Introduction

Multiple sclerosis (MS) is a chronic, progressive, autoimmune disease of the central nervous system, ‘characterized by inflammation, demyelination, followed by neurodegeneration’ [1]. MS is caused by damage to the myelin sheath, i.e., the protective covering of nerve fibers (axons) disrupting in that way the transmission of the nerve impulses to and from the central nervous system leading to specific clinical symptoms [2]. It also damages the nerve cell bodies (and their axons) in the brain, spinal cord and optic nerves affecting the transmission of visual information from the eye to the brain [3]. MS has traditionally been characterized as a persistent inflammatory ailment affecting the central nervous system [4]. This results in significant focal lesions in the white matter of the brain and spinal cord, marked by primary demyelination and varying degrees of axonal loss [4]. The presence of a dense glial scar in long-standing established lesions is associated with a profound astroglia reaction in the brain of individuals with MS, which in turn is linked to demyelination and neurodegeneration [4,5,6]. As the disease progresses the cerebral cortex shrinks leading to cortical atrophy [5, 6]. In addition, neuronal impairment has the potential to impact various bodily functions such as vision, sensation, coordination, movement, and bladder or bowel control [7]. This can lead to a range of neurogenic lower urinary tract symptoms in individuals with multiple sclerosis (MS), which have been reported to significantly impact their quality of life [8, 9]. MS can be diagnosed at any age; it most commonly manifests itself between ages 20 to 40, while the average age of onset is 30 years [10]. MS affects women twice as much as men [11]. MS etiology is still unclear, but it seems to be a disorder with great heterogeneity both among patients as well as within the same patient [12]; it is multifactorial attributed to both environmental and genetic factors [13].

There is a great variety of signs and symptoms in MS and many patterns have been identified: benign; a relapsing–remitting course; a secondary progressive type; and a primary progressive type [14]. In MS patients, the initial neurological signs and symptoms are subclinical, lasting for at least 24 h. This clinical presentation is known as clinically isolated syndrome (CIS) [14]. MS patients exhibit a range of symptoms reflecting multifocal lesions within the central nervous system affecting the motor, sensory, and visual systems. Thus, MS has a great impact on quality of life, as patients suffer from fatigue and mental difficulties [15], emotional distress [16], including depression, anxiety, negative mood, and trauma symptoms [17], alongside the sensory and motor limitations. Impairment of the motor system can subsequently affect the quality of communication [18].

The literature indicated that a large proportion of the MS population is impaired on standard neuropsychological tests, including verbal skills [18, 19]. Regarding the neuroanatomical pathophysiology of MS, cortical and subcortical brain structures have been identified to play a crucial role in the adjustment and coordination of the movement aspects of speech [20]. Communication is disrupted by the occurrence of motor speech disorders (dysarthria) that potentially affects all speech subsystems including, respiration, phonation, resonance, articulation, and prosody, along with impairments in receptive and expressive language [21]. Dysarthria is a prevalent motor speech disorder observed in individuals with MS, which restricts their communicative capacity in social situations and consequently impacts their overall quality of life [22, 23]. According to existing literature [24], individuals with MS may exhibit three distinct types of dysarthria, namely spastic dysarthria, ataxic dysarthria, and mixed dysarthria. Unique patterns of speech symptoms characterize these types of dysarthria. Nevertheless, it is worth noting that dysarthria is not the sole speech impairment that can be observed in the speech of individuals with multiple sclerosis. [24]. In addition, in MS, the lack of voluntary coordination of muscle movements is referred to as ataxia, which can cause speech problems [25]. Many difficulties concerning other domains such as voice, fluency and rate of speech are also noted which in turn have a significant effect on patients’ everyday communication [26]. The crucial responsibility of speech and language pathologists is to not only recognize and tackle speech impairments, but also to assist individuals in engaging in their daily routines by acquiring and implementing compensatory techniques to mitigate these challenges and enhance their overall well-being. Moreover, a recent meta-analysis has indicated that respiratory muscle training can enhance lung volumes and respiratory muscle strength in neuromuscular conditions such as multiple sclerosis (MS)[27].

Assessment tools

Given that the preliminary indications of the ailment often become apparent during the initial stages of adulthood, it is imperative to conduct neurological and neuropsychological assessments at an early stage of the diagnostic procedure for individuals who have been diagnosed with MS or are suspected to have MS. In this field, comprehensive neuropsychological test like the Minimal Assessment of Cognitive Function in MS [28], the Brief International Cognitive Assessment for MS [29] and the shortened version of Rao's Brief Repeatable Battery [30] attempting to cover the cognitive domains most commonly affected by MS. Moreover, in studies with MS patients [31, 32] the National Adult Reading Test [33, 34] Second Edition was used. The National Adult Reading Test is a test of premorbid intellectual functioning [35]. The Pyramids and Palm Trees Test was created in 1992 by Howard and Patterson [36] to measure the capacity to access detailed semantic information about words and objects and was used by researchers in patients with MS [31, 37].

Furthermore, The Expanded Disability Status Scale (EDSS) [38] is the most widely used instrument for evaluating disability in MS patients [39].

The evaluation of speech and language parameters, particularly verbal learning, has been conducted using various assessment tools in studies with MS patients. One such tool is the Rey Auditory Verbal Learning Test [40, 41]. Furthermore, in 2020 developed the Communication and Language Assessment Questionnaire for persons with Multiple Sclerosis [42], a reliable and valid tool that assesses self-perceived communication and language function in MS [42]. Furthermore, the Addenbrooke’s Cognitive Examination Revised [43] was used in studies for MS patients [44, 45] and contains 5 domains, between them one fluency domain and one language domain [43].

More specifically, to evaluate speech in MS patients, studies used many tools for that scope such as (a) the Assessment of Intelligibility of Dysarthric Speech Sentence Intelligibility Task [46, 47], (b) the speech pathology-specific questionnaire for patients with multiple sclerosis [48, 49], (c) the Dysphonia Severity Index [50], the GRBAS scale [51,52,53], the Voice Handicap Index, a self-reporting tool [54], the standardized speech tasks [55, 56], the Formant Centralization Ratio [57, 58] has been used as an acoustic metric of dysarthric speech. In terms of tongue control and function in MS a study [59] suggest that the quantitative motor measurement [60] of tongue function might proof useful efficient method to assess motor dysfunction in MS. To assess ataxia in MS patients, the Scale for the Assessment and Rating of Ataxia was developed [25, 61]. Each word is presented individually, and subjects are required to read each aloud [35]. The aforementioned assessment tools and measures contribute to a comprehensive evaluation of speech (voice, motor control) in individuals with MS, providing valuable insights for diagnosis, monitoring, and treatment planning.

This narrative review aimed to identify, enhance, update, and delve into our present comprehension of the type of speech deficits observed in patients with MS and the methodology (assessment tools) that studies followed that were published from May 2015 to June 2022.

Materials and methods

Α literature search was conducted on the MedLine and Scopus bases in June 2022 with the keywords ‘multiple sclerosis’, ‘speech disorders’, ‘dysarthria’, ‘communication disorders’, ‘phonological disorders’, ‘speech pathology’, ‘anomia’, ‘dysphonia’ and ‘voice problems’. We did not use PRISMA guidelines while conducting our review as it is a narrative review of the existing literature.

The dataset of the current study spanned from May 2015 to June 2022. There were specific eligibility criteria applied for the inclusion of studies in the current review. These were the following: For a study to be included it had to (1) have original data, (2) be conducted on patients with MS who exhibit speech difficulties, (3) focus on speech deficits that patients with MS face, (4) be written and presented in English, and (5) be published from 2015 to 2022. The exclusion criteria were the following: (1) no original data (letters to editor or other reviews were excluded), (2) the study targeted language and cognitive disorders, (3) the study targeted dysphagia, (4) the study targeted treatment methods, and (5) the study was published before 2015. Following database screening, titles and abstracts were reviewed to verify the inclusion criteria. An additional literature search was conducted for related references included in the manuscripts. After duplicates were removed, the suitability of the scanned abstracts was assessed by two independent individuals. Then the full texts were retrieved and read making sure that they met the eligibility criteria applied for this review. Conflicts were resolved after discussion between the authors. Following this, the results of the studies were compiled and presented in two different tables. In the first table, general information and sample characteristics of each study are provided. The second table includes a summary of the results obtained from speech assessments along with the main findings of the studies.

Results

The data sets of the studies reviewed in this paper were presented according to the following variables: type, mean age of the participants, gender, tests used to establish diagnosis and additional details. With regard to results, all studies used control group of healthy participants. Most of studies reported that EDSS was used to establish diagnosis. Furthermore, almost all studies clearly reported that the patients involved were over 18 years of age except from two studies that had no reference that the patients were over 18 years of age. Summary in the studies 281 males and 628 females recruited. Regarding the exclusion criteria, an acute upper airway respiratory infection, patients to be relapse-free for a month at least prior to testing, voice disorders, larynx malignance, no other neurological disorders other than MS, no vision or hearing problems were required. In addition, the inclusion criteria, patients’ ability to fill in questionnaires, a neurologically confirmed diagnosis of MS, symptoms of dysarthria. More information is shown in Table 1.

Table 1 Study information and sample characteristics of the articles included in the review
Full size table

For each study under review, the data obtained from the tests of speech assessment along with the results that were drawn, discussed and the main conclusions are presented in Table 2.

Table 2 Summary of the aims, speech assessments, and main findings of the studies
Full size table

Most of studies referred about the effects of MS on motor movement are which includes muscle weakness and what is the reason for muscle weakness, spasticity and loss of coordination and what are the symptoms of MS. As a result, the motor system is impaired and MS patients in their majority also deal with different forms of dysarthria, although not all studies have reached the same conclusion. Dysarthria is considered the primary cause of communication deficit in MS, yet patients with MS present with concurrent cognitive deficits that can interfere with effective communication.

Moreover, the articulation system is impaired. The studies indicated that MS affects the articulators per se and consequently patients’ speech rate. Articulation was analyzed across the studies and was characterized by consonant imprecision, decreased word output rate and slow vowel transitions likely due to slow tongue movements. Findings recognized that MS is a condition that can negatively affect the phonation system. MS patients face dysphonic problems, although there remains to define the severity level and its correlation to other factors. Voice quality was studied in studies [51, 62]. The studies that encompassed the documentation of perception primarily adopted a descriptive approach. The majority of multiple sclerosis patients were evaluated as having vocal impairments by speech pathologists. However, respiratory issues and voice impairments were comparatively given less attention and were not thoroughly examined. Furthermore, studies reported on the presence of prosody in patients [51, 63]. The speech rate of patients with MS was observed to be reduced and slower, as evidenced by a decrease in the number of syllables per second and a lower production of words per minute when compared to healthy control groups. In relation to tasks involving reading and speech, it was observed through acoustic analysis that individuals with multiple sclerosis exhibited a greater frequency and duration of pauses.

With regard to the main outcome of the studies, dysarthria seems to be a common symptom in MS patients in most of the studies as well as in two studies [62, 69]. In the studies [62, 68] there was an evaluation of acoustic analysis along with a vowel metric analysis to evaluate speech features and identify significant patterns in voice samples of patients with MS [68]. The authors considered to better define disordered voices against healthy ones, acoustic analytic approaches have been suggested for implementation of this procedure. In one study [70] problems with speech extraction were identified. Speech timing was significantly slower for MS patients with dysarthria compared to MS patients without dysarthria. In addition, silent pause durations also significantly differed for MS patients with both dysarthria and cognitive impairment compared to MS patients without either impairment [70]. Furthermore, in other studies [55, 67], Expanded Disability Status Score outcomes were used in support of the general idea that speech impairment is strongly correlated to the level of MS severity. However, in this study [51] due to the limited number of subjects and due to the limited parameters, that they investigated could not draw any solid conclusions about the level of severity of dysphonia that MS patients exhibited.

Evidence from the studies [55, 61] suggests that MS patients demonstrate abnormalities in their speech rate. Specifically, the studies revealed that the pace of speech was comparatively reduced and there was a rise in its variability. Furthermore, it was observed that individuals with multiple sclerosis exhibited an augmentation and extension of the pauses present in their speech. The amplitude of the vocal inflections exhibited a reduced variance, resulting in a uniform and unvarying quality of speech. Cerebellar dysfunction was found to be associated with subclinical voice and tremors.

Finally, the pronunciation of vowels and consonants was inaccurate. A study [69] found that MS patients exhibited slow articulation during reading and reduced sequence rhythm during rapid syllable repetition, as tests for joint rhythm, auditory speech and sequence rhythm indicated. Furthermore, 2 studies [59, 66] addressed the fact that there are motor problems, trembling and problems with the joints in MS patients. It was found that there were reduced ratios of maximal velocity displacement of the lower lips and jaw reduced peak velocities of the tongue. Based on these results it was pointed out that the ability to move the tongue with adequate speed during speech was significantly impaired in patients with MS, providing thus an explanation for their slowed speech rate. Therefore, it was suggested that compensatory strategies are in need during speech treatment in order to ‘maximize speech clarity in the presence of the impaired tongue motor performance’.

Discussion

The aim of the current literature review was to identify, record, discuss, and delve into the type of speech deficits observed in patients with MS. MS patients can exhibit a range of symptoms in various domains including speech as well. From the aforementioned studies, it is apparent that MS patients face difficulties concerning a number of components of speech including phonation, oral diadochokinesis, articulation, and prosody. These findings have been previously reported in the MS literature and it seems that they are prominent in the majority of patients. Considering the speech-related findings of the studies reviewed above, one of the most commonly identified symptoms of MS was articulation difficulties. In some of the studies reviewed the patients included, exhibited dysarthria [63, 68, 70], while in another study, actual cases of dysphonia were reported [51]. In other studies, it was also noted that MS patients also face a range of deficiencies regarding speech articulators per se as a result of an impaired motor system [59, 66]. However, more research needs to be carried out in order to further address the issue of the way that these deficiencies affect the articulation system of MS patients.

The second most reported deficit in MS patients was impaired phonation and a slow speech rate [51]. These findings are in line with the MS literature [71] according to which phonation difficulties such as vocal deficits, breathiness, volume abnormalities, etc., are present in many MS patients. Slow speech rate and long/extended pauses are also commonly observed and described in MS patients [69, 70]. Even though respiratory problems and resonatory impairments were less commonly described than other deficits [67], they still significantly impact patients' everyday lives. However, studies investigating these aspects are scarce, and because they do not follow a standard methodology, no solid conclusions can be drawn. The objective of this review is to enhance and revise our present comprehension of dysarthria in individuals with multiple sclerosis (PwMS),

Conclusion

The current literature review aimed to enhance, update, and delve into our present comprehension of a) the type of speech deficits observed in patients with MS, and b) the methodology (assessment tools) studies followed. In the literature on MS, speech difficulties are notable among patients due to an impairment of the motor system and its underlying anatomical structures. Nonetheless, the main bulk of studies indicates that patients with MS develop dysarthric characteristics. It is important for speech and language therapists working with MS patients to be aware of possible cognitive-linguistic impairments and take this into account when assessing, managing, and intervening. Taking in mind these, there is a need for more studies to be conducted that will apply a more systematic methodological approach and similar inclusion criteria to categorize various speech manifestations better and enhance our understanding of the patterns that may be (or not) associated with the specific clinical subtypes of MS. By doing so better intervention and treatment methods can be discovered and applied that will improve the communicational function, the psychological well-being, and the quality of MS patients’ life.

Future directions

Finally, it would be interesting for future research to systematically investigate possible correlations between the different clinical types of MS and speech deficits. It is essential, though, to stress that to achieve common ground among other studies, similar methodologies and inclusion criteria should be applied. To this end, it is evident that more research must be carried out about the etiology of MS, the neuroanatomical correlates, along with the definition of clinical, cognitive, and linguistic patterns present in each phase to develop better methods of intervention and treatment of MS patients and improve their communication ability and quality of life.