extent. Acute SDH accumulates moderately rapidly but seldom as rapid as the epidural haematoma. When acute SDH is associated with other significant brain injuries such as contusion and lacerations, the mortality rate can be as high as 60-70%. Figure 3. Acute subdural haematoma: Acute right frontal and temporal subdural haematoma as demonstrated by the presence of convex hyperdense blood along the inner concave surface of the right frontal and temporal fossa. In subacute SDH, as blood ages, it becomes less dense, so that by about 7-10 days, it approximates the density of the adjacent brain tissue. At this stage the density of the collection is term isodense to the adjacent brain. This can be difficult to detect especially when the SDH is small but careful scrutiny of the inward displacement of the cortical grey/white matter often help in arriving at the diagnosis. On occasion, this isodense collection is better demonstrated after intravenous contrast and will appear less dense than the adjacent enhanced brain. Chronic SDH may appear as a complex collection with layering of the more dense material posteriorly in gradual transition. Eventually, chronic SDH becomes a hypodense crescent shape collection with or without mass effect. Figure 4. Acute subdural haematoma: Acute right subdural haematoma along the tentorium incisura which appears as hyperdense irregular thickening of the edge of the tentorium.

5 180 Hong Kong j. emerg. med.! Vol. 8(3)! Jul 2001 Figure 5. Chronic subdural haematoma: Chronic bilateral subdural haematoma which appears as hypodense crescent shape collections along both fronto-parietal regions. The density of the subdural collection is similar to that in the ventricles. Figure 7. Acute on chronic subdural haematoma: Right subdural collection demonstrates foci of hyperdensities indicating the presence of fresh blood and these are often due to tearing of vessels transversing the space. Left subdural collection represents subacute haematoma in the process of becoming a chronic collection. Subarachnoid haemorrhage The subarachnoid space extends between the subdural space and the cortical/pia mater surface of the brain. This space tends to widen in the elderly with accompanying cerebral and cerebellar atrophy. Subarachnoid haemorrhage (SAH) occurs most commonly from closed head injury. Veins, venules, arteries or arterioles can be lacerated. The haemorrhage occurs in the cerebrospinal spaces in the sulci or in the basal cisterns and appear as fluid with increased attenuation. This may occur alone or in association with other intracerebral or extracerebral haematomas (including intraventricular haemorrhage). Figure 6. Subdural haematoma: Bilateral subacute subdural haematoma seen in the parietal regions. The density of the collection is almost similar to the adjacent cerebral parenchyma and this represents blood in the process of degradation. Two smaller hypodensities is seen on the left side of the frontal aspect of the cerebri falx which probably suggest previous episode of subdural haematoma being loculated by scarring or septations. In patients with head injury, presence of significant SAH is almost always associated with cortical contusions. In addition, SAH may block arachnoid villus leading to communicating hydrocephalus. On occasion, there may be difficulty in determining between SAH due to trauma and a ruptured aneurysm

6 Tan et al./cranial computed tomography in trauma 181 or arteriovenous malformation. In these instances, history from either the patient or eye witness is important. the surface of the brain beneath the site of impact while the contre-coup contusion occurs on the surface of the brain contra-lateral to site of impact. It is therefore important to ascertain the site of impact to the head (scalp wound) in the initial assessment of the patient in order to determine whether a contusion is coup or contre-coup. Regardless of the mechanism of injury, superficial frontal and temporal contusions are most frequently encountered. This has been attributed to the relatively irregular floor of the anterior and middle cranial fossae. The CT appearance of brain contusion is a focus of high density (blood) and associated lower density of cerebral oedema. When cerebral contusion is not haemorrhagic and the accompanying oedema is too subtle to be detected by CT, MRI may be required to demonstrate these foci of injuries. Figure 8. Acute subarachnoid haemorrhage: Acute bilateral extensive subarachnoid haemorrhage with hyperdense fluid (fresh blood) in the subarachnoid space which normally contain the hypodense cerebro-spinal fluid. Compare the density of the subarchnoid space fluid surrounding the cerebral hemisphere and the sulci between the gyri with the cerebro-spinal fluid density in the ventricles. Both frontal cerebral lobes density appears lower that the rest of the brain suggesting presence of accompanying infarct/ischaemia. Generalised cerebral oedema is indicated by reduced gyri and sulci. Contusions Contusions often occur in closed head injury from a blunt force that cause the surface of the brain to hit against either the skull or dura (tentorium and falx cerebri). This usually results in bruising or haemorrhage at the site of impact. The most common type of contusion is the coup and contre-coup contusion. The coup contusion occurs on Figure 9. Cerebral contusion with subdural haematoma: Hyperdense focus representing contusion. If large, it may become a haematoma with a rim of hypodensity which is due to the associated oedema over the left vertex. In addition, there is an acute left subdural haematoma adjacent to the parietal bone.

7 182 Hong Kong j. emerg. med.! Vol. 8(3)! Jul 2001 Diffuse axonal injury Diffuse axonal injury (DAI) is one of the most serious forms of injury in patients with severe head trauma. It represents an immediate, irreversible, and nonsurgical injury to the brain. Patients with DAI usually present with severe impairment of consciousness from the moment of impact and clinical outcome are inevitably poor. Diffuse axonal injury is caused by shearing of the white matter due to the difference in density or fixation between two structures and response to rotation and deceleration. The lobar white matter, centrum semiovale, brainstem, and corpus callosum are most often affected with no correlation to external injury or skull fractures. CT imaging often fails to demonstrate any abnormality to account for patient's severe clinical condition. However, if present, these foci of injuries appear as focal ovoid or elongated regions of decreased density in predisposed areas of the brain. velocity bullet with sufficient kinetic energy exits the skull resulting in more damage. Injuries sustained secondary to transitory cavitation formation includes remote fractures of the skull, brain contusion, brain parenchyma injuries and possible herniation. CT scan is the diagnostic procedure of choice for patients with gunshot wounds and other penetrating trauma. The course of the projectile and location of foreign bodies may be identified. Any abnormalities that require urgent neurosurgical intervention can be readily demonstrated on CT such as large haemotoma with mass effect. Despite the prevalence of these injuries especially in some cities with high crime rates, the morbidity and mortality of penetrating head injury remains high. Improvements in the understanding of the mechanisms of injury and appropriate management of patients with these injuries may eventually lead to improve outcomes. On the other hand, MRI is the preferred imaging technique to detect these non-haemorrhagic or haemorrhagic foci of injuries. Lesions often appear as 1-15 mm ovoid foci with their long axis being parallel to the direction of the involved axonal tracts. Despite the improvement of CT in detecting these foci of injury, histopathological extent of axonal injury invariably exceeds that visualised macroscopically. Penetrating trauma Technically, any wound in which the projectile breaches the cranium but does not exit is described as penetrating. An injury in which the projectile passes though the head and having an entrance and exit wound is termed as perforating. This distinction, according to some authors has some prognostic implications. In gunshot wounds, low velocity bullet penetrates skull with limited penetration of brain. However, high Figure 10. Gun wound: Left frontal cerebral contusion/ haemorrhage with multiple hyperdense oval, rounded and elongated bullet fragments. The hypodense (black) areas associated with the injury site represent gas pneumocephalus. The falx cerebri (anterior aspect) is hyperdense suggesting presence of accompanying subdural haematoma.

8 Tan et al./cranial computed tomography in trauma 183 Miscellaneous Head injuries may be associated with other injuries, especially if the injuries are severe, the cervical spine may be involved. In patients with moderate or severe head injuries, about one third of them had associated cervical spine and/or spinal cord injuries. Those patients with upper cervical injury are at greater risk of suffering from skull base fractures and severe intracranial haematomas compared to those with mid to lower cervical injury. Brain damage was more frequently associated with upper cervical injury. Conclusion Head injuries are common and the understanding of the mechanism of injury and their associated abnormalities will assist us in our management of this common condition. Currently, CT provides the best initial diagnostic work-up of patients with head injuries and is readily repeatable to allow follow up of the progress of the injury. On the other hand, MRI is often performed when the clinical findings are not commensurating with the CT findings. MRI being more sensitive to cerebral parenchyma changes, is also used to detect subtle abnormalities not often well visualised on CT. Recommended reading 1. The British Society of Rehabilitation Medicine. Rehabilitation after traumatic brain injury. London: British Society of Rehabilitation Medicine, Royal College of Surgeons of England. Report of the working party on the management of patients with head injuries. London: Royal College of Surgeons of England, The Society of British Neurological Surgeons. Guidelines for the initial management of head injuries: recommendations form the Society of British Neurological Surgeons. Br J Neurosurg 1998;12(4): Teasdale GM. Head injury. J Neurol Neurosurg Psych 1995;58(5): Gentry LR: Imaging of closed head injury. Radiology 1994;191(1): Hackney DB. Skull radiography in the evaluation of acute head trauma: a survey of current practice. Radiology 1991;181(3): Ogawa T, Inugami A, Fujita H, et al. MR diagnosis of subacute and chronic subarachnoid haemorrhage: comparison with CT. AJR Am J Roentgenol 1995;165 (5): Stone JA, Slone SW, Yu JS. Gunshot wounds of the brain: influence of ballistics and predictors of outcome by CT. Emergency Radiol 1997: Zimmerman RA, Bilaniuk LT, Gennarelli T, et al. Cranial computed tomography in the diagnosis and management of acute head trauma. Am J Roentgenol 1978;131(1): Masters SJ, McCLean PM, Arcarese JS,et al. Skull X-ray examination after head trauma. N Engl J Med 1987;316(2): Holodny AI, Visvikis GA, Schlenk RP, et al. Bilateral subdural haematomas exactly isodense to the subjacent gray matter. J Emerg Med 2001;20(4): Valadka AB, Gopinath SP, Robertson CS. Midline shift after severe head injury: pathophysiologic implications. J Trauma 2000;49(1): Diaconis JN, Rao KC. CT in head trauma: a review. J Comput Tomogr 1980;4(4): Koo AH, LaRoque RL. Evaluation of head trauma by computed tomography. Radiology 1977;123(2): Kobayashi S, Nakazawa S, Otsuka T. Clinical value of serial computed tomography with severe head injury. Surg Neurol 1983;20(1): Johnson MH, Lee SH.Computed tomography of acute cerebral trauma. Radiol Clin North Am 1992;30(2): Cooper PW, Kassel EE. CT of the cranium in head injury. J Can Assoc Radiol 1983;34(3): Zee CS, Go JL.CT of head trauma. Neuroimaging Clin N Am 1998;8(3): Iida H, Tachibana S, Kitahara T, et al. Association of head trauma with cervical spine injury, spinal cord injury, or both. J Trauma 1999;46(3): Macpherson BC, MacPherson P, Jennett B. CT evidence of intracranial contusion and haematoma in relation to the presence, site and type of skull fracture. Clin Radiol 1990;42(5):321-6.