Axonal Amyloid Precursor Protein in Traumatic Axonal Injury

APP can be demonstrated immunohistochemically in damaged axons within 35 minutes of the insult.¹³ APP is normally anterogradely transported along the axon by fast axoplasmic transport as a membrane-bound vesicular protein that accumulates rapidly proximal to the site of injury.¹⁴ Reversible APP-β immunoreactive axonal changes have been shown in some experimental animal studies, but whether this also occurs in humans is unknown.

Axonal Amyloid Precursor Protein Patterns in Traumatic Axonal Injury

Multifocal (diffuse) traumatic axonal injury is defined as axonal swellings and bulbs scattered throughout the white matter of cerebral hemispheres, brainstem, and cerebellum as individually affected axons. A spectrum of change is seen that is usually multifocal rather than truly diffuse. Vascular axonal injury (VAI) is defined as axonal swellings and bulbs that cluster around infarct or in ischemic brain in a distribution of vascular compromise associated with raised ICP. The affected axons are often arranged in clusters that have a zigzag, irregular, or geographic pattern¹⁵ (Fig. 325-2).

FIGURE 325-2 Zigzag pattern of amyloid precursor protein (APP)-immunopositive axonal injury secondary to internal herniation dueto raised intracranial pressure (APP immunostain, corpus callosum; ×100).

Amyloid precursor protein immunoreactivity is not specific for trauma and is often present around focal lesions such as infarcts and hemorrhages. Axonal damage in most fatal cases comprises a variable mixture of AI due to mechanical deformation and secondary ischemia, and APP immunopositivity per se is unable to distinguish between axonal damage secondary to mechanical trauma or ischemia.

Recent studies in humans have confirmed that APP-immunopositive axons may persist for years after the injury and that this may be associated with the formation of intra-axonal amyloid-β but without evidence of extracellular amyloid-β plaque deposition.^16,17 This is in contrast to previous studies that have shown extracellular deposition of amyloid-βcontaining plaque-like structures close to damaged axons just hours after trauma^18,19 linking TBI to the development of Alzheimer’s disease.²⁰

Pathogenesis of Traumatic Axonal Injury

Detailed experimental investigation of the pathobiology of TAI has revealed a spectrum of axonal damage.²¹ Most traumatically injured axons are not mechanically severed at the time of impact as originally believed; instead, they progress gradually to axonal disconnection over several hours.^22–24

Axonal deformation at the moment of injury results in a focal impairment of axoplasmic transport and subsequent focal swelling of the axon due to abnormal accumulation of neurofilaments and membranous organelles, and over the next 6 to 12 hours, there is disconnection of the proximal axonal segment from the distal segment. The separated distal segment undergoes wallerian degeneration over time. The transient focal disruption of the axonal membrane allows an influx of Ca²⁺, which activates multiple Ca²⁺-dependent deleterious cascades that involve calpains, caspases, and calcineurin and cause disruption of the cytoskeleton. Calcium-linked dephosphorylation of neurofilament side arms results in focal neurofilament compaction (NFC) type of axonal injury.^25,26

Diffuse Axonal Injury

Diffuse axonal injury (DAI) was first described as a clinicopathologic syndrome in patients unconscious from the time of trauma, with widespread traumatic axonal damage throughout the brain in the absence of intracranial mass lesions. Similar less severe axonal changes were also found in mild and moderate TBI resulting in the concept that these axonal changes were also the substrate for the transient disorders of consciousness associated with mild and moderate TBI.²⁷ Thus, it was conceptualized that there was a spectrum of DAI, with the severe end of the spectrum correlating with post-traumatic dementia and the mild end of the spectrum correlating with the concussive syndromes.²⁸

The application of more sensitive immunocytochemical techniques such as APP has expanded the spectrum of axonal damage demonstrable in mild, moderate, and severe TBI.²⁹

The severity of DAI has been graded on the basis of the combination of macroscopic and microscopic lesions using silver impregnation techniques to identify axonal swellings and bulbs.¹² In grade 1 DAI, widespread axonal damage is present in the corpus callosum, white matter of the cerebral hemispheres, brainstem, and cerebellum. In grade 2 DAI, there are additional focal abnormalities (usually small hemorrhages) in the corpus callosum (Fig. 325-3). In grade 3 DAI, there are, in addition to the findings of grade 2, small focal lesions in the rostral brainstem (Fig. 325-4). Focal lesions in the corpus callosum and dorsolateral rostral brainstem in grades 2 and 3 DAI may be visible on neuroimaging, but in grade 1 DAI without macroscopic focal marker lesions, conventional imaging techniques may not reveal any abnormalities. Neuroimaging of the small focal hemorrhagic lesions in the deep white matter, corpus callosum, and rostral brainstem have been used as surrogate markers of DAI.

FIGURE 325-3 Splenium showing scattered petechial hemorrhages in diffuse axonal injury type 2. Note crescent of subcortical petechial hemorrhages beneath congested cerebral cortex and hemorrhage in occipital horns.

FIGURE 325-4 Rostral pons showing dorsolateral hemorrhagic lesions in diffuse axonal injury type 3. Recent hemorrhage is seen in aqueduct.

The principal mechanical loading associated with the induction of DAI is rotational acceleration of the unrestricted head, resulting in shear, tensile, and compressive strains that produce dynamic deformation of brain tissue.³⁰ The large size of the human brain plays an important role in the generation of relatively high shear strains between different regions of tissue. The concentration of AI in midline structures may be due to dural barriers such as falx cerebri and tentorium cerebelli, which act as partial barriers to motion of the brain in a given direction.

Although DAI may occur in the absence of impact (contact) forces, most fatal human head injuries are the result of head impacts.³¹ The contact forces when the head is struck by or strikes a hard object characteristically produce focal lesions such as contusions, but they may also induce rapid acceleration-deceleration, potentially damaging axons. Nonimpact rotational acceleration of the head during car crashes may be followed by single or multiple head impacts against the interior of the motor car or road in the case of pedestrians, motor cyclists, and pedal cyclists.

Cerebral Contusions

Cerebral contusions (bruises) are focal injuries that result when mechanical forces damage the small blood vessels (capillaries, veins, or arteries) and other tissue components (nerve and glial cells and their processes) of the neural parenchyma. The bleeding from damaged blood vessels is usually the most obvious feature on macroscopic and microscopic examination, the manifestations ranging from microhemorrhages to confluent hemorrhage disrupting the tissue (Fig. 325-5). Contusions are typically surface lesions of the brain, but some also include similar hemorrhagic lesions in the deeper structures of the brain.

FIGURE 325-5 Multiple recent contusions of varying size and depth involving inferior frontal lobes. Intermediate contusions of central white matter and small gliding contusions are seen in left parasagittal subcortical white matter.

In a simple contusion, the overlying pial-glial membrane is intact. Disruption of this membrane with tearing of the underlying tissue constitutes a laceration. Contusions and lacerations form a continuum of tissue injury. Surface contusions of the brain show a wide spectrum of morphologic appearances, varying from microhemorrhages visible only under the microscope to confluent hemorrhagic necrotic lesions extending through the cortex into the subcortical white matter. Surface contusions typically affect the crests of gyri and involve the outer layers of the cortex, usually in association with subarachnoid hemorrhage.

Contusions are dynamic lesions that evolve with time. The progressive expansion or “blossoming” of contusions is demonstrated well by computed tomography (CT) and magnetic resonance (MRI) imaging.³² The damage to the blood vessels sets in train an intertwined cascade of events leading to hemorrhage, breakdown of the blood-brain barrier, and infarction secondary to compromise of the microcirculation (including compromise resulting from thrombotic occlusion of blood vessels).³³ This produces a spectrum of macroscopic changes varying from focally dilated blood vessels to burst brain lobes.

Acute surface contusions are characterized by focal vascular damage leading to punctate hemorrhages or small linear hemorrhages aligned at right angles to the cortical surface due to extension of hemorrhage along the perivascular plane. Occasionally, local subarachnoid hemorrhage adjacent to a contusion accumulates within the sulcus to form a sulcal hematoma. This may lead to an erroneous diagnosis of intracerebral hemorrhage on head CT scan. The radiating streak-like cortical hemorrhages on microscopy consist of perivascular accumulations of red cells, and serial sectioning may show evidence of focal traumatic rupture or tearing of the affected blood vessel with bleeding into the perivascular space or neural parenchyma. Damaged blood vessels may thrombose, leading to additional ischemic complications. Contusions often increase in size over hours to days owing to the evolving events related to the interplay of hemorrhage, vasogenic edema, and ischemic necrosis. In the first 24 hours after trauma, contused brain tissue biopsies show an inflammatory response, which is predominantly intravascular and consists of vascular margination of polymorphonuclear leukocytes. Extravascular polymorphonuclear leukocytes can be demonstrated in injured brain tissue only a few minutes after TBI. Three to 5 days later, the inflammation is predominantly parenchymal and consists of monocyte-macrophages, reactive microglia, polymorphonuclear cells, and CD4 and CD8 T-lymphocytes, correlating with delayed postcontusional brain swelling. Inflammatory cells produce free radicals and cytokines, such as interleukin-1β (IL-1β) and tumor necrosis factor-α, which mediate blood-brain barrier injury that leads to brain swelling and induce DNA fragmentation in neurons and oligodendrocytes.³⁴

Surgical and autopsy studies have also provided evidence that apoptosis (an active process requiring energy and protein synthesis) also occurs in human cerebral contusions in addition to necrosis.³⁵ Although necrosis and apoptosis have been considered as distinct separate mechanisms, it is possible that in TBI they represent poles in a continuum of cell injury.³⁶ TUNEL-positive neurons and oligodendrocytes have been identified in human contusions.³⁷ Increased expression of the antiapoptotic protein Bcl-2 has been observed in surviving neurons after human TBI.³⁸ Bcl-2 proteins may participate in the control of cell death and survival by regulating the release of mitochondrial cytochrome c, which is involved in the activation of caspases, especially caspase-3, which cleaves substrates associated with DNA damage and repair, including DNA-fragmentation factor (DFF45/40), poly (ADP-ribose) polymerase (PARP), and the cytoskeletal proteins actin and laminin. Caspase-3 is activated in the injured cerebral cortex of human TBI.³⁸

The next phase is that of resorption of damaged tissue and progressive reactive gliosis. Very small hemorrhages may be completely resorbed within 2 to 3 weeks, whereas larger hemorrhages may take many weeks or months to resorb. The extravasated red blood cells are sequentially broken down to various blood pigments, including hemosiderin. Necrotic brain tissue is phagocytosed by macrophages derived from monocytes at sites where there has been disruption of the blood-brain barrier and lipid macrophages appear 2 to 5 days after the insult. The end result of these processes of resorption is a shrunken brown cystic lesion involving the crests of gyri (plaque jaunt), often with fibrous scarring of adjacent meninges with the formation of a meningocerebral cicatrix (Fig. 325-6).

FIGURE 325-6 Old hemosiderin-stained areas of gliotic and meningeal fibrotic scarring representing “healed” contusions of inferior frontal and temporal lobes. Note loss of olfactory bulbs and tracts. This type of meningocerebral cicatricial scarring may be epileptogenic.

Contusions most frequently involve the inferior frontal lobes and the inferolateral temporal lobes and poles where brain tissue comes in contact with the irregular bony surfaces of the anterior and middle cranial fossae due to the relative motion of the brain and skull at these sites.³⁹ The occipital lobes and cerebellum are rarely contused in the absence of skull fractures.

Types of Contusion

Coup contusions are contusions that occur beneath the site of impact. Coup contusions are a type of contact injury and are produced by compressive forces operating beneath an area of skull in-bending or tensile forces generated by the negative pressure produced beneath an area of skull in-bending suddenly snapping back into place. Contrecoup contusions occur opposite the impact site. Intermediate coup contusions are intracerebral lesions that occur deeply within the neural parenchyma between the impact site and the opposite side of the brain. Fracture contusions occur beneath the site of a fracture. Gliding contusions occur in the parasagittal regions and are believed to be due to the rostrocaudal movement of the brain resulting from impact or impulsive loading forces. The hemorrhages involve the deeper layers of the cortex and the convolutional white matter and spare the surface of the gyrus. Gliding contusions are often associated with DAI.⁴⁰ Herniation contusions involve the medial temporal lobes and the cerebellar tonsils and are produced by movement of the brain impacting on the rigid tentorium cerebelli or the bony margins of the foramen magnum.

Contusion Patterns and Head Impacts