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Language Disorders in Huntington Disease

A Systematic Literature Review

Gagnon, Maude, MSc*; Barrette, Jasmine, MSc*; Macoir, Joël, PhD*,†

Cognitive and Behavioral Neurology: December 2018 - Volume 31 - Issue 4 - p 179–192
doi: 10.1097/WNN.0000000000000171
Review Article
Free

Objective: A systematic review examining the presence and nature of language disorders associated with Huntington disease (HD).

Background: HD is characterized by gradual motor dysfunction, psychiatric problems, and cognitive decline. Communication abilities in HD may be affected not only by dysarthria but also by language impairment. However, the nature and type of this impairment is not well defined.

Methods: We searched the PubMed and PsycINFO databases and selected studies on the basis of the original language of the article, peer-review status, and specificity of the results regarding language and communication disorders.

Results: Thirty-one articles meeting the selection criteria were selected for this review. According to most of the studies, individuals with HD present with primary deficits of language. However, a few authors suggested that language deficits in HD result from nonlinguistic impairments, or that language abilities are largely preserved. More specifically, studies showed that HD is associated with difficulties in producing and understanding sentences and discourse, processing semantic representations of object and action concepts, retrieving lexical forms of nouns, and applying morphological and syntactic rules. There is some disagreement regarding whether HD affects reading abilities, sentence production, semantic processing, and application of morphological rules in verb conjugation.

Conclusions: Although people with HD present with language impairment, further studies are needed to identify their functional origin. Clinical studies are also needed to determine the impact of such impairments on an individual’s functional communication in daily living and to chart the progression of the impairments over the course of the disease.

*Speech-Language Pathology Program, Department of Rehabilitation, Faculty of Medicine, Laval University, Quebec City, Quebec, Canada

CERVO Brain Research Centre, Mental Health University Institute of Quebec, Quebec City, Quebec, Canada

The authors declare no conflicts of interest.

Correspondence: Joël Macoir, PhD, Department of Rehabilitation, Faculty of Medicine, Laval University, Pavillon F-Vandry, Quebec City, Quebec, Canada G1K 7P4 (e-mail: joel.macoir@fmed.ulaval.ca).

Received December 19, 2017

Accepted September 14, 2018

BNT: Boston Naming Test

HD: Huntington disease

PRISMA: Preferred Reporting Items for Systematic reviews and Meta-Analyses

Huntington disease (HD) is an autosomal dominant, inherited neurodegenerative disease caused by a mutation (repeating CAG triplet series in the huntingtin gene) of chromosome 4. The huntingtin gene encodes the huntingtin protein known to have postdevelopmental roles in axonal trafficking, regulation of gene transcription, and cell survival (Schulte and Littleton, 2011). HD leads to the progressive atrophy of brain neurons, especially in the striatum (Vonsattel et al, 1985).

Many recent studies have improved the neuropathological knowledge of HD and have shown that the brain distribution pattern of neurodegeneration also includes the cerebral neocortex and allocortex, thalamus, pallidum, brain stem, and cerebellum (Rub et al, 2016). In Western populations, the prevalence of HD is ∼10.6 to 13.7 people per 100,000 (McColgan and Tabrizi, 2018). HD affects both men and women, the first symptoms are usually reported between 35 and 45 years of age, and death typically follows within 15 to 20 years of the diagnosis (Sturrock and Leavitt, 2010). The clinical diagnosis is essentially based on the identification of an extrapyramidal movement disorder associated with a positive genetic test or a confirmed family history of HD.

HD is characterized by gradual motor dysfunction, psychiatric problems, and cognitive decline. Chorea (ie, abnormal involuntary movement characterized by brief, abrupt, irregular, unpredictable, nonstereotyped movements) affecting various body parts is the most prominent motor symptom of HD. As the disease progresses, other motor manifestations such as dysarthria, dysphagia, ocular dysfunction, hypotonia, and bradykinesia appear.

Psychiatric problems accompanying HD include depression, obsessive-compulsive disorders, anxiety, irritability, and apathy (Paoli et al, 2017). With respect to cognition, studies have shown that impairments may be detected at least 15 years before establishment of the motor diagnosis of HD (Paulsen et al, 2008). In fact, various studies have shown that the striatum not only plays a role in motor control but also is involved in cognitive functions, particularly executive functions (Hedden and Gabrieli, 2010) and working memory (Landau et al, 2009). Compelling evidence from studies conducted with healthy controls and individuals with brain damage suggests that the striatum is involved in various language domains, including phonology (Tettamanti et al, 2005), word retrieval (Crosson, 1985; Crosson et al, 2003), morphology (Marangolo and Piras, 2008; Vannest et al, 2005), and syntax (Damasio et al, 1982; Friederici and Kotz, 2003). Still controversial, however, is whether the striatum plays a primary role in language processing or, instead, has a modulating role on language via general cognitive functions.

Language is the cognitive ability to communicate and manipulate linguistic symbols and should be distinguished from speech, which is a motor ability. Compared with speech, language has been much less studied in HD, with no clear consensus on either the characteristics of language impairment in HD or their functional origin. Speech-language pathologists usually prioritize speech symptoms and overlook the potential language difficulties that people with HD may experience. However, language impairment can have a major impact on an individual’s quality of life as well as the efficiency of rehabilitation. Therefore, identifying and characterizing language disorders associated with HD is valuable to both frontline clinicians and rehabilitation professionals.

Although a few older studies mentioned the presence of language impairment in individuals with dementia, including dementia associated with HD (Albert, 1978; Bayles, 1982), we aimed our review at studies specifically investigating the nature of language impairment in HD at the different stages of the disease.

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METHODS

Review Protocol

As a guideline for conducting this review, we used the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) Statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions (Liberati et al, 2009). The PRISMA Statement consists of a 27-item checklist and a four-phase flow diagram designed to help authors improve the reporting of systematic reviews and meta-analyses.

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Eligibility Criteria

To be included in this review, studies had to meet the following criteria:

  • Be conducted with adults with presymptomatic HD (pre-HD), symptomatic HD, or dementia associated with HD.
  • Be focused on language and communication disorders.
  • Be published in peer-reviewed journals.
  • Be written in English or French.

Studies were excluded if they had any of the following features:

  • Included participants not diagnosed with HD.
  • Appeared in a literature review.
  • Targeted general cognitive skills.
  • Targeted dysarthria and orofacial manifestations.
  • Were limited to verbal fluency tasks, which are mainly used to measure executive functions.
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Information Sources and Search Strategy

We identified studies by searching PubMed and PsycINFO from the beginning of the databases up to April 2018. In both databases, we used the following key words in combination with the Boolean operator AND: “Huntington disease,” “language disorders,” “language,” “speech disorders,” or “speech” (the key word “speech” was used because the distinction between speech and language is not consistently made in studies). We also reviewed the reference lists of all identified articles for additional potentially relevant studies.

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Study Selection

We screened the titles and abstracts of the studies for potential eligibility. We then retrieved full texts of potentially eligible studies to ensure that they met the inclusion criteria. Agreement among the authors on included and excluded studies was 100%.

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RESULTS

Study Selection

We identified 317 studies in PubMed and 63 in PsycINFO, for a total of 380. After excluding 37 duplicates, we screened the titles and abstracts of the remaining 343 studies for eligibility. Of these, we excluded 306 and earmarked 37 for closer reading. We selected one additional study from the reference lists of the selected articles for closer reading. From these 38 studies, we excluded 7 according to eligibility criteria, leaving a total of 31 studies included in this review. Figure 1 illustrates the article selection process.

FIGURE 1

FIGURE 1

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Main Characteristics of the Studies

We first extracted data items using a chart containing the following variables: number of participants, country of origin, diagnostic method, mean age, sex, education, and stage of the disease (Table 1).

TABLE 1

TABLE 1

For each of the studies, we gathered data related to language tests and experimental tasks, results, and main conclusions. The results of this in-depth analysis are shown in Table 2. The studies were conducted in 12 different countries (Argentina, Australia, Brazil, Colombia, France, Germany, Hungary, New Zealand, Spain, Sweden, United Kingdom, and United States). The studies had relatively small sample sizes, ranging from 6 to 56 patients. In all, the 31 studies reported in this review cover 598 participants with HD, including 29 pre-HD participants and 38 HD first-degree relatives. All 31 studies had a control group: 29 of the control groups included only healthy individuals, one contained individuals at risk for developing HD, and one contained individuals with Friedreich ataxia. In most of the studies, the diagnosis of HD was established through family history, neurologic examination, and standardized rating scales (Dementia Rating Scale [Mattis, 1988], Global Deterioration Scale [Reisberg et al, 1982], Total Functional Capacity Scale [Shoulson, 1981], Unified Huntington’s Disease Rating Scale [The Huntington Study Group, 1996], Shoulson and Fahn Scale [Shoulson and Fahn, 1979]). The diagnosis of HD was confirmed by genetic testing in only six of the studies.

TABLE 2

TABLE 2

TABLE 2

TABLE 2

Primary deficits of language were reported in 24 of the studies (Azambuja et al, 2012; Caine et al, 1986; Chenery et al, 2002; de Diego-Balaguer et al, 2008; Frank et al, 1996; Garcia et al, 2018; Gordon and Illes, 1987; Hinzen et al, 2018; Hodges et al, 1990; Illes, 1989; Kargieman et al, 2014; Lepron et al, 2009; Murray, 2000; Nemeth et al, 2012; Péran et al, 2004; Saldert et al, 2010; Sambin et al, 2012; Smith et al, 1988; Teichmann et al, 2005, 2006, 2008a, 2008b, 2009; Ullman et al, 1997). For the seven other studies (Hodges et al, 1991; Jensen et al, 2006; Longworth et al, 2005; Murray and Lenz, 2001; Murray and Stout, 1999; Podoll et al, 1988; Wallesch and Fehrenbach, 1988), the authors reported that language deficits in individuals with HD were not primary but were the product of nonlinguistic impairments affecting perceptual, motor, or cognitive functions.

In all of the studies, various instruments were used to assess the participants’ language function. Test batteries included the Boston Diagnostic Aphasia Examination (Goodglass et al, 2001) and the Aachen Aphasia Test (Huber et al, 1984), as well as specific tests such as the Token Test (De Renzi and Faglioni, 1978) and experimental tasks (de Diego-Balaguer et al, 2008; Hinzen et al, 2018; Kargieman et al, 2014; Lepron et al, 2009; Longworth et al, 2005; Nemeth et al, 2012; Péran et al, 2004; Sambin et al, 2012; Smith et al, 1988; Teichmann et al, 2005, 2006, 2008a, 2008b, 2009; Ullman et al, 1997). For this reason, the results of the studies are extremely variable and cannot be easily compared.

As shown in Table 2, we divided the studies into four main categories:

  • Studies (n=5) providing an overview of general deficits of language processing in individuals with HD.
  • Studies (n=9) specifically targeting production and comprehension impairment in sentence processing, spontaneous speech, and narrative discourse.
  • Studies (n=9) on lexical-semantic deficits.
  • Studies (n=10) specifically related to impairment in linguistic rule application.

There is some overlap between categories in which studies were grouped according to their main focus. In addition, two studies (Garcia et al, 2018; Jensen et al, 2006) focused on two distinct language domains and are therefore addressed in two categories.

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DISCUSSION

Individuals with HD present with various symptoms, among which chorea is the most characteristic. Motor dysfunction also affects these individuals’ communication abilities in that individuals with HD show dysarthria very early in the course of the disease. With respect to language, a wide variety of impairments were reported in the 31 studies we reviewed.

We grouped the studies into four distinct categories and here discuss general conclusions and controversies about each of the four. We also offer our conclusions on the limitations of the studies and their clinical implications.

Included in the first two categories are studies in which the authors proposed a general description of impaired performance in various language domains and tasks without, however, stating their functional origin, ie, primary or secondary deficits of language. For the studies in the other two categories, the authors explored the impairment of specific linguistic domains in individuals with HD and clearly linked them to functional localizations, ie, lexical-semantic processing and rule application.

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Category 1: General Deficit of Language Processing

In five of the reviewed studies (Azambuja et al, 2012; Caine et al, 1986; Chenery et al, 2002; Podoll et al, 1988; Wallesch and Fehrenbach, 1988), the authors provided a general characterization of language impairment in individuals with HD using comprehensive batteries of language assessments, eg, Boston Diagnostic Aphasia Examination and Aachen Aphasia Test. With respect to spoken production, the studies reported deficits of word-finding (all five studies), sentence generation (Chenery et al, 2002), sentence repetition (Azambuja et al, 2012), and narrative discourse and spontaneous speech, ie, loss of initiative and reduction of syntactic complexity (all five studies). As regards written expression, Azambuja et al (2012) reported impairment in narrative writing in 23 participants with HD, whereas Podoll et al (1988) observed the production of dysgraphic errors, mainly characterized by an omission of letters, in the writing to dictation task of the Aachen Aphasia Test battery, in 45 individuals with HD. HD also appeared to affect individuals’ receptive language, with deficits affecting the comprehension of words (Podoll et al, 1988), sentences (Azambuja et al, 2012; Podoll et al, 1988; Wallesch and Fehrenbach, 1988), commands (Azambuja et al, 2012; Chenery et al, 2002; Wallesch and Fehrenbach, 1988), and pragmatics (Chenery et al, 2002). Finally, Podoll et al (1988) reported impairment of visual processing in reading in the middle and later stages of HD, but Azambuja et al (2012) and Caine et al (1986) found reading abilities to be unaffected.

As a whole, the main conclusions of these studies, presented in Table 2, suggest that individuals with HD frequently have deficits affecting all domains of language functioning. However, the extent of these deficits, their functional origin, and their relationship with the disease severity and stage remain unclear.

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Category 2: Impairment in Sentence Processing, Spontaneous Speech, and Narrative Discourse

In the present literature review, six studies on language impairment in individuals with HD (Gordon and Illes, 1987; Hinzen et al, 2018; Illes, 1989; Jensen et al, 2006; Murray, 2000; Murray and Lenz, 2001) described participants’ spoken production abilities in spontaneous and narrative speech, and another three studies focused on participants’ spoken production abilities in sentence (Garcia et al, 2018) and discourse (Murray and Stout, 1999; Saldert et al, 2010) comprehension. In four (Gordon and Illes, 1987; Illes, 1989; Murray, 2000; Murray and Lenz, 2001) of the six studies focusing on the production of spontaneous and narrative speech, the authors reported the reduction of syntactic complexity (ie, production of shorter and syntactically simpler sentences) in individuals with HD. However, it is worth noting that the sample sizes of these four studies were quite small, making it difficult to generalize their results (Table 1). In addition to this syntactic impairment, Gordon and Illes (1987), Murray (2000), Murray and Lenz (2001), and Hinzen et al (2018) reported the presence of grammatical errors (eg, absence or inappropriate use of function words, agreement errors, missing or inappropriate referents) in the participants with HD.

Jensen et al (2006), however, suggested that syntactic complexity is preserved in individuals with HD and that the impairment only concerns the grammaticality of sentences. Moreover, according to Jensen et al (2006) and Murray and Lenz (2001), the impairment of narrative speech in individuals with HD is more suggestive of concomitant cognitive and speech deficits than primary language impairment. Using voxel-based morphometry to link specific brain degeneration patterns to narrative speech impairment, Hinzen et al (2018) also showed that, among the participants with HD, there was a significant positive correlation between quantitative measures of language (ie, mean length of utterances and number of words per minute) and the gray matter volume bilaterally in the dorsal basal ganglia. According to Hinzen et al (2018), this finding suggests that the impairment of narrative speech in individuals with HD might be explained in part by underlying alterations of the structures involved in the motor loop and that their linguistic and cognitive origin should be further explored.

In their study on sentence comprehension, Garcia et al (2018) reported that pre-HD and HD participants showed impairment in the syntactic processing of sentences. This impairment was not linked to the overall cognitive profile of the participants, suggesting that frontostriatal degeneration results in a grammatical impairment, even in the presymptomatic stage of the disease. The two studies (Murray and Stout, 1999; Saldert et al, 2010) examining discourse comprehension also reported difficulties in individuals with HD. In the Stout study (1999), nine participants with HD listened to short stories and responded to questions with yes/no answers. Compared to healthy controls, participants with HD showed specific difficulty answering questions about stated and implied details. Those authors also found positive correlations between discourse comprehension performances and dementia severity ratings as well as between discourse comprehension and measures of memory and attention. According to them, and in contrast to Garcia et al (2018), the comprehension impairment in individuals with HD might, therefore, reflect cognitive deficits. In 2010, Saldert et al also showed that participants with HD, even in the early stages of the disease, have difficulties in discourse comprehension for both explicit and ambiguous information. They found that these individuals also encounter difficulties in the comprehension of metaphors and lexical ambiguities in sentences, a deficit previously reported by Chenery et al (2002). According to Saldert et al (2010), such difficulties may result in a pragmatic impairment that might cause problems in individuals’ conversational interactions.

In summary, studies involved in the present review reported impairment for people with HD at the sentence and the discourse level in both the production and comprehension linguistic domains. In production, this impairment affects the syntactic complexity and/or grammatical form of sentences; in comprehension, difficulties are encountered for explicit and implicit information. As for the studies reported in the section on general deficit of language processing, the authors did not specify the functional origin of these difficulties and impairment. Moreover, the conclusions of some of these studies should be considered with caution considering their very small sample size, especially those studies exploring production deficits.

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Category 3: Lexical-Semantic Deficits

Nine studies in the present review specifically explored the functional origin of lexical-semantic deficits in individuals with HD.

In five of them (Frank et al, 1996; Garcia et al, 2018; Hodges et al, 1990; Kargieman et al, 2014; Smith et al, 1988), the authors specifically explored individuals’ semantic processing abilities. In one study conducted with 14 participants with HD, Hodges et al (1990) concluded that the participants’ semantic knowledge was relatively preserved. However, in the other four studies (Frank et al, 1996; Garcia et al, 2018; Kargieman et al, 2014; Smith et al, 1988), access to the semantic knowledge of objects was shown to be disrupted in individuals with HD. It must be noted that the studies of Frank et al (1996) and Smith et al (1988) were conducted with participants with mild and moderate dementia associated with HD. This may explain the presence of semantic impairment due to disease progression. Garcia et al (2018) and Kargieman et al (2014), however, found that participants with HD without dementia showed semantic impairment for both object and action concepts, whereas pre-HD participants were affected for action concepts only. According to these authors, such results suggest that frontostriatal degeneration leads to an action-semantic impairment that might constitute an early marker of HD. Moreover, Garcia et al (2018) suggested that the impairment for object knowledge may reflect the widespread atrophy in temporal and parietal regions observed in individuals with HD as the disease progresses.

In two of the nine studies on lexical-semantic deficits in HD, difficulties in retrieving lexical forms of concepts were reported in narrative and spontaneous speech (Gordon and Illes, 1987; Illes, 1989), whereas Jensen et al (2006) found that participants with HD did not display more word-finding difficulties than controls in a picture description task.

Lexical access was also specifically explored in six studies through word generation (Lepron et al, 2009; Péran et al, 2004) and picture naming (Frank et al, 1996; Hodges et al, 1990, 1991; Smith et al, 1988) tasks. Inconsistent results were reported for verb generation tasks. Péran et al (2004) showed that, as a whole, the performance of the participants with HD was impaired in noun and verb generation tasks; a specific deficit for verb production was found only in participants with HD with dementia. According to the authors, this verb production deficit could be explained by the impairment of the semantic representation of actions in HD, resulting from the pathology affecting the basal ganglia circuitry at later stages of the disease. Also using a word generation task, Lepron et al (2009) reported difficulties in 12 HD participants without dementia in terms of reaction time and proportion of errors, without a difference between nouns and verbs.

Positron emission tomography data also revealed that the brain regions (the anterior cingulate and inferior frontal gyri) usually involved in lexical retrieval were not activated in the participants with HD, but activation was found in those regions involved in the phonological loop (left supramarginal gyrus) and effortful retrieval (right inferior frontal gyrus), suggesting that they compensate for word selection processes.

It is worth noting that Jensen et al (2006) also found that, compared to the control group, participants with HD (n = 6) produced fewer verbs in a picture description task.

Finally, impairment on the Boston Naming Test (BNT) (Kaplan et al, 1983) was found in four studies in this category. However, picture naming tests such as the BNT include various processing steps involving visual perception and recognition, activation of semantic representations, and lexical retrieval, which makes it difficult to identify the functional locus of impairment. In two of the studies (Frank et al, 1996; Smith et al, 1988), the authors reported impairment on the BNT in HD participants with mild and moderate dementia. They attributed this impairment to a breakdown in the organization of lexical-semantic representations (Smith et al, 1988) or to semantic access disruption (Frank et al, 1996). In their 2-year longitudinal study, Hodges et al (1990) also reported impairment on the BNT by participants with HD. However, the authors concluded that, because the participants’ performance on tasks exploring semantic memory remained stable over time, their semantic knowledge was unimpaired and was not at the origin of their naming deficit. The performance of the participants with HD on the BNT was also impaired in the Hodges et al (1991) study. There the participants with HD produced significantly more visual-based errors than did the healthy participants. The authors interpreted this result as reflecting a disruption of perceptual analysis rather than a semantic impairment.

In summary, except for Hodges et al (1990, 1991), the studies reviewed here suggest that the semantic processing abilities of object and action concepts are impaired in individuals with HD, including the presymptomatic stage of the disease. As a whole, these studies also show impairment of lexical access to concrete nouns in individuals with HD, although the results remain inconsistent for verbs.

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Category 4: Impairment in Linguistic Rule Application

According to the declarative/procedural model proposed by Ullman (2001) and Ullman et al (1997), declarative memory is sustained by medial temporal lobe structures and underlies the mental lexicon in which the phonological forms of words are stored. Procedural memory, rooted in the frontostriatal circuits of the brain, sustains the acquisition and computation of rule-based linguistic procedures. Support for this model comes from studies conducted with individuals suffering from poststroke aphasia (Ullman et al, 2005), Parkinson disease (Ullman et al, 1997), and Alzheimer disease (Walenski et al, 2009). In 10 studies included in the present review, HD was used as a clinical model to test the assumptions of the declarative/procedural model (Ullman, 2001).

In the only study on the perception of phonemes and the application of phonological rules in words and sentences, Teichmann et al (2009) found reduced abilities of phoneme discrimination in sentences in the 15 participants with HD. The authors concluded that the striatum does not play a role in phonological rule application but rather underlies phonological short-term memory capacities engaged in sentence listening.

In 2008, de Diego-Balaguer and colleagues assessed the ability to extract lexical rules in 13 pre-HD and 43 HD participants using a simplified artificial language. The performance of the pre-HD participants was unimpaired, but the participants with HD showed significant impairment in word-rule learning. According to the authors, these results suggest that striatal degeneration in individuals with HD impairs the earliest stages of language learning when no semantic information is available. Moreover, this impairment was significantly correlated with neuropsychological measures of working memory and attentional control, suggesting their involvement in language acquisition.

In the morphological domain, five studies showed that individuals with HD have difficulty applying infrequent morphological rules, leading to errors in verb conjugation (Nemeth et al, 2012; Teichmann et al, 2005, 2008b; Ullman et al, 1997) and acceptability judgment on conjugated verbs (Teichmann et al, 2006). The authors attributed this impairment to striatal damage that may reduce inhibition of the most frequent linguistic rules, thereby leading to the production of overregularization errors. In these five studies, the conjugation of irregular verbs, which is retrieved from lexical memory, was less impaired than the conjugation of regular verbs, which is rule-governed. This dissociation was interpreted as support for the declarative/procedural model. In contrast, Longworth et al (2005) did not find any differences between regular and irregular verbs in past tense production and comprehension tasks in a study conducted with 10 individuals with HD. These patients had difficulties suppressing semantically appropriate alternatives when trying to conjugate novel verbs. This result led the authors to conclude that the striatum does not have an essential role in morphological rule application, but rather plays a restricted, non-language-specific, inhibitory role in language processing.

Finally, the syntactic abilities of individuals with HD were explored in four of the studies we reviewed (Sambin et al, 2012; Teichmann et al, 2005, 2008a, 2008b). The authors of these four studies found that participants with HD had difficulties applying syntactic rules, suggesting that the striatum is involved in syntactic rule processing.

In summary, according to 8 of the 31 studies reviewed in this article, individuals with HD present with difficulties in word-rule extraction and morphological and syntactic rule application resulting from striatal damage. However, involvement of the striatum in morphological rule application was challenged in the Longworth et al (2005) study.

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CONCLUSIONS

HD has detrimental effects on an individual’s speech, mainly due to dysarthria. Based on the studies reported in this literature review, one can also conclude that language is impaired in individuals with HD, whatever the disease stage.

Results from the studies presenting a general overview of language suggested that individuals diagnosed with HD frequently have deficits affecting all domains of language functioning. Other studies focusing on specific language domains showed that individuals with HD may have difficulties producing and understanding sentences and discourse, processing semantic representations of object and action concepts, retrieving lexical forms of concrete nouns, and extracting and applying linguistic rules. Moreover, deficits affecting the production of narrative discourse (Hinzen et al, 2018), sentence comprehension (Garcia et al, 2018), semantic processing of action concepts (Garcia et al, 2018), and verb conjugation (Nemeth et al, 2012) have been reported in individuals at the presymptomatic stage of the disease. At the other end of the disease continuum, a worsening of language impairment was found in participants with dementia associated with HD in tasks exploring lexical access to nouns and verbs (Frank et al, 1996; Péran et al, 2004; Smith et al, 1988) and discourse comprehension (Murray and Stout, 1999). Figure 2 is a graphic summary of the main results of our literature review.

FIGURE 2

FIGURE 2

The studies revealed some inconsistencies in findings related to reading abilities, syntactic complexity in sentence production, semantic processing, and application of morphological rules in verb conjugation. A possible explanation may lie in the heterogeneity of the studies with respect to clinical samples (eg, diagnostic process, size) and methods used. As a whole, the research samples in the studies were small, and participants were at different stages of the disease. In HD, the neurodegeneration extends mainly from the striatum to a wide cortical degeneration, so the deficits in one domain of language may precede others over the course of the disease, thus explaining some of the seeming discrepancies between studies. Various standardized tests were used to measure language abilities. In addition, experimental tasks were used in a few of the studies to measure very specific aspects of language and to test theoretical models of language processing. Therefore, further studies conducted within a cognitive framework of assessment are needed to identify the functional origin of language disturbances in individuals with HD. These studies should focus particularly on discourse organization and planning, pragmatics, written production, and reading abilities, for which the picture remains unclear.

Considering the important contribution of the frontostriatal regions to cognitive processes such as executive functions and working memory (Hedden and Gabrieli, 2010; Landau et al, 2009), it is notable that only a few of the studies in our review explored the primary nature of language impairment in individuals with HD. Further studies are needed to disentangle specific language impairment in individuals with HD from those resulting from general cognitive disruptions.

Finally, clinical studies are also needed to determine the impact of language impairment in individuals with HD on functional communication in daily living, as well as to chart their progression over the course of the disease.

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                    Keywords:

                    Huntington disease; language impairment; assessment

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