Plum and posner’s diagnosis of stupor and coma fourth Edition series editor sid Gilman, md, frcp
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nerve), and there is buildup of axoplasm on the retinal side of the disk. The swollen optic axons obscure the disk margins, beginning at the su- perior and inferior poles, then extending later- ally and finally medially. 8 The size of the optic disk increases, and this can be mapped as a larger ‘‘blind spot’’ in the visual field. Some pa- tients even complain of a visual scotoma in this area. If ICP is increased sufficiently, the gan- glion cells begin to fail from the periphery of the retina in toward the macula. This results in a concentric loss of vision. Because papilledema reflects the back- pressure on the optic nerves from increased ICP, it is virtually always bilateral. A rare excep- tion occurs when the optic nerve on one side is itself compressed by a mass lesion (such as an olfactory groove meningioma), thus resulting in optic atrophy in one eye and papilledema in the other eye (the Foster Kennedy syndrome). On the other hand, optic nerve injury at the level of the optic disk, either due to demyelinating disease or vascular infarct of the vasa nervorum (anterior ischemic optic neuropathy), can also block axonal transport and venous return, due to retrobulbar swelling of the optic nerve. 9 The resulting papillitis can look identical to papille- dema but is typically unilateral, or at least does not involve the optic nerves simultaneously. In addition, papillitis is usually accompanied by the relatively rapid onset of visual loss, partic- ularly focal loss called a scotoma, so the clinical distinction is usually clear. The origin of headache in patients with in- creased ICP is not understood. CSF normally leaves the subarachnoid compartment mainly by resorption at the arachnoid villi. 10 These structures are located along the surface of the superior sagittal sinus, and they consist of in- vaginations of the arachnoid membrane into the wall of the sinus. CSF is taken up from the subarachnoid space by endocytosis into vesi- cles, the vesicles are transported across the arachnoid epithelial cells, and then their con- tents are released by exocytosis into the venous sinus. Imbalance in the process of secretion and resorption of CSF occurs in cases of CSF- secreting tumors as well as in pseudotumor cerebri. In both conditions, very high levels of CSF pressure, in excess of 600 mm of water, may be achieved, but rather little in the way of brain dysfunction occurs, other than headache. Experimental infusion of artificial CSF into the subarachnoid space, to pressures as high as 800 or even 1,000 mm of water, also does not cause cerebral dysfunction and, curiously, often does not cause headache. 11,12 However,
conditions that cause diffusely increased ICP such as pseudotumor cerebri usually do cause headache, 13 suggesting that they must cause some subtle distortion of pain receptors in the cerebral blood vessels or the meninges. 14 On the other hand, when there is obstruction of the cerebral venous system, increased ICP is often associated with signs of brain dysfunction as well as severe headache. The headache is lo- calized to the venous sinus that is obstructed (superior sagittal sinus headache is typically at the vertex of the skull, whereas lateral sinus headache is usually behind the ear on the af- fected side). The headache in these conditions is thought to be due to irritation and local dis- tortion of the sinus itself. Brain dysfunction is produced by back-pressure on the draining veins that feed into the sinus, thus reducing the perfusion pressure of the adjacent areas of the brain, to the point of precipitating venous in- farction (see page 154). Small capillaries may be damaged, producing local hemorrhage and focal or generalized seizures. Superior sagittal sinus thrombosis produces parasagittal ischemia in the hemispheres, causing lower extremity pa- resis. Lateral sinus thrombosis typically causes infarction in the inferior lateral temporal lobe, which may produce little in the way of signs, other than seizures. The most important mechanism by which diffusely raised ICP can cause symptoms is by impairment of the cerebral arterial supply. The brain usually compensates for the increased ICP by regulating its blood supply as described in Chapter 2. However, as ICP reaches and ex- ceeds 600 mm of water, the back-pressure on cerebral perfusion reaches 45 to 50 mm Hg, which becomes a major hemodynamic chal- lenge. Typically, this is seen in severe acute liver failure, 15 with vasomotor paralysis follow- ing head injury, or occasionally in acute en- cephalitis. When perfusion pressure falls below the lower limit required for brain function, neu- rons fail to maintain their ionic gradients due to energy failure, resulting in additional swell- ing, which further increases ICP and results in a downward spiral of reduced perfusion and further brain infarction. 92 Plum and Posner’s Diagnosis of Stupor and Coma Decreased perfusion pressure can also occur when systemic blood pressure drops, such as when assuming a standing position. Some patients with increased ICP develop brief bi- lateral visual loss when they stand, called vi- sual obscurations, presumably due to failure to autoregulate the posterior cerebral blood flow. Failure of perfusion pressure can also occur focally (i.e., in a patient with an otherwise asymptomatic carotid occlusion who develops symptoms in the ipsilateral carotid distribu- tion on standing because of the resulting small drop in blood pressure). If the patient has bi- lateral chronic carotid occlusions, transient loss of consciousness may result. 16 Patients with elevated ICP from mass lesions often suffer sudden rises in ICP precipitated by changes in posture, coughing, sneezing, or straining, or even during tracheal suctioning (plateau waves). 17 The sudden rises in ICP can reduce cerebral perfusion and produce a variety of neurologic symptoms including confusion, stupor, and coma 18 (Table 3–2). In general, the symptoms last only a few minutes and then re- solve, leading some observers to confuse these with seizures. Finally, the loss of compliance of the intra- cranial system to further increases in volume and the rate of change in ICP plays an important role in the response of the brain to increased ICP. Compliance is the change in pressure caused by an increase in volume. In a normal brain, increases in brain volume (e.g., due to a small intracerebral hemorrhage) can be com- pensated by displacement of an equal volume of CSF from the compartment. However, when a mass has increased in size to the point where there is little remaining CSF in the compart- ment, even a small further increase in volume can produce a large increase in compartmental pressure. This loss of compliance in cases where diffuse brain edema has caused a critical in- crease in ICP can lead to the development of plateau waves. These are large, sustained in- creases in ICP, which may approach the mean arterial blood pressure, and which occur at in- tervals as often as every 15 to 30 minutes. 19,20 They are thought to be due to episodic arterial vasodilation, which is due to systemic vasomo- tor rhythms, but a sudden increase in vascular volume in a compartment with no compliance, even if very small, can dramatically increase ICP. 21
thus cause a wide range of neurologic parox- ysmal symptoms (see Table 3–2). When pres- sure in neighboring compartments is lower, this imbalance can cause herniation (see be- low). 22
Increase in Intracranial Pressure Impairment of consciousness Opisthotonus, trismus Trancelike state Rigidity and tonic extension/flexion Unreality/warmth of the arms and legs Confusion, disorientation Bilateral extensor plantar responses Restlessness, agitation Sluggish/absent deep tendon reflexes Disorganized motor activity, carphologia Generalized muscular weakness Sense of suffocation, air hunger Facial twitching Cardiovascular/respiratory disturbances Clonic movements of the arms and legs Headache
Facial/limb paresthesias Pain in the neck and shoulders Rise in temperature Nasal itch Nausea, vomiting Blurring of vision, amaurosis Facial flushing Mydriasis, pupillary areflexia Pallor, cyanosis Nystagmus Sweating Oculomotor/abducens paresis Shivering and ‘‘goose flesh’’ Conjugate deviation of the eyes Thirst External ophthalmoplegia Salivation Dysphagia, dysarthria Yawning, hiccoughing Nuchal rigidity Urinary and fecal urgency/incontinence Retroflexion of the neck Adapted from Ingvar. 18 Structural Causes of Stupor and Coma 93 Conversely, when a patient shows early signs of herniation, it is often possible to reverse the situation by restoring a small margin of com- pliance to the compartment containing the mass lesion. Hyperventilation causes a fall in arterial pCO
2 , resulting in arterial and venous vaso- constriction. The small reduction in intracranial blood volume may reverse the herniation syn- drome dramatically in just a few minutes. The Role of Vascular Factors and Cerebral Edema in Mass Lesions As indicated above, an important mechanism by which compressive lesions may cause symp- toms is by inducing local tissue ischemia. Even in the absence of a diffuse impairment of ce- rebral blood flow, local increases in pressure and tissue distortion in the vicinity of a mass le- sion may stretch small arteries and reduce their caliber to the point where they are no longer able to supply sufficient blood to their targets. Many mass lesions, including tumors, inflam- matory lesions, and the capsules of subdural hematomas, are able to induce the growth of new blood vessels (angiogenesis). 23 These blood ves- sels do not have the features that characterize normal cerebral capillaries (i.e., lack of fenes- trations and tight junctions between endothelial cells) that are the basis for the blood-brain bar- rier. Thus, the vessels leak; the leakage of Tight junction Astrocyte foot Vesicular transport across endothelial cells Capillary endothelial cells A C
Astrocyte foot Edematous astrocyte Edematous neuron
Edematous capillary endothelial cells
Opened tight junctions and escaping plasma Figure 3–1. A schematic drawing illustrating cytotoxic versus vasogenic edema. (A) Under normal circumstances, the brain is protected from the circulation by a blood-brain barrier, consisting of tight junctions between cerebral capillary endo- thelial cells that do not permit small molecules to penetrate the brain, as well as a basal lamina surrounded by astrocytic end-feet. (B) When the blood-brain barrier is breached (e.g., by neovascularization in a tumor or the membranes of sub- dural hematoma), fluid transudates from fenestrated blood vessels into the brain. This results in an increase in fluid in the extracellular compartment, vasogenic edema. Vasogenic edema can usually be reduced by corticosteroids, which decrease capillary permeability. (C) When neurons are injured, they can no longer maintain ion gradients. The increased intracel- lular sodium causes a shift of fluid from the extracellular to the intracellular compartment, resulting in cytotoxic edema. Cytotoxic edema is not affected by corticosteroids. (From Fishman, RA. Brain edema. N Engl J Med 293 (14):706–11, 1975. By permission of Massachusetts Medical Society.) 94 Plum and Posner’s Diagnosis of Stupor and Coma contrast dyes during CT or MRI scanning pro- vides the basis for contrast enhancement of a lesion that lacks a blood-brain barrier. The vas- cular leak results in the extravasation of fluid into the extracellular space and vasogenic edema
24,25 (see Figure 3–1B). This edema further displaces surrounding tissues that are pushed progressively farther from the source of their own feeding arteries. Because the large arteries are tethered to the circle of Willis and small ones are tethered to the pial vascular system, they may not be able to be displaced as freely as the brain tissue they supply. Hence, the disten- sibility of the blood supply becomes the limiting factor to tissue perfusion and, in many cases, tissue survival. Ischemia and consequent energy failure cause loss of the electrolyte gradient across the neuronal membranes. Neurons depolarize but are no longer able to repolarize and so fail. As neurons take on more sodium, they swell (cyto- toxic edema), thus further increasing the mass effect on adjacent sites (see Figure 3–1C). In- creased intracellular calcium meanwhile results in the activation of apoptotic programs for neu- ronal cell death. This vicious cycle of swelling produces ischemia of adjacent tissue, which in turn causes further tissue swelling. Cytotoxic edema may cause a patient with a chronic and slowly growing mass lesion to decompensate quite suddenly, 24,25
with rapid onset of brain failure and coma when the lesion reaches a criti- cal limit. HERNIATION SYNDROMES: INTRACRANIAL SHIFTS IN THE PATHOGENESIS OF COMA The Monro-Kellie doctrine hypothesizes that because the contents of the skull are not com- pressible and are contained within an unyield- ing case of bone, the sum of the volume of the brain, CSF, and intracranial blood is constant at all times. 26 A corollary is that these same re- strictions apply to each compartment (right vs. left supratentorial space, infratentorial space, spinal subarachnoid space). In a normal brain, increases in the size of a growing mass lesion can be compensated for by the displacement of an equal volume of CSF from the compart- ment. The displacement of CSF, and in some cases blood volume, by the mass lesion raises ICP. As the mass grows, there is less CSF to be displaced, and hence the compliance of the in- tracranial contents decreases as the size of the compressive lesion increases. When a mass has increased in size to the point where there is little remaining CSF in the compartment, even a small further increase in volume can produce a large increase in compartmental pressure. When pressure in neighboring compartments is lower, this imbalance causes herniation. Thus, intracranial shifts are of key concern in the di- agnosis of coma due to supratentorial mass le- sions (Figure 3–2). The pathogenesis of signs and symptoms of an expanding mass lesion that causes coma is rarely a function of the increase in ICP itself, but usually results from imbalances of pressure between different compartments leading to tis- sue herniation. To understand herniation syndromes, it is first necessary to review briefly the structure of the intracranial compartments between which herniations occur. Anatomy of the Intracranial Compartments The cranial sutures of babies close at about 18 months, encasing the intracranial contents in a nondistensible box of finite volume. The intra- cranial contents include the brain tissue (approx- imately 87%, of which 77% is water), CSF (ap- proximately 9%), blood vessels (approximately 4%), and the meninges (dura, arachnoid, and pia that occupy a negligible volume). The dural septa that divide the intracranial space into com- partments play a key role in the herniation syn- dromes caused by supratentorial mass lesions. The falx cerebri (Figures 3–2 and 3–3) sepa- rates the two cerebral hemispheres by a dense dural leaf that is tethered to the superior sagittal sinus along the midline of the cranial vault. The falx contains the inferior sagittal sinus along its free edge. The free edge of the falx normally rests just above the corpus callosum. One result is that severe head injury can cause a contusion of the corpus callosum by violent upward dis- placement of the brain against the free edge of the falx. 33 The pericallosal branches of the an- terior cerebral artery also run in close proximity to the free edge of the falx. Hence, displace- ment of the cingulate gyrus under the falx by a hemispheric mass may compress the pericallo- sal artery and result in ischemia or infarction of Structural Causes of Stupor and Coma 95
the cingulate gyrus (see falcine herniation, page 100).
The tentorium cerebelli (Figure 3–3) sepa- rates the cerebral hemispheres (supratentorial compartment) from the brainstem and cere- bellum (infratentorial compartment/posterior fossa). The tentorium is less flexible than the falx, because its fibrous dural lamina is stret- ched across the surface of the middle fossa and is tethered in position for about three-quarters of its extent (see Figure 3–3). It attaches an- teriorly at the petrous ridges and posterior clinoid processes and laterally to the occipital bone along the lateral sinus. Extending poster- iorly into the center of the tentorium from the posterior clinoid processes is a large semioval opening, the incisura or tentorial notch, whose diameter is usually between 25 and 40 mm me- diolaterally and 50 to 70 mm rostrocaudally. 34 The tentorium cerebelli also plays a key role in the pathophysiology of supratentorial mass le- sions, as when the tissue volume of the supra- tentorial compartments exceeds that compart- ment’s capacity, there is no alternative but for tissue to herniate through the tentorial opening (see uncal herniation, page 100). Tissue shifts in any direction can damage structures occupying the tentorial opening. The midbrain, with its exiting oculomotor nerves, traverses the opening from the posterior fossa to attach to the diencephalon. The superior por- tion of the cerebellar vermis is typically applied closely to the surface of the midbrain and occu- pies the posterior portion of the tentorial open- ing. The quadrigeminal cistern, above the tectal plate of the midbrain, and the peduncular and interpeduncular cisterns along the base of the midbrain provide flexibility; there may be con- siderable tissue shift before symptoms are pro- duced if a mass lesion expands slowly (Figure 3–2). The basilar artery lies along the ventral sur- face of the midbrain. As it nears the tentorial opening, it gives off superior cerebellar arteries bilaterally, then branches into the posterior ce- rebral arteries (Figure 3–4). The posterior cere- bral arteries give off a range of thalamoperforat- ing branches that supply the posterior thalamus and pretectal area, followed by the posterior communicating arteries. 35 Each posterior cere- bral artery then wraps around the lateral surface of the upper midbrain and reaches the ventral surface of the hippocampal gyrus, where it gives off a posterior choroidal artery. 36 The posterior choroidal artery anastomoses with the anterior choroidal artery, a branch of the internal carotid artery that runs between the dentate gyrus and the free lateral edge of the tentorium. The A B
Herniation Central
Herniation Falcine
Herniation Midline
Shift Figure 3–2. A schematic drawing to illustrate the different herniation syndromes seen with intracranial mass effect. When the increased mass is symmetric in the two hemispheres (A), there may be central herniation, as well as herniation of either or both medial temporal lobes, through the tentorial opening. Asymmetric compression (B), from a unilateral mass lesion, may cause herniation of the ipsilateral cingulate gyrus under the falx (falcine herniation). This type of compression may cause distortion of the diencephalon by either downward herniation or midline shift. The depression of consciousness is more closely related to the degree and rate of shift, rather than the direction. Finally, the medial temporal lobe (uncus) may herniate early in the clinical course. 96 Plum and Posner’s Diagnosis of Stupor and Coma posterior cerebral artery then runs caudally along the medial surface of the occipital lobe to supply the visual cortex. Either one or both posterior cerebral arteries are vulnerable to compression when tissue herniates through the tentorium. Unilateral compression causes a homonymous hemianopsia; bilateral compres- sion causes cortical blindness (see Patient 3–1). The oculomotor nerves leave the ventral sur- face of the midbrain between the superior cerebellar arteries and the diverging posterior cerebral arteries (Figure 3–3). The oculomotor Yüklə 9,02 Mb. Dostları ilə paylaş:
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