1. IntroductionMyotonic dystrophy type 2 (DM2) is an autosomal dominant, late-onset, slowly progressive, disease [1Clinical and genetic heterogeneity in myotonic dystrophies., 2Hilbert JE Kissel JT Luebbe EA Martens WB McDermott MP Sanders DB et al.Methods and data from the National Registry of Myotonic Dystrophy (DM) and Facioscapulohumeral Muscular Dystrophy (FSHD)., 3The myotonic dystrophies: Molecular, clinical, and therapeutic challenges.]. It is caused by the expansion of tetranucleotide CCTG repeats in a non-coding region of the CNBP gene [[4]Liquori CL Ricker K Moseley ML Jacobsen JF Kress W Naylor SL et al.Myotonic dystrophy type 2 caused by a CCTG expansion in intron 1 of ZNF9.]. The disease is typically manifested by mild proximal muscle weakness, mild and usually fluctuating myotonia, presenile cataracts, cardiac problems, and endocrinopathy [[3]The myotonic dystrophies: Molecular, clinical, and therapeutic challenges.].Previous studies also indicated the central nervous system (CNS) involvement in DM2. Histopathological post-mortem analysis showed loss of nerve cells, presence of eosinophilic intracellular inclusions, neurofibrillary tangles, as well as Marinesco bodies in different regions of the DM2 brain [[5]Maurage CA Udd B Ruchoux MM Vermersch P Kalimo H Krahe R Delacourte A Sergeant N. Similar brain tau pathology in DM2/PROMM and DM1/Steinert disease.]. Using MRI techniques, the most significant macrostructural impairments were observed within the white matter including white matter hyperintensity lesions (WMHL) and atrophy, while the grey matter showed less volume decrease or even remained unchanged [6Weber YG Roebling R Kassubek J Hoffmann S Rosenbohm A Wolf M et al.Comparative analysis of brain structure, metabolism, and cognition in myotonic dystrophy 1 and 2., 7Minnerop M Weber B Schoene-Bake JC Roeske S Mirbach S Anspach C et al.The brain in myotonic dystrophy 1 and 2: Evidence for a predominant white matter disease., 8Peric S Rakocevic-Stojanovic V Meola G. Cerebral involvement and related aspects in myotonic dystrophy type 2.]. Functional neuroimaging indicated reduced glucose uptake and decreased blood flow mostly in the frontal and temporal lobes [[6]Weber YG Roebling R Kassubek J Hoffmann S Rosenbohm A Wolf M et al.Comparative analysis of brain structure, metabolism, and cognition in myotonic dystrophy 1 and 2.,[9]Peric S Brajkovic L Belanovic B Ilic V Salak-Djokic B Basta I et al.Brain positron emission tomography in patients with myotonic dystrophy type 1 and type 2.]. Neuropsychological testing showed that executive and visuospatial dysfunction are the most common impairments in DM2. Main limitation of the previous studies examining cognitive impairment in DM2 was a small sample of subjects (from nine in the study of Weber et al. to 46 in the study of Peric et al.) [[6]Weber YG Roebling R Kassubek J Hoffmann S Rosenbohm A Wolf M et al.Comparative analysis of brain structure, metabolism, and cognition in myotonic dystrophy 1 and 2.,[10]Romeo V Pegoraro E Ferrati C Squarzanti F Sorarù G Palmieri A et al.Brain involvement in myotonic dystrophies: Neuroimaging and neuropsychological comparative study in DM1 and DM2.,[11]Peric S Rakocevic Stojanovic V Mandic Stojmenovic G Ilic V Kovacevic M Parojcic A et al.Clusters of cognitive impairment among different phenotypes of myotonic dystrophy type 1 and type 2.]. Another limitation is that the influence of cognitive impairment on patients’ everyday functioning was not examined thoroughly, although our group found that general cognitive performance, and especially memory sub-score correlated with quality of life in DM2 [[12]Rakocevic Stojanovic V Peric S Paunic T Pesovic J Vujnic M Peric M et al.Quality of life in patients with myotonic dystrophy type 2.]. Also, it is not clear what tests to use to detect cognitive impairment in DM2.

The aim of our study was to assess cognitive functions in a large cohort of patients with DM2 using an extensive battery of neuropsychological tests, and to assess percentage of patients with significant cognitive impairment that affects patients’ activities of daily living. We also sought to see which sub-group of DM2 patients is particularly prone to have significant cognitive impairment and how it may be easily diagnosed.

2. Patients and MethodsWe included DM2 patients that were neuropsychologically tested at the time of diagnosis of DM2 at the Neurology Clinic of the University Clinical Center of Serbia in the period from 2009 to 2019. All patients were adults. Final number of tested patients was 76 (having at least one test performed in each cognitive domain described below), whil 68 patients had all tests for different cognitive domains performed. Regarding cognitive screening tests, Addenbrooke's Cognitive Examination Revised (ACE-R) was performed in 66 patients, while Raven's Standard Progressive Matrices (RSPM) as a measure of general intellectual level were performed in 65 cases. It is of note that patients reported in our previous paper [[11]Peric S Rakocevic Stojanovic V Mandic Stojmenovic G Ilic V Kovacevic M Parojcic A et al.Clusters of cognitive impairment among different phenotypes of myotonic dystrophy type 1 and type 2.] were also included in this analysis if it was their first neuropsychological assessment at the time of diagnosis and if they underwent the same battery of tests. So, 37 of 46 patients from the previous paper plus 39 new patients were included in this study. In all patients, the diagnosis of DM2 was confirmed by molecular genetic analysis of the expansion of CCTG repeats in the CNBP gene using the repeat-primed polymerase chain reaction [[13]Kamsteeg EJ Kress W Catalli C Hertz JM Witsch-Baumgartner M Buckley MF et al.Best practice guidelines and recommendations on the molecular diagnosis of myotonic dystrophy types 1 and 2.]. This method is not able to inform on the number of CCTG repeats. Since neuropsychological testing was conducted as part of a standard diagnostic procedure, and anonymous patient data were used for the purpose of this study, patients were not required to sign informed consent to participate. The research was approved by the Ethical Board of the Neurology Clinic, Clinical Center of Serbia.Sociodemographic data including age, gender and education were obtained by completing a general questionnaire prior to the neuropsychological testing. Main clinical parameters were obtained from patients’ medical records. Muscle strength was assessed using the Medical Research Council (MRC) scale [[14]Kleyweg RP van der Meche FGA Schmitz PI. Interobserver agreement in the assessment of muscle strength and functional abilities in Guillain-Barré syndrome.], including following muscle groups: shoulder adductors and abductors; elbow flexors and extensors; wrist and fingers flexors and extensors; hip flexors, extensors, adductors, and abductors; knee flexors and extensors, and plantar and dorsal ankle and toe flexors. We summed the scores of the weakest muscle group of proximal and distal upper limbs, and proximal and distal lower limbs to obtain the final MRC score with the maximum score being 20 [[15]Peric S Mandic-Stojmenovic G Stefanova E Savic-Pavicevic D Pesovic J Ilic V et al.Frontostriatal dysexecutive syndrome: A core cognitive feature of myotonic dystrophy type 2.,[16]Peric S Maksimovic R Banko B Durdic M Bjelica B Bozovic I et al.Magnetic resonance imaging of leg muscles in patients with myotonic dystrophies.]. Presence of myotonia and myopathy was examined using needle electromyography (EMG). Cardiac examination consisted of physical examination, electrocardiography (ECG), and echocardiography, where ejection fraction below 55% was considered pathological. Pulmonary function tests were performed using spirometry, where forced vital capacity (FVC) below 80% was considered pathological. The presence of glucose metabolism impairment, thyroid function disorders, as well as levels of triglycerides, total cholesterol, low density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol were also analyzed. The levels of triglycerides above 1.7 mmol/l, total cholesterol above 5.2 mmol/l, LDL above 3.4 mmol/l and HDL below 1.0 mmol/l were considered pathological, according to the norms of the University Clinical Centre of Serbia laboratory.For neuropsychological testing, we used an extensive battery of classic pen-and-pencil neuropsychological tests, performed by experienced neuropsychologists in a pleasant and calm environment in the morning, after breakfast. Before these tests, Beck Depression Inventory (BDI) was administered and interview with psychologist was made to exclude significant depression that can interfere with cognitive testing. No one of the patients had BDI above 20 suggesting at least moderate depression. Neuropsychologist did not find significant clinical depression in any of the tested patients. Also, no DM2 patients used antidepressants. RSPM test was used as a measure of the general intellectual level. The obtained scores were converted into percentiles based on patients’ age, and scores below the 25th percentile were considered pathological [[17]Raven's Progressive Matrices
More on norms, reliability, and validity.]. Global cognitive status was assessed using the Mini Mental State Examination (MMSE) and ACE-R [[18]Mioshi E Dawson K Mitchell J Arnold R Hodges JR. The Addenbrooke's Cognitive Examination Revised (ACE-R): a brief cognitive test battery for dementia screening.]. A score less than 24 on MMSE and score less than 83 on ACE-R indicated the presence of a cognitive impairment [[18]Mioshi E Dawson K Mitchell J Arnold R Hodges JR. The Addenbrooke's Cognitive Examination Revised (ACE-R): a brief cognitive test battery for dementia screening.,[19]Dijagnostički testovi u neuropsihologiji.].For the remaining neuropsychological tests, results were considered abnormal if they were two standard deviations (SDs) out of the mean value of healthy population according to the following references. Patients underwent analysis of five cognitive domains: attention, executive function, visuospatial function, memory, and language. Attention was assessed using the Trail Making Test A (TMT-A) [[20]Chang L Anderson T Migneco OA Boone K Mehringer CM Villanueva-Meyer J et al.Cerebral blood flow, magnetic resonance imaging, and neuropsychological tests.]. Executive functions were assessed by the Trail Making Test B (TMT-B) [[21]A compendium of neuropsychological tests: Administration, norms and commentary.], the Wisconsin Card Sorting Test (WCST) [[21]A compendium of neuropsychological tests: Administration, norms and commentary.], including number of categories and sets, and Phonemic Fluency (PF) [[19]Dijagnostički testovi u neuropsihologiji.]. Executive dysfunction was defined as two SDs below norm on WCST set and/or categories and/or TMT-B. Copy of the Rey-Osterrieth Complex Figure (ROCF) was used to assess visual construction abilities, while recall of ROCF was used as a measure of visual memory [[21]A compendium of neuropsychological tests: Administration, norms and commentary.]. Immediate and delayed verbal memory were examined using the Rey Auditory Verbal Learning test (RAVLT) [[21]A compendium of neuropsychological tests: Administration, norms and commentary.,[22]Geffen G Moar KJ O'Hanlon AP Clark CR Geffen LB Performance measures of 16- to 86-year-old males and females on the auditory verbal learning test.]. Memory impairment was considered if patients scored two SDs below norm on recall of ROCF (if copy of ROCF was normal) and/or RAVLT immediate memory total score and/or RAVLT recognition. Boston Naming Test and the Categorical Fluency (CF) were used to examine language functions [[23]The assessment of aphasia and related disorders.]. Impaired language function was defined as a result two SDs below norm in any of these two tests. Cognitive impairment was defined as affection of at least two different cognitive domains for at least two SDs out of norm.

Everyday functioning of our patients during at least last six months was assessed by a free interview with patients and their family members / caregivers in all cases. If discrepancy between patients and caregivers was noted, additional discussion was performed to achieve the most realistic answer that was finally estimated by a neuropsychologist. Following activities were included: memory and learning (important dates, names, phone numbers), finding personal belongings, conversation, handling the household equipment, handling new devices, do household tasks, preparing food, watching TV, washing clothes, washing dishes, taking medications, reading, completing forms, visuospatial orientation in known and new space, reading maps, driving, shopping, explaining a problem, planning activities, and doing at two simple cognitive tasks at the same time. Patients and family members were strictly asked to differentiate if certain activity was limited to their cognitive problems and not by physical disability caused by their disease. Significant cognitive impairment in DM2 was defined as A) cognitive impairment as defined above + B) functional incapacity in everyday life in at least two above-mentioned activities lasting at least six months reported by patients themselves or by their family members. We further analysed differences between DM2 patients with vs. without significant cognitive impairment, including their MMSE and ACE-R scores.

2.1 Data analysis

Among the descriptive statistical methods, means, standard deviations and proportions were used. Mann-Whitney U test and Student's t test were used to examine differences between two groups. Spearman's rank coefficient was used to examine correlations. The level of statistical significance was p<0.05.

3. ResultsSociodemographic, clinical and laboratory data of patients are presented in Table 1. The study included 76 patients (69.7% women). Patients’ mean age at testing was 51.6 ± 11.5 years with mean disease duration of 12.9 ± 12.1 years.

Table 1Main sociodemographic, clinical, and laboratory data of DM2 patients

MRC Medical Research Council scale, EMG electromyography, ECG electrocardiography, EF left ventricle ejection fraction, FVC forced vital capacity, HDL high-density lipoprotein, LDL low-density lipoprotein

RSPM showed that half (50.8%) of DM2 patients had average intelligence. Only four (6.2%) patients were below average, and as many as 28 (43.1%) were above (Table 2). Cognitive screening tests indicated presence of cognitive deficits in only four (5.5%) patients according to MMSE and 17 (25.8%) patients according to ACE-R (with mean ACE-R score of 74.4 ± 9.6 in this group).

Table 2Results of the general intellectual level and cognitive screening tests in DM2 patients

RSPM Raven's Standard Progressive Matrices, MMSE Mini Mental State Examination test, ACE-R Addenbrooke's Cognitive Examination-Revised

Results of neuropsychological tests are presented in Table 3. About quarter of DM2 patients had impairment on BNT, copy of ROCF, and TMT-B.

Table 3Results on neuropsychological tests in DM2 patients

Data are presented as mean ± standard deviation or as a percentage (%) of patients.

ROCF Rey-Osterrieth Complex Figure, TMT-B Trail Making Test B, WCST Wisconsin Card Sorting Test, TMT-A Trail Making Test A, RAVLT Rey Auditory Verbal Learning Test, BNT Boston Naming Test

Numerous correlations of neuropsychological test findings with sociodemographic and clinical data were observed (Table 4), including with patients’ education, their age at onset and at testing, and muscle strength.

Table 4Significant correlations between sociodemographic / clinical data and neuropsychological achievements in patients with DM2

Moderate correlations (Spearman's rho 0.50-0.69) are given in red, low correlations (0.30-0.49) in orange, and absence of correlations are given in green.

Abbreviations: MRC, Medical Research Council; RSPM, Raven's Standard Progressive Matrices; MMSE, Mini Mental State Examination test; ACE-R, Addenbrooke Cognitive Examination - Revised; TMT-A, Trail Making Test A; TMT-B, Trail Making Test B; ROCF, Rey-Osterrieth complex figure; RAVLT, Rey Auditory Verbal Learning Test; PF, phonemic fluency; CF, Categorial fluency; WCST, Wisconsin Card Sorting Test; BNT, Boston Naming Test.

*p<0.05

**p<0.01, n.s. not significant.

Twenty-four (35.3%) of 68 patients had cognitive impairment (having at least two tests with two SDs beyond norm), and 17 (25%) had significant cognitive impairment with affection of patients ADLs. The most affected domains in patients with significant cognitive impairment were visuospatial (82.4%), executive (70.6%) and language domain (64.7%) while memory and attention were less commonly impaired (35.3% and 23.5% respectively). Among patients with significant cognitive impairment, two domains were affected in 9 (53%) patients, three in 4 (23.5%) patients, and more than three domains were affected in 4 (23.5%) patients. Figure 1 represents patterns of impaired domains that we encountered in our cohort of DM2 patients with significant cognitive impairment.

Fig. 1Patterns of cognitive profiles among DM2 patients with dementia

Show full caption

V visuospatial function, E executive functions, A attention, M memory, L language. Gray rectangle represents impairment of the corresponding cognitive domain.

TMT-B test was able to detect 14 (73.7%) patients with executive dysfunction, WCST number of categories 4 (21.1%) patients, and WCST number of sets 1 (5.3%) patient. Interestingly, we observed no DM2 patients with executive dysfunction confirmed with at least two tests. Regarding memory domain, we did not find patients with visual memory impairment. RAVLT delayed recall was more sensitive to detect memory deficit than RAVLT immediate memory total score (90.9% vs 27.3%). Only one patient had decrease in immediate total score without decrease in delayed recognition. In the language domain, BNT was more sensitive than CF test registering 19 (95%) patients vs. 1 (5%) patient with language dysfunction, respectively.

Patients with significant cognitive impairment were older at testing and at disease onset compared to those without it (61.3 ± 5.7 vs. 48.1 ± 11.0 and 48.3 ± 13.9 vs. 36.5 ± 9.7 years, p<0.01), they were less educated (10.1 ± 3.1 vs. 13.0 ± 2.9 years, p<0.01), and had more severe muscle weakness (16.2 ± 2.2 vs. 18.2 vs. 1.5, p<0.01). Other sociodemographic/clinical features were not associated with the presence of cognitive impairment in DM2.

MMSE score was worse in patients with significant cognitive impairment (26.2 ± 3.0 vs. 28.6 ± 1.3, p<0.01). No patient without significant cognitive impairment had MMSE below 24 while 17.6% of those with it had score below 24 (p<0.05). This suggests high specificity (100.0%), but very low sensitivity of MMSE to detect significant cognitive impairment in DM2. ACE-R was worse in demented DM2 patients (77.5 ± 11.1 vs. 91.8 ± 5.0, p<0.01). Among patients without significant cognitive impairment 8.5% had score below 83 on ACE-R compared to 76.9% patients with it (p<0.01). This suggests ACE-R sensitivity to detect significant cognitive impairment in DM2 is 76.9% and specificity was 91.5%.

4. Discussion

Our neuropsychological study conducted in the largest cohort of DM2 patients so far detected significant cognitive impairment in one quarter of patients and offered a set of tests that can be easily administered to detect it in DM2.

It is of note that our patients had preserved general intellectual level, with almost half of them achieving even above-average/superior scores on the RSPM test. Only one in twenty patients had a below-average/defective result, which confirms the findings of previous studies showing that mental retardation is not described in DM2 [[11]Peric S Rakocevic Stojanovic V Mandic Stojmenovic G Ilic V Kovacevic M Parojcic A et al.Clusters of cognitive impairment among different phenotypes of myotonic dystrophy type 1 and type 2.,[15]Peric S Mandic-Stojmenovic G Stefanova E Savic-Pavicevic D Pesovic J Ilic V et al.Frontostriatal dysexecutive syndrome: A core cognitive feature of myotonic dystrophy type 2.,[24]Bosco G Diamanti S Meola G Angeard N Bassez G Ekström AB et al.]. Thus, DM2 cognitive impairments detected in our and previous studies are not neurodevelopmental but likely neurodegenerative. In line with this, we found correlations between cognitive deficit and age in our DM2 patients. Our patients with significant cognitive impairment were on average 13 years elder and have on average 12 years later disease onset compared to patients without it with no difference in disease duration between these groups. Associations between cognition and muscle strength speaks in favour the hypothesis that both brain and muscle degeneration is a part of similar accelerated aging process [[15]Peric S Mandic-Stojmenovic G Stefanova E Savic-Pavicevic D Pesovic J Ilic V et al.Frontostriatal dysexecutive syndrome: A core cognitive feature of myotonic dystrophy type 2.,[25]Sansone V Gandossini S Cotelli M Calabria M Zanetti O Meola G. Cognitive impairment in adult myotonic dystrophies: A longitudinal study.]. Longitudinal studies are needed to further resolve this issue. It is of note that lower education was more common in DM2 patients with cognitive impairment. Similar have been reported in patients with dementia, particularly Alzheimer disease [26Nicolas B Alessandra D Daniela P Osman R Sara T Giovanni B F et al.Basal forebrain metabolism in Alzheimer's disease continuum: relationship with education., 27Kim KW Woo SY Kim S Jang H Kim Y Cho SH et al.Disease progression modeling of Alzheimer's disease according to education level., 28Anderson EL Howe LD Wade KH Ben-Shlomo Y Hill WD Deary IJ et al.Education, intelligence and Alzheimer's disease: Evidence from a multivariable two-sample Mendelian randomization study.].The most impaired cognitive domains in DM2 were visuospatial, executive and language, since the worst achievements were noticed on copying of ROCF (reflective of both visuospatial and executive functions), TMT-B (executive test), and BNT (language naming test). In accordance with this, other studies reported dysexecutive syndrome of different degree in up to 60% of DM2 patients, including problems in conditional-associative learning, decision making, planning, divided attention and attentional control, interference, alertness with and without warning signals, verbal fluency with the change of categories, conceptual reasoning and set shifting [[6]Weber YG Roebling R Kassubek J Hoffmann S Rosenbohm A Wolf M et al.Comparative analysis of brain structure, metabolism, and cognition in myotonic dystrophy 1 and 2.,[10]Romeo V Pegoraro E Ferrati C Squarzanti F Sorarù G Palmieri A et al.Brain involvement in myotonic dystrophies: Neuroimaging and neuropsychological comparative study in DM1 and DM2.,[25]Sansone V Gandossini S Cotelli M Calabria M Zanetti O Meola G. Cognitive impairment in adult myotonic dystrophies: A longitudinal study.,29Schneider-Gold C Bellenberg B Prehn C Krogias C Schneider R Klein J et al.Cortical and subcortical grey and white matter atrophy in myotonic dystrophies type 1 and 2 is associated with cognitive impairment, depression and daytime sleepiness., 30Ates S Deistung A Schneider R Prehn C Lukas C Reichenbach JR et al.Characterization of Iron Accumulation in Deep Gray Matter in Myotonic Dystrophy Type 1 and 2 Using Quantitative Susceptibility Mapping and R2* Relaxometry: A Magnetic Resonance Imaging Study at 3 Tesla., 31Sansone V Meola G Perani D Fazio F Garibotto V Cotelli M et al.Glucose metabolism and dopamine PET correlates in a patient with myotonic dystrophy type 2 and parkinsonism., 32Meola G Sansone V Perani D Scarone S Cappa S Dragoni C et al.Executive dysfunction and avoidant personality trait in myotonic dystrophy type 1 (DM-1) and in proximal myotonic myopathy (PROMM/DM-2).]. Visuo-spatial impairment is observed in up to one half of DM2 patients [[11]Peric S Rakocevic Stojanovic V Mandic Stojmenovic G Ilic V Kovacevic M Parojcic A et al.Clusters of cognitive impairment among different phenotypes of myotonic dystrophy type 1 and type 2.,[15]Peric S Mandic-Stojmenovic G Stefanova E Savic-Pavicevic D Pesovic J Ilic V et al.Frontostriatal dysexecutive syndrome: A core cognitive feature of myotonic dystrophy type 2.,[25]Sansone V Gandossini S Cotelli M Calabria M Zanetti O Meola G. Cognitive impairment in adult myotonic dystrophies: A longitudinal study.