Chapter 7: The Cerebral Hemispheres and Vascular Syndromes

Chapters 4 and 6 mapped the primary motor cortex (precentral gyrus of the frontal lobe), primary somatosensory cortex (postcentral gyrus of the parietal lobe), and the primary visual cortex (calcarine cortex of the posterior occipital lobe) onto the cerebral hemispheres. The primary auditory cortex is housed in the superior temporal gyrus of the temporal lobe. Knowing the locations of the motor cortex and these three primary sensory cortices allows for a logical deduction of the functions of the rest of the cortical surface as is discussed below.

The hemisphere contralateral to the side of handedness is considered the dominant hemisphere (e.g., the left hemisphere in a right-handed patient), and the hemisphere ipsilateral to the side of handedness is considered the nondominant hemisphere (e.g., the right hemisphere in a right-handed patient). Most patients are right-handed, so their left hemisphere is the dominant hemisphere. Language dysfunction is most commonly due to lesions in the dominant (usually left) hemisphere, whereas neglect (see “Attention” below) is most commonly due to lesions in the nondominant (usually right) hemisphere (causing left-sided neglect).

Parietal Lobes: Spatial Attention and Praxis

Attention

The parietal lobe regions bounded by the somatosensory cortex anteriorly and the visual cortex posteriorly are ideally situated to combine visual and spatial information, playing roles in awareness of the body in space, spatial reasoning, and mathematical processing (Fig. 7–1). The projection from the occipital lobe superiorly to the parietal lobe (the dorsal stream) is referred to as the “where” pathway: Visual information is processed here to determine where things are in space with respect to the body. Lesions here can cause neglect: The patient is unaware of one half of the world. Neglect is more common with lesions in the nondominant parietal lobe, which is most commonly the right parietal lobe causing left-sided neglect. Examination findings in patients with neglect may include extinction to double simultaneous stimulation (see Ch. 4), lack of awareness of deficits (anosognosia; e.g., not acknowledging that a paretic limb is weak despite inability to move it), and in severe cases, inability to recognize the neglected body parts as one’s own.

Lesions in the angular gyrus of the dominant (usually left) parietal lobe can cause Gerstmann’s syndrome: left-right confusion, inability to count (acalculia), inability to name the fingers (finger agnosia), and inability to write (agraphia).

Praxis

Parietal lesions can also cause difficulty performing a complex learned action (apraxia). This can be demonstrated by asking a patient to mime an action (e.g., “pretend you are taking out a pack of matches and lighting one,” or “pretend you are brushing your teeth”). There are several types of apraxia:

• Limb-kinetic apraxia refers to a loss of dexterity in performing actions.

• Ideational apraxia refers to inability to conceive of the idea of how to accurately perform an action, leading to errors in how the action is performed.

• Ideomotor apraxia refers to inability to convert an idea about how to do something into a motor plan. Affected patients may be able to explain the intended action but are unable to perform it normally, making errors in sequencing and/or timing of the component movements of complex learned actions.

Although these terms are commonly confused, their names provide clues to what they signify: kinetic is difficulty with movements themselves, ideational is loss of the idea of how to perform an action, and ideomotor is difficulty translating an idea into a motor plan. The latter two types of apraxia are generally caused by lesions of the parietal lobe in the dominant (usually left) hemisphere.

Temporal Lobes: Recognition Memory

The temporal lobes are ideally located to combine sensory input from olfactory, auditory, visual, and somatosensory cortices. The temporal lobes are thus ideally suited to play a role in recognition memory, since memories are internal representations of sensory experiences. Lesions of the medial temporal lobes (including the hippocampus) can cause amnesia. The flow of visual information inferiorly to the temporal lobe (the ventral stream) is referred to as the “what pathway:” visual information is processed here to determine what things are (recognition memory). The dominant (usually left) inferior temporal lobe houses the visual word form area necessary for reading, and the nondominant (usually right) inferior temporal lobe houses the face recognition area. Inability to read is called alexia and inability to recognize faces is referred to as prosopagnosia.

Frontal and Temporal Lobes: Language

The inferior frontal gyrus lies in proximity to the auditory and motor cortices and is adjacent to the premotor cortex for the face. It is therefore ideally positioned to combine auditory and motor functions for speech production. The inferior frontal gyrus houses Broca’s area for speech production. Wernicke’s area for speech recognition lies at the junction of the auditory cortex (superior temporal gyrus) and the parietal cortex, where auditory regions are adjacent to parietal regions involved in awareness of one’s surroundings. In most right-handed patients, language is localized to the left hemisphere, and this is true in many left-handed patients as well. However, in some patients (more commonly left-handed patients), language may localize to the right hemisphere. Lesions in and around Broca’s and Wernicke’s areas lead to speech disturbances (aphasia).

The aphasias can be categorized based on the patient’s ability to produce speech, comprehend speech, and repeat words and phrases (Table 7–1). In a pure Broca’s aphasia, the primary deficit is in production of speech (nonfluent or expressive aphasia), but the patient can generally still comprehend. However, patients with Broca’s aphasia may have difficulty with comprehension of grammatically complex phrases (e.g., “The tiger was eaten by the lion. Who survived?”). In the most severe Broca’s aphasias, the patient is mute. When less severe, the patient may have effortful speech with frequent errors. Since comprehension is generally largely preserved in Broca’s aphasia, the patient is aware of and frustrated by the inability to speak. In a pure Broca’s aphasia, the patient cannot repeat phrases stated by the examiner but can comprehend (i.e., can follow commands). If a patient has an expressive aphasia with preserved repetition, this is called a transcortical motor aphasia.

In pure Wernicke’s aphasia, comprehension is impaired (receptive aphasia), and although the prosody (melody and rhythm) of speech is preserved (fluent aphasia), the content is nonsensical. The patient cannot understand his or her own nonsensical speech, and so may not appear concerned by the deficit. In pure Wernicke’s aphasia, a patient cannot repeat phrases. If repetition is preserved in a receptive aphasia, this is called a transcortical sensory aphasia.

If both production and comprehension are impaired, this is called a global aphasia. Rarely, patients with both productive and receptive aphasia are still able to repeat what they hear, a scenario called mixed transcortical aphasia.

Note that all of the transcortical aphasias are characterized by preserved repetition, and named for the primary language deficit: transcortical motor aphasia is characterized by a deficit in speech production (motor output), transcortical sensory aphasia is characterized by a deficit in speech comprehension (“sensation” of speech), and mixed transcortical aphasia is characterized by a mix of both expressive and receptive aphasia.

If a patient’s only language deficit is repetition with preserved comprehension and production, this is called a conduction aphasia, since conduction between Wernicke’s area and Broca’s area (via the arcuate fasciculus) is disrupted.

Sudden-onset aphasia is most commonly due to stroke in the left middle cerebral artery territory, but can also be due to a seizure or postictal state if the seizure activity originates in or spreads to language regions. More subacute development of aphasia can be seen with a left-sided chronic subdural hematoma or a tumor affecting language regions. Aphasia can also develop even more insidiously due to neurodegenerative diseases such as primary progressive aphasia (see Ch. 22).

In addition to regions involved in language and motor control, the frontal lobes support executive functions including working memory, decision making, abstract reasoning, and emotional processing. Frontal lobe lesions can cause abulia (decreased initiative, motivation, speech, and emotional response), behavioral disinhibition, and/or impairments in any of the above executive functions.

The higher order functions of the occipital lobes are discussed in Chapter 6.

The thalamus and basal ganglia are “islands” of gray matter in the subcortical white matter (Fig. 7–2). Both are nodes in a variety of circuits that begin and/or end in the cortex, brainstem, and/or cerebellum.

The Thalamus

The left and right thalamus are positioned on either side of the third ventricle, just superior to the midbrain. The thalamus is a collection of nuclei, most of which project to one or more cortical regions (only the reticular nuclei do not project to the cortex, but rather to other thalamic nuclei) (Table 7–2). Four basic types of circuitry pass through thalamic nuclei en route to the cortex:

1. Sensory pathways. All sensory pathways synapse in the thalamus, which transmits sensory information to the respective sensory cortices. Smell is the only sensory modality that reaches the cortex before the thalamus (transmitted directly to the olfactory cortex, which then transmits smell information to the thalamus (dorsomedial nucleus)).

2. Motor control pathways. The ventral anterior (VA) and ventral lateral (VL) nuclei of the thalamus participate in cortical–basal ganglia–cortical loops and cerebellar–cortical pathways.

3. Consciousness/arousal pathways. These pathways begin in the brainstem reticular activating system and project to both thalami, which in turn project diffusely throughout the cortex.

4. Cognition/emotion pathways. Corticocortical loops pass through the thalamus, playing roles in diverse cognitive functions. One such loop is the circuit of Papez which participates in memory and emotion: hippocampus→fornix→mamillary bodies→anterior nucleus of the thalamus→anterior cingulate→entorhinal cortex→hippocampus.

Individual thalamic nuclei can be affected by small strokes (e.g., lacunar stroke in ventral posterior medial/ventral posterior lateral [VPM/VPL] thalamic nuclei causing contralateral sensory loss). Larger lesions of the thalamus can cause decreased level of consciousness. The thalamus is commonly affected by hypertensive hemorrhage, which also often affects the adjacent posterior limb of the internal capsule, leading to contralateral hemiparesis/hemiplegia (in addition to contralateral hemisensory loss and depressed level of consciousness due to thalamic involvement). Given its diffuse connections with diverse cortical regions, lesions of the thalamus are said to be able to “do anything” (i.e., cause any type of deficit), including causing “cortical” signs (e.g., aphasia, neglect, cognitive deficits) and eye movement abnormalities (in part due to effects on nearby midbrain pathways for eye movements).

The Basal Ganglia

The basal ganglia include the caudate, putamen, globus pallidus, and subthalamic nucleus (see Fig. 7–2). The caudate and putamen together are referred to as the striatum, and the putamen and globus pallidus together are referred to as the lenticular nuclei. The basal ganglia are part of circuits that initiate and coordinate movements, and dysfunction in the basal ganglia leads to movement disorders (e.g., Parkinson’s disease; see Ch. 23). When the basal ganglia are affected by cerebrovascular disease, the surrounding internal capsule is also often affected, causing the predominant manifestation to be contralateral weakness, with movement disorders being relatively uncommon in this scenario. One exception is stroke of the subthalamic nucleus, which can produce contralateral hemiballismus (unilateral ballistic movements). Slower growing lesions involving the basal ganglia (e.g., tumors, toxoplasmosis) can cause contralateral movement disorders.

The brain, brainstem, and cerebellum are supplied by arteries arising from the paired internal carotid arteries (the anterior circulation) and the paired vertebral arteries (the posterior circulation) (Figs. 7–3 and 7–4). The internal carotid arteries arise from the common carotid arteries, which themselves arise from the aortic arch (from the brachiocephalic trunk on the right and directly from the aortic arch on the left). Each carotid artery ultimately gives rise to a middle cerebral artery (MCA) and an anterior cerebral artery (ACA), and these arteries together supply the majority of the cerebral hemispheres including the frontal lobes, parietal lobes, and superior and lateral temporal lobes.

Each internal carotid artery also gives rise to an ophthalmic artery (which supplies the retina) and an anterior choroidal artery (which supplies the posterior thalamus and internal capsule).

The vertebral arteries arise from the subclavian arteries, join to form the basilar artery at around the level of the pontomedullary junction, and end by giving off the posterior cerebral arteries (PCAs) at the level of the upper midbrain. The PCAs supply the regions of the cerebral hemispheres not supplied by the MCAs and ACAs: the occipital lobes and inferior and medial temporal lobes.

Before giving rise to the PCAs, the vertebrobasilar system gives off three paired circumferential arteries that supply the lateral brainstem and cerebellum (superior cerebellar arteries [SCAs], anterior inferior cerebellar arteries [AICAs], and posterior inferior cerebellar arteries [PICAs]), which are discussed in relation to the brainstem in Chapter 9.

The anterior circulation and posterior circulation are linked by the posterior communicating arteries, and the ACAs are linked by the anterior communicating artery. These connections form the circle of Willis on the inferior surface of the brain, which provides routes for collateral flow. Not all patients have a complete circle of Willis, and some patients have anatomic variants, the most common of which are introduced here. The clinical implications of some of these variants are discussed later in this chapter.

• Hypoplastic vertebral artery. Many patients have one dominant vertebral artery and a smaller hypoplastic nondominant vertebral artery. When this occurs, the basilar artery appears to swing to the side of the nondominant vertebral artery, and the vertebral canal is smaller on the side of the congenitally smaller vertebral artery. These features help to distinguish a congenitally smaller vertebral artery from a pathologically smaller one (e.g., due to dissection or atherosclerosis). In Figure 7–4A, the left vertebral artery (on the right of the image) is dominant, the right vertebral artery (on the left of the image) is nondominant, and the basilar artery swings toward the nondominant right vertebral artery (toward the left of the image). Note that although it is common for the vertebral arteries to be different in caliber, the carotid arteries should always be symmetric. Any asymmetry of the internal carotid arteries is therefore abnormal.

• Azygous ACA. In this variant, both ACAs emerge from a common trunk.

• Fetal PCA. In this variant, the PCA arises from the internal carotid artery rather than the top of the basilar. This variant may occur unilaterally or bilaterally (Fig. 7–5).

• Artery of Percheron. In this variant, a single artery from one of the PCAs supplies both thalami (rather than an individual supply on each side).

The Vascular Territories of the ACA, MCA, and PCA

Most generally, the ACAs and MCAs supply the anterior, medial, and lateral aspects of the hemispheres, and the PCAs supply the posterior and inferior aspects (Fig. 7–6). On the cortical surface, the MCAs supply the lateral surface of the frontal, temporal, and parietal lobes, the ACAs supply the medial surface of the frontal and parietal lobes, and the PCAs supply the occipital lobes and the inferior temporal lobes. Subcortically, the MCAs supply the majority of the hemispheres, creating a trapezoidal shape in the axial plane—anteriorly and medially to this trapezoid are supplied by the ACAs, and posteriorly and inferiorly are supplied by the PCAs (including the thalamus, which is supplied by penetrating vessels arising from the PCAs and posterior communicating arteries).

Watershed (Borderzone) Territories

The watershed (borderzone) territories are the regions at the border of two arterial territories (Fig. 7–7). The MCA-ACA and MCA-PCA borderzones are the most commonly discussed borderzones, but there is also a borderzone between AICA and PICA, as well as a deep borderzone territory between the lenticulostriate branches of the MCA (penetrating from below) and the leptomeningeal branches of the MCA (penetrating from above). Although the common teaching is that borderzone infarction is due to hypoperfusion, borderzone strokes can also be caused by emboli: If the smallest possible emboli travel as distally as possible before causing an occlusion, they will arrive at the end-arterial territories, which are the borderzones. So although borderzone infarction can certainly be due to hypoperfusion, this is not always the cause, and an embolic etiology should also be considered as the etiology of borderzone infarction (for an interesting discussion of this topic, see Caplan and Hennerici, 1998).

Any artery or arterial branch may be affected by ischemic stroke, with corresponding symptoms related to the location and size of the infarct. The diagnosis and management of stroke are discussed in Chapter 19.

MCA Territory Infarction

The MCA territory includes the majority of the cerebral hemisphere, including portions of the frontal, temporal, and parietal lobes with the exception of the anterior, medial, and superior frontal lobes and the medial and superior parietal lobes (supplied by the ACA), and the occipital and inferior temporal lobes (PCA territory) (Fig. 7–8). The functional regions supplied by the MCA therefore include the motor and premotor regions, somatosensory cortex, the frontal eye fields, the language areas (found on the left in the majority of patients), parietal regions responsible for spatial attention (right parietal lesions may cause left-sided neglect), and the superior and inferior radiations of the visual pathways as they pass through the parietal and temporal lobes, respectively. Therefore, a full left MCA syndrome causes right hemiplegia and hemisensory loss, aphasia, gaze deviation toward the left, and right homonymous hemianopia. A full right MCA syndrome causes left hemiplegia and hemisensory loss, left-sided neglect, gaze deviation to the right, and left-sided homonomyous hemianopia. Gaze deviation is discussed in Chapter 11 and visual field deficits in Chapter 6.

The MCA stem is called the M1 segment of the MCA. The MCA stem gives off the lenticulostriate penetrating branches that supply the basal ganglia and internal capsule before dividing into superior and inferior branches known as the M2 segments. The superior M2 branch of the MCA supplies Broca’s area (most commonly on the left), the motor cortex, and the superior visual radiation, whereas the inferior M2 branch supplies Wernicke’s area (most commonly on the left) and the inferior visual radiation. The M2 segments divide into further branches—the smaller and more distal the vessel occluded in ischemic stroke, the smaller the corresponding deficit. However, if a small penetrating (lenticulostriate) branch of the MCA stem is affected, this can affect the convergence of descending motor fibers and lead to a complete contralateral hemiparesis despite a small infarct (pure motor lacunar syndrome; see “Lacunar Strokes” below).

The MCA stem is quite poorly collateralized, and MCA stem occlusion (proximal to the origin of the lenticulostriate branches) can lead to a large subcortical infarct with sparing of more distal regions if there is good collateral flow distally (Fig. 7–9).

ACA Territory Infarction

The ACAs supply the anterior, superior, and medial frontal lobes and the superior and medial parietal lobes—essentially all of the frontal and parietal lobes not supplied by the MCAs (Fig. 7-10). Due to the position of the leg area in the motor homunculus (medial; see Ch. 4), ACA strokes cause contralateral leg weakness and sensory loss more so than face and arm weakness and sensory loss. ACA strokes can also cause cognitive changes such as abulia. The ACAs are connected by the anterior communicating artery. Proximal to the anterior communicating artery, the ACAs are labeled A1 segments; distal to the anterior communicating artery, they are labeled A2 segments.

In some patients, both ACAs arise from a common trunk (azygous ACA). Occlusion of both ACAs simultaneously can cause acute paraplegia, mimicking acute spinal pathology. The presence of cognitive symptoms usually distinguishes bilateral ACA infarction from acute spinal cord pathology.

The ACAs can also be compromised by subfalcine herniation (see Ch. 25).

One typically learns that the MCA supplies the face and arm areas on the lateral surface of the homunculus and the ACA supplies the leg area, so that MCA strokes cause contralateral face and arm weakness much more so than leg weakness, and ACA strokes cause contralateral leg weakness much more so than face and arm weakness. This is true for strokes affecting the cortical surface. However, the motor fibers join subcortically and travel together, so a stroke that affects the subcortical white matter pathways can cause a complete contralateral hemiparesis or hemiplegia affecting the face, arm, and leg (e.g., a lacunar stroke in the posterior limb of the internal capsule or a full MCA territory infarct affecting the cortex and subcortical white matter).

Recurrent Artery of Huebner Territory Infarction

The recurrent artery of Huebner is a branch of the ACA that supplies the head of the caudate and the adjacent internal capsule. Infarction can cause contralateral hemiparesis and/or movement disorder, which may be accompanied by cognitive deficits.

Anterior Choroidal Artery Territory Infarction

The anterior choroidal artery branches directly from the internal carotid artery and supplies the posterior thalamus (including the lateral geniculate nucleus) and the internal capsule (including descending motor and ascending thalamocortical pathways). Infarction in the territory of the anterior choroidal artery can cause contralateral visual field defects, contralateral hemiparesis, and/or contralateral hemisensory loss, and can also cause cortical signs (e.g., neglect if the right hemisphere is affected) due to interruption of thalamocortical pathways.

PCA Territory Infarction

The PCAs supply the occipital lobes, inferior medial temporal lobes, and the thalami (Fig. 7–11). Depending on the extent of infarction in the PCA territory, deficits can include contralateral homonymous hemianopia or superior quadrantanopia (see Ch. 6), impaired short-term memory (if there is medial temporal/hippocampal involvement), inability to read with spared ability to write (alexia without agraphia) (if there is left inferior temporal involvement), decreased ability to recognize faces (prosopagnosia) (if there is right inferior temporal involvement), and/or changes in cognition and/or level of arousal (if there is thalamic involvement).

The PCAs are connected to the anterior circulation by the posterior communicating arteries. Proximal to each posterior communicating artery, the PCA is called the P1 segment, and distal to the posterior communicating artery, it is called the P2 segment. In some patients, one or both PCAs arise from the internal carotids rather than the top of the basilar artery, a variant referred to as a fetal PCA (see Fig. 7–5). Although strokes in the PCA territory are generally considered posterior circulation strokes, if a patient has a PCA stroke in the setting of a fetal PCA, the stroke would be considered to have arisen from the anterior circulation. This is important to recognize in patients with PCA stroke and ipsilateral carotid stenosis: A fetal PCA on the side of a PCA stroke and carotid stenosis suggests that the stenotic carotid is symptomatic (see Ch. 19).

In some patients, the left and right thalami are both supplied by a single artery that arises from the PCA, referred to as the artery of Percheron. Occlusion of this artery can lead to bithalamic infarction causing acutely altered mental status, a rare global, rather than focal, stroke syndrome (Fig. 7–12).

Lacunar Strokes

Lacunar strokes are caused by occlusion of small penetrating arteries affecting the subcortical white matter (internal capsule), subcortical gray matter (basal ganglia, thalamus [Fig. 7–13]), or anterior pons. Lacunar stroke syndromes include:

• Pure motor stroke: unilateral hemiparesis/hemiplegia due to involvement of the posterior limb of the internal capsule or the anterior pons.

• Pure sensory stroke: unilateral hemisensory loss due to involvement of the VPL/VPM nuclei of the thalamus.

• Ataxia-hemiparesis: unilateral hemiparesis/hemiplegia (due to involvement of the corticospinal tract) with ataxia in the weak limb(s) (due to interruption of the corticopontocerebellar fibers destined for the middle cerebellar peduncles (see Ch. 8)). This can occur due to lacunar stroke in either the internal capsule or the anterior pons, both of which are places where the corticospinal tract and corticopontocerebellar fibers run together.

• Dysarthria–clumsy hand: dysarthria and unilateral upper limb ataxia; localization is the same as for ataxia-hemiparesis (internal capsule or anterior pons).

Infarction in the Watershed (Borderzone) Territories

The MCA-ACA watershed regions span the “stripes” at the border of the two territories (Fig. 7-14). Recalling the homunculus (Fig. 4–1), the part of the motor homunculus supplied by the MCA-ACA watershed region includes the proximal arm and leg, which are joined at the shoulder and hip in the homunculus. Therefore, infarction in the MCA-ACA borderzone can cause proximal arm and leg weakness with preserved strength distally in the hands and feet. When this occurs bilaterally, it causes what is called the “person in a barrel” syndrome since the distal arms and legs function well but the proximal limbs are weak (simulating a person in a barrel with the hands and feet sticking out).

The MCA-PCA watershed region is at the parieto-occipital junction. When the MCA-PCA watershed region is affected bilaterally, the patient will often have deficits in visual attention that can include some or all of the elements of Balint’s syndrome: optic ataxia, ocular apraxia, and simultanagnosia (see Ch. 6).

The evaluation and management of patients with cerebral infarction is discussed in detail in Chapter 19.