Definitions

Aphasia is defined as a disorder of language acquired secondary to brain damage. This definition, adapted from Alexander and Benson (1997), separates aphasia from several related disorders. First, aphasia is distinguished from congenital or developmental language disorders, called dysphasias. (In contrast with British usage, in the United States the term dysphasia applies to developmental language disorders rather than partial or incomplete aphasia.)

Second, aphasia is a disorder of language rather than speech. Speech is the articulation and phonation of language sounds; language is a complex system of communication symbols and rules for their use. Aphasia is distinguished from motor speech disorders (the subject of Part B of this chapter), which include dysarthria, dysphonia (voice disorders), stuttering, and speech apraxia. Dysarthrias are disorders of articulation of single sounds; causes of these disorders may include mechanical disturbance of the tongue or larynx and neurological disorders such as dysfunction of the muscles, neuromuscular junction, cranial nerves, bulbar anterior horn cells, corticobulbar tracts, cerebellar connections, or basal ganglia. Apraxia of speech is a syndrome of misarticulation of phonemes, especially consonant sounds. Unlike dysarthria, in which certain phonemes are consistently distorted, apraxia of speech is characterized by inconsistent distortions and substitutions of phonemes. The disorder is called an apraxia because there is no primary motor deficit in articulation of individual phonemes. Clinically, speech-apraxic patients produce inconsistent articulatory errors, usually worse on the initial phonemes of a word and with polysyllabic utterances. Apraxia of speech, so defined, is commonly involved in speech production difficulty in the aphasias.

Third, aphasia is distinguished from disorders of thought. Thought involves the mental processing of images, memories, and perceptions, usually but not necessarily involving language symbols. Psychiatric disorders derange thought and alter the content of speech without affecting its linguistic structure. Schizophrenic patients, for example, may manifest bizarre and individualistic word choices, with loose associations and a loss of organization in discourse together with vague or unclear references and communication failures (Docherty et al., 1996). Elementary language and articulation, however, are intact. Abnormal language content in psychiatric disorders is therefore not considered to represent aphasia, because the disorder is more one of thought than of language. Language disorders associated with diffuse brain diseases such as encephalopathies and dementias do qualify as aphasias, but the involvement of other cognitive functions distinguishes them from aphasia secondary to focal brain lesions.

An understanding of language disorders requires an elementary review of linguistic components. Phonemes are the smallest meaning-carrying sounds; morphology is the use of appropriate word endings and connector words for tenses, possessives, and singular versus plural; semantics refers to word meanings; the lexicon is the internal dictionary; and syntax is the grammatical construction of phrases and sentences. Discourse refers to the use of these elements to create organized and logical expression of thoughts. Pragmatics refers to the proper use of speech and language in a conversational setting, including pausing while others are speaking, taking turns properly, and responding to questions. Specific language disorders affect one or more of these elements.

Relevant Neuroanatomy

Language processes have a clear neuroanatomical basis. In simplest terms, the reception and processing of spoken language take place in the auditory system, beginning with the cochlea and proceeding through a series of way stations to the auditory cortex, the Heschl gyrus, in each superior temporal gyrus. Decoding sounds into linguistic information involves the posterior part of the left superior temporal gyrus, the Wernicke area or Brodmann area 22, which gives access to a network of cortical associations to assign word meanings. For both repetition and spontaneous speech, auditory information is transmitted to the Broca area in the posterior inferior frontal gyrus. This area of cortex “programs” the neurons in the adjacent motor cortex subserving the mouth and larynx, from which descending axons travel to the brainstem cranial nerve nuclei. The inferior parietal lobule, especially the supramarginal gyrus, also may be involved in phoneme processing in language comprehension and in phoneme production for repetition and speech (Hickok and Poeppel, 2000). These anatomical relationships are shown in Figs. 12A.1 and 12A.2. Reading requires perception of visual language stimuli by the occipital cortex, followed by processing into auditory language information via the heteromodal association cortex of the angular gyrus. Writing involves activation of motor neurons projecting to the arm and hand. A French study that used aphasia testing and magnetic resonance imaging (MRI) scans to evaluate 107 stroke patients confirmed the general themes of nearly 150 years of clinical aphasia research: that frontal lesions caused nonfluent aphasia, whereas posterior temporal lesions affected comprehension (Kreisler et al., 2000).

Fig. 12A.1 Lateral surface of the left hemisphere, showing a simplified gyral anatomy and the relationships between the Wernicke area and Broca area. Not shown is the arcuate fasciculus, which connects the two cortical speech centers via the deep subcortical white matter.

Fig. 12A.2 Coronal plane diagram of the brain, indicating the inflow of auditory information from the ears to the primary auditory cortex in both superior temporal regions (xxx), and then to the Wernicke area (ooo) in the left superior temporal gyrus. The motor outflow of speech descends from the Broca area (B) to the cranial nerve nuclei of the brainstem via the corticobulbar tract (dashed arrow). In actuality, the Broca area is anterior to the Wernicke area, and the two areas would not appear in the same coronal section.

These pathways, and doubtless others, constitute the cortical circuitry for language comprehension and expression. In addition, other cortical centers involved in cognitive processes project into the primary language cortex, influencing the content of language. Finally, subcortical structures play increasingly recognized roles in language functions. The thalamus, a relay for the reticular activating system, appears to alert the language cortex, and lesions of the dominant thalamus frequently produce fluent aphasia. Nuclei of the basal ganglia involved in motor functions, especially the caudate nucleus and putamen, participate in expressive speech. No wonder, then, that language disorders are seen with a wide variety of brain lesions and are important in practical neurological diagnosis and localization.

In right-handed people, and in a majority of left-handers as well, clinical syndromes of aphasia result from left hemisphere lesions. Rarely, aphasia may result from a right hemisphere lesion in a right-handed patient, a phenomenon called crossed aphasia (Bakar et al., 1996). In left-handed persons, language disorders are usually similar to those of right-handed persons with similar lesions, but occasional cases manifest with atypical syndromes that suggest a right hemisphere capability for at least some language functions. For example, a patient with a large left frontotemporoparietal lesion may have preserved comprehension, suggesting right hemisphere language comprehension. For the same reason, recovery from aphasia may be better in some left-handed than in right-handed patients with left hemisphere strokes.

Diagnostic Features

Muteness, a total loss of speech, may represent severe aphasia (see Aphemia later in the chapter). Muteness also can be a sign of dysarthria, frontal lobe dysfunction with akinetic mutism, severe extrapyramidal system dysfunction (as in Parkinson disease), non-neurological disorders of the larynx and pharynx, or even psychiatric syndromes such as catatonia. Caution must therefore be taken in diagnosing the mute patient as aphasic. A good rule of thumb is that if the patient can write or type and the language form and content appear normal, the disorder is probably not aphasic in origin. If the patient cannot speak or write but makes apparent effort to vocalize, and if there is also evidence of deficient comprehension, aphasic muteness is likely. Associated signs of a left hemisphere injury, such as right hemiparesis, also aid in diagnosis. Finally, if the patient gradually begins to make sounds containing paraphasic errors, aphasia can be identified with confidence.

Hesitant speech is a feature of aphasia but also of motor speech disorders such as dysarthria or stuttering, and it may be a manifestation of a psychogenic disorder. A second rule of thumb is that if the utterances of a hesitant speaker can be transcribed into normal language, the patient is not aphasic. Hesitancy occurs in many aphasia syndromes for various reasons, including difficulty in speech initiation, imprecise articulation of phonemes, deficient syntax, and word-finding difficulty.

Anomia, or inability to produce a specific name, is generally a reliable indicator of language disorder, although it also may reflect memory loss. Anomia is manifested in aphasic speech by word-finding pauses and circumlocutions, or use of a phrase when a single word would suffice (e.g., “the thing you tell time with” for watch).

Paraphasic speech refers to the presence of errors in the patient’s speech output. Paraphasic errors are divided into literal or phonemic errors involving substitution of an incorrect sound (e.g., “shoon” for “spoon”) and verbal or semantic errors involving substitution of an incorrect word (e.g., “fork” for “spoon”). A related language symptom is perseveration, the inappropriate repetition of a previous response. Occasionally, aphasic utterances involve nonexistent word forms called neologisms. A pattern of paraphasic errors and neologisms that so contaminate speech that the meaning cannot be discerned is called jargon speech.

Another cardinal symptom of aphasia is failure to comprehend the speech of others. Most aphasic patients also have difficulty with comprehension and production of written language (reading and writing). Fluent paraphasic speech usually makes an aphasic disorder obvious. The chief considerations in the differential diagnosis here include aphasia, psychosis, acute encephalopathy or delirium, and dementia. Aphasic patients usually do not appear confused and do not exhibit inappropriate behavior; they are not agitated and do not misuse objects, with occasional exceptions in acute syndromes of Wernicke or global aphasia. In contrast, most psychotic patients speak in an easily understood, grammatically appropriate manner, but their behavior and speech content are abnormal. Only rarely do schizophrenics speak in “clang association” or “word salad” speech. Sudden onset of fluent paraphasic speech in a middle-aged or elderly patient should always be suspected of representing a left hemisphere lesion with aphasia.

Patients with acute encephalopathy or delirium may manifest paraphasic speech and “higher” language deficits, such as the inability to write, but the grammatical expression of language is less disturbed than its content. These language symptoms, moreover, are less prominent than accompanying behavioral disturbances such as agitation, hallucinations, drowsiness, or excitement, and cognitive difficulties such as disorientation, memory loss, or delusional thinking.

Chronic encephalopathies, or dementias, pose a more difficult diagnostic problem because involvement of the language cortex produces readily detectable language deficits, especially involving naming, reading, and writing. These language disorders (see Language in Dementing Diseases later in this chapter) differ from aphasia secondary to focal lesions mainly by the involvement of other cognitive functions such as memory and visuospatial processes.

Broca Aphasia

In 1861, the French physician Paul Broca described two patients, establishing the aphasia syndrome that now bears his name. The speech pattern is nonfluent; on bedside examination, the patient speaks hesitantly, often producing the principal meaning-containing nouns and verbs but omitting small grammatical words and morphemes. This pattern is called agrammatism or “telegraphic speech.” An example is “wife come hospital.” Patients with acute Broca aphasia may be mute or may produce only single words, often with dysarthria and apraxia of speech. They make many phonemic errors, inconsistent from utterance to utterance, with substitution of phonemes usually differing only slightly from the correct target (e.g., /p/ for /b/). Naming is deficient, but the patient often manifests a “tip-of-the-tongue” phenomenon, getting out the first letter or phoneme of the correct name. Paraphasic errors in naming more frequently are of literal than of verbal type. Auditory comprehension seems intact, but detailed testing usually reveals some deficiency, particularly in the comprehension of complex syntax. For example, for persons with Broca aphasia, sentences with embedded clauses involving prepositional relationships cause difficulty in comprehension as well as in expression (“The rug that Bill gave to Betty tripped the visitor”). A positron emission tomography (PET) study in normal persons (Caplan et al., 1998) showed activation of the Broca area in the frontal cortex during tests of syntactic comprehension; the Broca area thus appears to be involved in syntactical operations, both expressively and receptively. Repetition is hesitant in these patients, resembling their spontaneous speech. Reading often is impaired despite relatively preserved auditory comprehension. Benson termed this reading difficulty of Broca aphasics the “third alexia,” in contradistinction to the two classical types of alexia (see Aphasic Alexia later in the chapter). Patients with Broca aphasia may have difficulty with syntax in reading, just as in auditory comprehension and speech. Writing is virtually always deficient in Broca aphasics. Most patients have a right hemiparesis necessitating use of the nondominant left hand for writing, but this left-handed writing is far more abnormal than the awkward renditions of a normal right-handed person attempting to write left-handed. Many patients can scrawl only a few letters.

Associated neurological deficits of Broca aphasia include right hemiparesis, hemisensory loss, and apraxia of the oral apparatus and the nonparalyzed left limbs. Apraxia in response to motor commands is important to recognize because it may be mistaken for comprehension disturbance. As mentioned earlier, comprehension should also be tested by responses to yes/no questions or commands to point to an object. The common features of Broca aphasia are listed in Table 12A.1.

Table 12A.1 Bedside Features of Broca Aphasia

An important clinical feature of Broca aphasia is its frequent association with depression (Robinson, 1997). Patients with Broca aphasia typically are aware of and frustrated by their deficits. At times they become withdrawn and refuse help or therapy. Usually the depression lifts with recovery from the deficit, but it may be a limiting factor in rehabilitation.

The lesions responsible for Broca aphasia usually include the traditional Broca area in the posterior part of the inferior frontal gyrus, along with damage to adjacent cortex and subcortical white matter. Most patients with lasting Broca aphasia, including Broca’s original cases, have much larger left frontoparietal lesions, including most of the territory of the upper division of the left middle cerebral artery. In such patients, the deficit typically evolves from global to Broca aphasia over weeks to months. Patients who manifest Broca aphasia immediately after their strokes, by contrast, have smaller lesions of the inferior frontal region, and their deficits generally resolve quickly. In computed tomography (CT) scan analyses at the Boston Veterans Administration Medical Center, lesions restricted to the lower precentral gyrus produced only dysarthria and mild expressive disturbance. Lesions involving the traditional Broca area (Brodmann areas 44 and 45) resulted in difficulty initiating speech, and lesions combining the Broca area, the lower precentral gyrus, and subcortical white matter yielded the full syndrome of Broca aphasia. In other studies at the center, damage to two subcortical white matter sites—the rostral subcallosal fasciculus deep to the Broca area and the periventricular white matter adjacent to the body of the left lateral ventricle—was required to cause permanent nonfluency. These concepts concerning the Broca area and its mainly temporary role in Broca aphasia have been confirmed by a recent MRI study, indicating that MRI lesions in the Broca area correlate with Broca or global aphasia in acute stroke, but not in the chronic period (Ochfeld et al., 2010). Fig. 12A.3 shows an MRI scan of the brain from a patient with Broca aphasia.

Fig. 12A.3 Magnetic resonance imaging study of the brain of a patient with Broca aphasia. In this patient, the cortical Broca area, subcortical white matter, and the insula all were involved in the infarction. The patient made a good recovery.

Aphemia

A rare variant of Broca aphasia is aphemia, a nonfluent syndrome in which the patient initially is mute and then becomes able to speak with phoneme substitutions and pauses. All other language functions are intact, including writing. This rare and usually transitory syndrome results from small lesions of the Broca area or its subcortical white matter or of the inferior precentral gyrus. Because written expression and auditory comprehension are normal, aphemia is not a true language disorder; aphemia may be equivalent to pure apraxia of speech.

Wernicke Aphasia

Wernicke aphasia may be considered the opposite of Broca aphasia in that expressive speech is fluent, but comprehension is impaired. The speech pattern is effortless and sometimes even excessively fluent (“logorrhea”). A speaker of a foreign language would notice nothing amiss, but a listener who shares the patient’s language detects speech empty of meaning, containing verbal paraphasias, neologisms, and jargon productions. Neurolinguists refer to this pattern as paragrammatism. In milder cases, the intended meaning of an utterance may be discerned, but the sentence goes awry with paraphasic substitutions. Naming in Wernicke aphasia is deficient, often with bizarre, paraphasic substitutions for the correct name. Auditory comprehension is impaired, sometimes even for simple nonsense questions. Deficient semantics is the major cause of the comprehension disturbance in Wernicke aphasia, along with disturbed access to the internal lexicon. Repetition is impaired; whispering a phrase in the patient’s ear, as in a hearing test, may help cue the patient to attempt repetition. Reading comprehension is usually affected in a fashion similar to that observed for auditory comprehension, but occasionally patients show greater deficit in one modality than in the other. The discovery of spared reading ability in Wernicke aphasics is important in allowing these patients to communicate. In addition, neurolinguistic theories of reading must account for the access of visual language images to semantic interpretation, even in the absence of auditory comprehension. Writing also is impaired, but in a manner quite different from that of Broca aphasia. The patient usually has no hemiparesis and can grasp the pen and write easily. Written productions are even more abnormal than oral ones, however, in that spelling errors are also evident. Writing samples are especially useful in the detection of mild Wernicke aphasia.

Associated signs are limited in Wernicke aphasia; most patients have no elementary motor or sensory deficits, although a partial or complete right homonymous hemianopia may be present. The characteristic bedside examination findings in Wernicke aphasia are summarized in Table 12A.2.

Table 12A.2 Bedside Features of Wernicke Aphasia

The psychiatric manifestations of Wernicke aphasia are quite different from those of Broca aphasia. Depression is less common; many Wernicke aphasics seem unaware of or unconcerned about their communicative deficits. With time, some patients become angry or paranoid about the inability of family members and medical staff to understand them. This behavior, like depression, may hinder rehabilitative efforts.

The lesions of patients with Wernicke aphasia usually involve the posterior portion of the superior temporal gyrus, sometimes extending into the inferior parietal lobule. Fig. 12A.4 shows a typical example. The exact confines of the Wernicke area have been much debated. Damage to this area (Brodmann area 22) has been reported to correlate most closely with persistent loss of comprehension of single words, although only larger temporoparietal lesions have been found in patients with lasting Wernicke aphasia. In the acute phase, the ability to match a spoken word to a picture is quantitatively related to decreased perfusion of the Wernicke area on perfusion-weighted MRI, indicating less variability during the acute phase than after recovery has taken place (Hillis et al., 2001). Electrical stimulation of the Wernicke area produces consistent interruption of auditory comprehension, supporting the importance of this region for decoding auditory language. A receptive speech area in the left inferior temporal gyrus has also been suggested by electrical stimulation studies and by a few descriptions of patients with seizures involving this area (Kirshner et al., 1995), but aphasia has not been recognized with destructive lesions of this area. Extension of the lesion of Wernicke aphasia into the inferior parietal region may predict greater involvement of reading comprehension. In terms of vascular anatomy, the Wernicke area lies within the territory of the inferior division of the left middle cerebral artery.

Fig. 12A.4 Brain imaging in Wernicke aphasia. The patient was an elderly woman. A and B, Axial and coronal magnetic resonance images. C, Axial positron emission tomography (PET) scan. There is a large left superior temporal lobe lesion. The onset of the deficit was not clear, and the PET scan was useful in showing that the lesion had reduced metabolism, favoring a stroke over a tumor.