Neuro-ophthalmology of Bilingual Aphasia
Bilingual aphasia refers to the loss of the ability to understand and express speech as a result of brain damage in a bilingual or multilingual individual.  The degree and character of the language deficits vary widely between patients and depend on multiple factors such as the location and size of the lesion. Moreover, in bilingual aphasia, the nature of language impairment can be selective and differ in both languages. For instance, a patient who is unable to speak a primary and secondary language but retains the ability to understand the secondary language.
The causes of bilingual aphasia are similar to those of general aphasia. The leading cause of aphasia, in general, is cerebral infarction and is estimated to affect one-third of stroke patients. Other causes of aphasia include cerebral insults such as trauma, seizures, and toxic metabolic products.
The pathophysiology of bilingual aphasia is closely related to the cerebral representation of language. Hence, understanding the localization of both primary and secondary languages is crucial in managing these patients. Original theories on language representation suggested separate sites for each language that an individual acquired. According to this theory, a bilingual patient would be expected to have two different sites for each language. These theories were replaced by the shared language representation theories, which postulated that all languages were derived from the same cerebral sites. Under these theories, a cerebral insult would be expected to produce equal language deficits in all languages. However, these theories came under scrutiny with empirical evidence that showed heterogeneity in both symptomology and recovery in aphasic patients. As a result, the amalgamated (mixture of separate and shared localization) theory, which suggests that language representation includes both shared and specific sites, has taken prominence. Thus, language is represented with both specific and shared sites. Most bilingual individuals have distinct language sites that are absent in monolingual individuals, while some bilinguals (less than five percent) have been found to have only shared sites.
Language representation is generally located in the anterior (frontal lobe) motor region and posterior (temporal lobe) receptive region. Regarding primary and secondary language representation, both primary and secondary languages have been shown to occupy equal cerebral cortex extension. Notably, primary and secondary language sites differ in anatomical distribution. Although variations exist, the anterior expressive region has been shown to contain shared sites for both primary and secondary languages while the posterior receptive region contains sites that are specific to secondary languages. This is clinically important because insults in the anterior expressive region are more likely to result in multilingual language deficits than those in the posterior region.
The pathophysiology of aphasia is characterized by a lesion anywhere in the language pathway. The language pathway is described by the Broca-Wernicke-Lichtheim-Geschwind model that consists of the visual cortex, the auditory cortex, Wernicke’s area (superior temporal gyrus of the temporal lobe), Broca’s area (frontal gyrus of the frontal lobe), the motor cortex, and the Geschwind territory (inferior parietal lobule). The Geschwind territory consists of the supramarginal gyrus rostrally and the angular gyrus caudally. Written or sign language is first perceived in the visual cortex (occipital lobe) while spoken language is first perceived in the auditory cortex (temporal lobe). This information is then transmitted to the Wernicke’s area where it is matched to an individual’s memory and achieves comprehension. Signals from this area are then transmitted to Broca’s area via the arcuate fasciculus. Broca’s area is responsible for speech production and sends signals to the motor cortex which produces muscle movements necessary for speech. On the other hand, the Geschwind territory (inferior parietal lobule) is connected to all other components of the language pathway and performs a multimodal synthesis of information.
The general incidence of aphasia is estimated to be 300,000 per year with cumulative prevalence estimated to be over a million people in the United States. There is an increasing incidence of poststroke aphasia with advancing age, and the prevalence of aphasia following a first-ever ischemic stroke (FEIS) is 43 cases in every 100,000. Although there is a paucity of studies on the epidemiology of bilingual aphasia, Paridis et al approximated that 45,000 new cases of bilingual aphasia are expected annually.
The presentation of bilingual aphasia is variable among patients and different languages have varying levels of impairment. Hence, a detailed and comprehensive history is necessary to determine causes of the aphasia, as well as gauging the patient’s recovery.
A systematic evaluation of all languages that the patient speaks is required to gauge the level of impairment. Considering that bilingual aphasic patients do not manifest the same deficits in all languages, evaluations must be conducted in all languages known to the patient. Paradis et al created the Bilingual Aphasia Test (BAT), which is made up of three parts that evaluate language impairment in multilingual patients :
- Part A: consists of 50 items that evaluate the multilingual history of a patient
- Part B: consists of 472 items in each language that the patient speaks. This part provides a comparative evaluation of deficits among all languages spoken by the patient
- Part C: consists of 58 items that evaluate translation abilities and interference
When assessing patients who have varying residual abilities, each language must be evaluated on a separate day and the patient’s linguistic abilities before the insult should be verified by family members or friends. Bilingual aphasia evaluations should also take into consideration the three temporal phases: 1) the acute phase, which usually lasts four weeks after onset; 2) the lesion phase, which lasts for several weeks and up to 5 months after; 3) the late phase, which begins months after onset and can last the rest of the patient’s life. This is because some symptoms are usually prominent in different phases. For instance, the acute phase can exhibit severe deficits of primary language acquired during childhood with preservation of the secondary languages learned later in life.
A comprehensive neurological exam should also be performed, especially in the acute setting. Acute language deficits are usually a result of cerebral lesions that can be elucidated during a neurological exam. Considering that the occipital lobe is part of the language pathway, an ophthalmology examination that includes visual acuity, field, and pupillary reflexes should also be performed. Slit-lamp examination is usually unremarkable but visual field testing can reveal deficits depending on the extent of the cerebral lesion.
In the acute setting, patients should undergo a computerized tomography (CT) scan if there is a suspicion of a hemorrhagic infarct. In addition, all patients should undergo neuroimaging with cerebral MRI. Clinicians can also consider performing electroencephalography testing because some aphasic patients have been found to have underlying seizures.
Alexia without agraphia: It is characterized by the inability to read with retained ability to write. Patients can understand and produce sound. It is due to a lesion in the left occipital lobe that extends to the splenium of the corpus callosum.
Pure word mutism: Patients present with impaired speech production while retaining the ability to understand and produce speech.
Pure word deafness: This is a rare syndrome that is marked by the inability to understand spoken language, yet retaining the ability to understand written language.
Akinetic mutism: It is characterized by a poor response and a paucity in speech output. Akinetic mutism can result from a mesial frontal region lesion.
Management of bilingual aphasia marked by a multidisciplinary approach that addresses various language impairments. Although treatments used for monolingual aphasics have been used for bilingual aphasia, it is important to note that two conditions, while comparable, are not similar. When possible, patients should receive neurorehabilitation and speech therapy in all the languages that are impaired. Since there is a paucity of multilingual neurorehabilitation resources, cross-linguistic therapies (CLT) have been proposed. Cross-linguistic therapy leverages the fact that therapy in one language can affect deficits in another language. Considering that patients with aphasia can also have concomitant visual field deficits, these patients benefit from vision therapy.
The recovery of patients with aphasia is largely dependent on the extent of the initial injury. In addition, various recovery patterns have been observed in individuals with bilingual aphasia with great variability. For instance, a study that reviewed 132 patients with bilingual aphasia found that 61% had a parallel recovery of their two languages, 18% had a differential recovery, and 7% had blended recovery. Moreover, studies have shown that with extensive therapy, within one year, patients experienced a progression to a milder form of aphasia.
With the increasing globalization and the number of people who speak multiple languages coupled with the increasing vasculopathy diseases, the incidence of bilingual aphasia is likely to increase. Patients with bilingual aphasia can present with a visual impairment such as visual field deficits because the occipital lobe is involved in the language pathway. Hence, clinicians should be aware that aphasic patients can have ophthalmologic manifestation and that these patients can benefit from neurorehabilitation, speech therapy, and vision therapy.
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