Scalp Injury

The scalp consists of five layers. From superficial to deep these are the skin, subcutaneous fibrofatty tissue, galea aponeurotica, loose areolar connective tissue, and periosteum.²² Following head trauma, hemorrhage may accumulate in the subcutaneous, subgaleal, and subperiosteal compartments. Subcutaneous and subperiosteal hematomas are much more common in infants and may result from birth trauma. Subgaleal hematomas are the type most frequently seen in adults. Because the subgaleal space lies deep to the frontalis and occipitalis muscles but superficial to the temporalis muscles, subgaleal hemorrhages are often identified on noncontrast CT as crescentic or elliptical areas of high density superficial to the temporalis muscle (Fig. 25-1).

FIGURE 25-1 Scalp hematoma in a 23-year-old man. CT shows large high-density mass in scalp on right. Arrow indicates air in the wound.

Lacerations of the scalp may accompany both penetrating and closed-head trauma. Any lacerations identified should be scrutinized for foreign material, which may be a source of infection (Fig. 25-2). Extensive scalp avulsion, or “degloving,” is sometimes seen in unrestrained passengers ejected from moving vehicles (Fig. 25-3).

FIGURE 25-2 Scalp and skull laceration in a 30-year-old man. CT demonstrates soft tissue and bone laceration, as well as foreign material (arrow) in the wound.

FIGURE 25-3 Scalp avulsion in a 52-year-old woman. Bone window CT shows exposed calvaria. Arrows indicate scalp edges.

Skull Fractures

Skull fractures are present in only 25% of fatal head injuries at autopsy.²³ Patients with a skull fracture have a higher incidence of intracranial hematoma than those without fracture.¹⁴ When a blunt object strikes the head, the local inbending causes a compressive strain on the outer table and a tensile strain on the inner table. Because bone is more susceptible to tensile strain, the fracture begins in the inner table.¹⁶ It then propagates along the lines of least resistance. When the skull strikes a broad or flat object, as in a fall, a linear fracture commonly results (Fig. 25-4). Depressed fractures result from more intense forces delivered over a smaller area of impact. Diastatic fractures are linear fractures that intersect or occur along a suture line, causing diastasis of the suture. Reopening of a closed suture is a special form of fracture that may be called sutural diastasis. Basal skull fractures result from blows directly to the occiput or mastoid or from forces transmitted from blows to the face. They also frequently accompany gunshot injury.

FIGURE 25-4 Linear skull fractures. Postmortem photographs. A, A large temporalis muscle hematoma (H) has been reflected away from the linear fracture line (arrows) that extends across the temporal squama. (See Fig. 25-17 for the internal view of this fracture line as it crosses the grooves for the ascending branches of the middle meningeal artery.) B, Left occipital fracture (arrows) and contrecoup fractures of both orbital roofs secondary to the occipital impact.

(Courtesy of John Deck, MD.)

Noncontrast CT with bone algorithm is the study of choice for imaging all skull fractures (Figs. 25-5 and 25-6). It is important to study the scout image of the CT scan in addition to the individual slices, because linear skull fractures oriented precisely in the plane of CT section and concurrent trauma to the upper cervical spine may occasionally be imaged only on the scout view (Figs. 25-7 and 25-8). Linear fractures do not require specific treatment. However, they do indicate that substantial force was delivered to the skull, so detection of such fractures mandates careful search for accompanying intracranial contusion and hemorrhage. Linear fractures that cross the vascular grooves for the middle meningeal artery or dural venous sinuses are especially likely to cause epidural hematomas.

FIGURE 25-5 Linear skull fracture. A radiolucent line (arrow) is seen in the squamous temporal bone on bone window CT.

FIGURE 25-6 Diastatic lambdoid fracture in a 1-year-old boy. A, On bone window axial CT, the lambdoid suture is widened (arrow). B, 3D reformatted CT shows well-demonstrated fracture.

FIGURE 25-7 Linear skull fracture seen on CT scout image only (arrows). Fracture was in the CT scan plane.

FIGURE 25-8 C2 fracture that was missed on CT head scan. A, Scout radiograph shows fracture line (arrow). B, Follow-up CT of cervical spine demonstrates fracture (arrow).

Depressed fractures are readily seen on CT and are likely to be associated with underlying brain injury. Depressed fractures assume two forms: (1) the adjacent fragments may be angled inward at the fracture line without losing their relative position (Fig. 25-9) or (2) a section of bone may be displaced deep to the neighboring bone (subducted) leaving an open donor site and a double-thickness subduction zone (Fig. 25-10). Fractures depressed more than one full thickness of the skull need to be elevated,²⁴ so the degree of depression should be indicated in the report. Depressed fractures are most common in the frontal and parietal regions. Depressed fractures at the midline, crossing the superior sagittal sinus, carry risk for sinus thrombosis and subsequent venous infarction or hemorrhage. Infection, cerebrospinal fluid (CSF) leak, and post-traumatic epilepsy are other complications of depressed fractures.²⁵

FIGURE 25-9 Depressed right squamous temporal bone fracture. Fragments are bent inward but retain their relative position to the skull.

FIGURE 25-10 A 13-year-old male following an ATV accident. A, Note subducted bone fragment (arrow) embedded deeply in frontal lobe. B, Soft tissue windows show brain tissue (arrow) herniating out of the bony defect. Pneumocephalus is present. ATV, All terrain vehicle.

Basal skull fractures often require multiplanar thin-section CT for detection and display of their full extent. Axial slice thicknesses of 1 to 2 mm and coronal reformatted images are recommended. Cranial nerve injury and CSF rhinorrhea/otorrhea are common complications (Fig. 25-11). Pneumocephalus may follow fractures involving the frontal and sphenoidal sinuses (Fig. 25-12) or the mastoid portions of the temporal bones. Air-fluid levels in the paranasal sinuses suggest possible skull base fracture. Longitudinal fractures along the temporal bone may cause ossicular chain disruption (Fig. 25-13). Transverse fractures across the temporal bone may damage the seventh or eighth cranial nerves (Fig. 25-14).²⁶ Fractures of the cribriform plate may cause olfactory nerve injury and anosmia. The optic nerves, cavernous sinuses, and carotid arteries are susceptible to injury from sphenoid fracture. In cases of fracture through the carotid canal, CT angiography or MR angiography should be considered to rule out carotid dissection. Epidural abscess is a potential complication of fracture of the posterior wall of the frontal sinus or mastoid air cells. Meningitis, subdural empyema, and intracranial hypotension can result from meningeal tears.

FIGURE 25-11 Posterior skull base fracture in a 21-year-old man. A, Fracture line extends through occipital bone on right (white arrows) and anteriorly into right sphenoidal sinus (black arrow), which has an air-fluid level. B, Fracture involves the right hypoglossal canal (arrowhead).

FIGURE 25-12 Anterior skull base fracture. White arrows indicate fracture through the sella turcica and planum sphenoidale. A fracture through the frontal bone is also seen (arrowhead). There is a small amount of pneumocephalus (black arrow).

FIGURE 25-13 Longitudinal left temporal bone fracture in a 56-year-old man. A, Axial CT shows fracture line (arrows) extending from mastoid into middle ear cavity. Incus (I) is dislocated from malleus (M). B, Coronal reformatted CT shows dislocated incus (arrow) in mesotympanum.

FIGURE 25-14 Transverse left temporal bone fracture in a 15-year-old boy. Axial CT demonstrates fracture through basal turn of the cochlea (arrow), resulting in sensorineural hearing loss.

Growing Skull Fracture

Also known as a leptomeningeal cyst, this lesion occurs exclusively in childhood; 90% are in children younger than age 3 years.^27–29 Growing fractures result when the dura deep to the fracture is torn and retracts, exposing the bony margins of the fracture to the pulsations of the arachnoid and subarachnoid CSF. Continued pulsations then erode the bone margins, enlarge the fracture, and permit the arachnoid and subarachnoid CSF to protrude into and through the enlarging fracture line. The presence of meninges in the fracture line interferes with osteoblastic activity and prevents fracture healing.³⁰ Growing skull fractures often originate from a diastatic parietal fracture but may develop at any site with fracture and torn dura. On plain radiographs, leptomeningeal cysts appear as elongate to ovoid radiolucent areas with smooth margins (Fig. 25-15). CT confirms the presence of a CSF-density cystic area at the fracture site (Fig. 25-16). Underlying brain injury is common, as are neurologic deficits, such as a contralateral hemiparesis.

FIGURE 25-15 Growing skull fracture. A, Diastatic fracture of left parietal bone. B, Skull radiograph 3¹/₂ months later shows widening of the fracture line.

(Courtesy of Sandra Fernbach, MD.)

FIGURE 25-16 Intradiploic leptomeningeal cyst in a 73-year-old woman. A, Axial bone window CT shows radiolucent defect in inner table of right parietal bone, with cavity indicated as CSF density in diploic space (arrow). B, Axial T2W MR image shows temporal lobe cortex (arrow) herniating into diploic CSF-filled space.

Leptomeningeal cyst may be a sequela of a form of pediatric closed-head trauma known as the cranial burst fracture. This severe head injury consists of a widely diastatic fracture associated with dural laceration and extrusion of brain tissue outside the calvaria beneath an unbroken scalp.³¹ The herniated cerebral tissue and dural tear may not be apparent on CT and require MRI for diagnosis.²⁸ Surgical repair of the defect is recommended to prevent further neurologic deterioration.²⁹ Because of its therapeutic implications, some investigators recommend MRI for all skull fractures with more than 4 mm of diastasis to exclude brain hernia. If the dura is intact, the fracture will heal and a leptomeningeal cyst will not develop.²⁷

CT

On noncontrast CT scans, acute EDHs typically appear as biconvex, hyperdense masses adjacent to the inner table of the skull (Figs. 25-19 and 25-20). The CT attenuation of blood is related to the high electron density of the hemoglobin molecule and not the iron content. Clotted blood has a hematocrit of about 90%, whereas unclotted blood has a hematocrit of about 50%. On CT, areas of decreased attenuation within an EDH are thought to represent unclotted blood and indicate active bleeding (the swirl sign) (Fig. 25-21).³⁴ EDHs that result from a torn venous sinus may cross the falx or tentorium (Fig. 25-22) and thus be present in both the infratentorial and supratentorial compartments.³⁵

FIGURE 25-19 Epidural hematoma in an 18-year-old boy. A, Axial CT shows high-density biconvex collection in left frontal region. B, Bone window CT shows subtle linear fracture (arrow).

FIGURE 25-20 Epidural hematoma in a 16-year-old girl. Axial CT demonstrates high-density collection in the right middle cranial fossa (arrows).

FIGURE 25-21 Epidural hematoma in a 5-year-old boy. Unenhanced CT. Arrow indicates interface between brain and hematoma. Lower-density areas within the hematoma are the result of active bleeding (the swirl sign).

FIGURE 25-22 Venous epidural hematoma from transverse sinus laceration. A, Bone window CT shows linear fracture of occipital bone (arrow). B, CT through posterior fossa demonstrates large infratentorial hematoma on right. Fourth ventricle (arrow) is compressed and displaced. C, Higher slice shows supratentorial component of hematoma. Upward herniation of cerebellar vermis (arrows) through the tentorial incisura is seen.

MRI

On MRI, acute EDHs are usually isointense to brain on T1-weighted (T1W) images and isointense to hypointense on T2-weighted (T2W) images. Early subacute EDHs are hyperintense on T1W images and hypointense on T2W images (Fig. 25-23). Late subacute EDHs are typically hyperintense on both T1W and T2W imaging. The medially displaced dura is seen as a black line between the hematoma and compressed brain. MRI is rarely necessary to make the diagnosis of EDH, except occasionally for those that are located at the midline vertex, which are difficult to appreciate on CT.

FIGURE 25-23 Venous epidural hematoma, acute to early subacute phase. A, Unenhanced CT, day 2 post injury. High-density collection is seen in right occipital region (arrows). B, T1W MRI, day 4. Note bright signal in hematoma. C, T2W image shows low intensity in hematoma (arrows), consistent with intracellular methemoglobin.