There are seven major patterns of brain shift:
falcine herniation, lateral displacement of the diencephalon, uncal herniation, central transtentorial herniation, rostrocaudal brainstem deterioration, tonsillar herniation, and upward brainstem herniation. The first five patterns are caused by supratentorial mass lesions, whereas tonsillar herniation and upward brainstem herniation usually result from infratentorial mass lesions, as described below.
Falcine herniation occurs when an expanding lesion presses the cerebral hemisphere medially against the falx (Figure 3–2A). The cingulate gyrus and the pericallosal and callosomarginal arteries are compressed against the falx and may be displaced under it. The compression of the pericallosal and callosomarginal arteries causes ischemia in the medialwall ofthe cerebralhemispherethatswellsandfurtherincreas esthecompression. Eventually, the ischemia may advance to frank infarction, which increases the cerebral mass effect further.44
Lateral displacement of the diencephalon occurs when an expanding mass lesion, such as a basal ganglionic hemorrhage, pushes the diencephalon laterally (Figure 3–2B). This process may be monitored by displacement of the calcified pineal gland, whose position with respect to the midline is easily seen on plain CT scanning.45 This lateral displacement is roughly correlated with the degree of impairment of consciousness: 0 to 3 mm is associated with alertness, 3 to 5 mm with drowsiness, 6 to 8mm with stupor, and 9 to 13 mm with coma.1
Structural Causes of Stupor and Coma 115 Uncal herniation occurs when an expanding
mass lesion usually located laterally in one cerebral hemisphere forces the medial edge of the temporal lobe to herniate medially and downward over the free tentorial edge into the tentorial notch (Figure 3–2). In contrast to central herniation, in which the first signs are mainly those of diencephalic dysfunction, in uncal herniation the most prominent signs are due to pressure of the herniating temporal lobe on the structures that occupy the tentorial evidence of some impairment of ocular motility by this stage, but it may be less apparent to the examiner as the patient may not be sufficiently awake either to complain about it or to follow commands on examination (i.e., to look to the side or up or down), and some degree of exophoria is present in most people when they are not completelyawake. However,examining oculocephalic responses by rotating the head usually will disclose eye movement problems associated with third nerve compression.
A second key feature of uncal herniation that is sufficient to cause pupillary dilation is impaired level of consciousness. This may be due to the distortion of the ascending arousal systems as they pass through the midbrain, distortion of the adjacent diencephalon, or perhaps stretching of blood vessels perfusing the midbrain, thus causing parenchymal ischemia. Nevertheless, the impairment of arousal is so prominent a sign that in a patient with a unilateral fixed and dilated pupil and normal level of consciousness, the examiner must look for another cause of pupillodilation.
Pupillary dilation from uncal herniation with a preserved level of consciousness is rare enough to be the subject of case reports.46
Hemiparesis may also occur due to compression of the cerebral peduncle by the uncus. The paresis may be contralateral to the herniation (if the advancing uncus impinges upon the adjacent cerebral peduncle) or ipsilateral (if the uncus pushes the midbrain so that the opposite cerebral peduncle is compressed against the incisural edge of Kernohan’s notch,47 but see 48 ). Hence, the side of paresis is not helpful in localizing the lesion,
but the side of the enlarged pupil accurately identifies the side of the herniation over 90% of the time.49
An additional problem in many patients with uncal herniation is compression of the posterior cerebral artery in the tentorial notch, which may give rise to infarction in the territory of its distribution.50 Often this is overlooked at the time of the herniation, when the impairment of consciousness may make it impossible to test visual fields, but emerges as a concern after the crisis is past when the patient is unable to see on the side of space opposite the herniation.
Bilateral compression of the posterior cerebral arteries results in bilateral visual field infarction and cortical blindness (see Patient 3–
1, Figure 3–6).51 physicians planned to admit her to hospital, perform an elective cesarean section, and then operate on the tumor. She was admitted to the hospital the day before the surgery. During the night she complained of a more severe headache and rapidly became lethargic and then stuporous. An emergency CT scan disclosed hemorrhage into the tumor and transtentorial herniation, and at craniotomy a right frontal hemorrhagic oligodendroglioma was removed, and she rapidly recovered consciousness.
Upon awakening she complained that she was unable to see. Examination revealed complete loss of vision including ability to appreciate light but with retained pupillary light reflexes. Repeat MRI scan showed an evolving infarct involving the occipital lobes bilaterally (see Figure 3–6). Over the following week she gradually regained some central vision, after which it became clear that she had severe prosopagnosia (difficulty recognizing faces).52 Many months after recovery of vision she was able to get around and read, but she was unable to recognize her own face in the mirror and could only distinguish between her husband and her brother by the fact that her brother was taller.
Central transtentorial herniation is due to pressure from an expanding mass lesion on the diencephalon. If the mass effect is medially located, the displacement may be primarily downward, in turn pressing downward on the midbrain, although the mass may also have a substantial lateral component shifting the diencephalon in the lateral direction.31 The diencephalon is mainly supplied by small penetrating endarteries that arise directly from the
Figure 3–6. Bilateral occipital infarction in Patient 3–1.
Hemorrhage into a large frontal lobe tumor caused transtentorial herniation, compressing both posterior cerebral arteries. The patient underwent emergency craniotomy to remove the tumor, but when she recovered from surgery she was cortically blind.
vessels of the circle of Willis. Hence, even small degrees of displacement may stretch and compress important feeding vessels and reduce blood flow. In addition to accounting for the pathogenesis of coma (due to impairment of the ascending arousal system at the diencephalic level), the ischemia causes local swelling and eventually infarction, which causes further edema, thus contributing to gradually progressive displacement of the diencephalon. In severe cases, the pituitary stalk may even become partially avulsed, causing diabetes insipidus, and the
diencephalon may buckle against the midbrain.
The earliest and most subtle signs of impending central herniation tend to begin with compression of the diencephalon.
Less commonly, the midbrain may be forced downward through the tentorial opening by a mass lesion impinging upon it from the dorsal surface. Pressure from this direction produces the characteristic dorsal midbrain or Parinaud’s syndrome (loss of upgaze and convergence, retractory nystagmus; see below).
Rostrocaudal deterioration of the brainstem may occur when the distortion of the brainstem compromises its vascular supply. Downward displacement of the midbrain or pons stretches the medial perforating branches of the basilar artery, which itself is tethered to the circle of Willis and cannot shift downward (Figure 3–4).
Paramedian ischemia may contribute to loss of consciousness, and postmortem injection of the basilar artery demonstrates that the paramedian arteries are at risk of necrosis and extravasation. The characteristic slit-like hemorrhages seen in the area of brainstem displacement postmortem are called Duret hemorrhages53 (Figure 3–7). Such hemorrhages can be replicated experimentally in animals.54 It is also possible for the venous drainage of the brainstem to be compromised by compression of the great vein of Galen, which runs along the midline on the dorsal surface of the midbrain.
However, in postmortem series, venous infarction is a rare contributor to brainstem injury.55
Tonsillar herniation occurs in cases in which the pressure gradient across the foramen magnum impacts the cerebellar tonsils against the foramen magnum, closing off the fourth ventricular outflow and compressing the medulla (Figures 3–7 and 3–8). This may occur quite suddenly, as in cases of subarachnoid hemorrhage, when a large pressure wave drives the cerebellar tonsils against the foramen magnum, compressing the caudal medulla. The patient suddenly stops breathing, and blood pressure rapidly increases as the vascular reflex pathways in the lower brainstem attempt to perfuse the lower medulla against the intense local pressure. A similar syndrome is sometimes seen when lumbar puncture is performed on a patient whose intracranial mass lesion has exhausted the intracranial compliance.56 In patients with sustained
Structural Causes of Stupor and Coma 117
tonsillar herniation, the cerebellar tonsils are typically found to be necrotic due to their impaction against the unyielding edge of the foramen magnum. This problem is discussed further below.
Upward brainstem herniation may also occur through the tentorial notch in the presence of a rapidly expanding posterior fossa lesion.3 The superior surface of the cerebellar vermis and the midbrain are pushed upward, compressing the dorsal mesencephalon as well as the adjacent blood vessels and the cerebral aqueduct (Figure 3–8).
The dorsal midbrain compression results in impairment of vertical eye movements as well as consciousness. The pineal gland is typically displaced upward on CT scan.57 The compression of the cerebral aqueduct can cause acute hydrocephalus, and the superior cerebellar artery may be trapped against the tentorial edge, resulting in infarction and edema of the superior cerebellum and increasing the upward pressure.