Content » Vol 47, Issue 5

Original report

Development and testing psychometric properties of an ICF-based health measure: The Neuromuscular Disease Impact Profile

Isaäc Bos, MSc1, Jan B. M. Kuks, MD, PhD1 and Klaske Wynia, PhD1,2

From the 1Department of Neurology and 2Department of Health Sciences, Community and Occupational Health, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands

OBJECTIVES: To develop a measure that is based on the International Classification of Functioning, Disability and Health (ICF) and reflects the prevalence and severity of disabilities related to neuromuscular disorders, and to evaluate the psychometric properties of this measure.

METHODS: A preliminary questionnaire was developed, based on the categories of the ICF Core Set for Neuromuscular Diseases. Next a cross-sectional postal survey was carried out among 702 patients (70% response rate) diagnosed with a neuromuscular disease. Finally, psychometric properties were examined.

RESULTS: The preliminary Neuromuscular Disease Impact Profile (NMDIP) consisted of 45 items. Factor analysis showed that the NMDIP comprised domains representing 3 ICF-components: 5 factors in the Body Functions component, 2 factors in the Activities component, and 1 factor in the Participation component. Scales showed moderate to good internal consistency (α = 0.63–0.92) and mean inter-item correlation coefficients (0.38–0.77). Convergent and discriminant validity analysis indicated that the NMDIP measures the impact of neuromuscular disease on physical, mental, and social functioning. The NMDIP discriminates between groups who differ in extent of limitations.

CONCLUSION: The NMDIP is an ICF-based measure that reflects neuromuscular disease-related disabilities. It consists of 36 items divided over 8 scales with satisfactory psychometric properties and 4 single items.

Key words: neuromuscular disease; health measure; International Classification of Functioning Disability and Health; psychometric properties.

J Rehabil Med 2015; 47: 00–00

Correspondence address: Isaäc Bos, University Medical Center Groningen (UMCG), University of Groningen, Department of Neurology, PO Box 30001, NL-9700 RB Groningen, The Netherlands. E-mail: i.bos@umcg.nl

Accepted Oct 28, 2014; Epub ahead of print Feb 13, 2015

INTRODUCTION

Neuromuscular diseases (NMD) may be acquired or hereditary. Causes are dysfunction of the anterior horn cell or sensory ganglion cell (neuronopathy), peripheral nerve (neuropathy), neuromuscular junction (myasthenia), or muscle (myopathy) (1). Common symptoms of neuromuscular diseases include muscle weakness, impairment in muscle endurance, involuntary muscle activity (stiffness, myotonia, cramp, and fasciculation), sensory loss, autonomic dysfunction and impairment in control of voluntary movements. Sensations of pain and fatigue are common consequences of these muscle and nerve disturbances (2, 3). These symptoms have a profound impact on daily activities and participation in life situations (4, 5).

In clinical practice and research there is a need for reliable and validated assessment tools as well as outcome measures that cover the broad range of health problems in neuromuscular patients (6).

Over the last 2 decades many health measurement instruments have been developed for use both in clinical practice and in research. As a result, there are generic health-related quality of life (HRQoL) instruments with a broad scope, for example, the Medical Outcome study Short Form Questionnaire (SF-36) (7). An example of a disease-specific HRQoL instrument with a broad scope is the Amyotrophic Lateral Sclerosis Assessment Questionnaire (ALSAQ-40) (8). In addition, there are generic and domain-specific measures to assess limitations in daily living, for example the Groningen Activity Restriction Scale (GARS) (9), or to assess participation in life situations, for example, the Impact on Participation and Autonomy Questionnaire (IPAQ) (10).

The International Classification of Functioning, Disability, and Health (ICF) is a classification developed by the World Health Organization (WHO) and aims at providing a unified and standardized language for describing and classifying health domains and health-related states, and hence providing a common framework for the development of health outcome measures (11, 12).

The ICF comprises 4 key components. The first component, Body Functions and Structures, refers to functions of body systems, and to anatomic parts. The second component, Activities, refers to “task or action execution by the individual”. The third component, Participation, refers to “involvement in life situations”. The Environmental Factors that interact with these 3 components, are described in the fourth component of the ICF (11).

In the model of functioning that underlies the ICF classification system, the components body functions and structures, activities and participation are summarized under the concepts “functioning” and “disability”. These are associated both with health status and with personal and environmental factors. Functioning is an umbrella term encompassing all body functions, activities and participation. Similarly, disability is an umbrella term for impairments in body functions, limitations in activities and restrictions in participation (11). ICF Core Sets have been the first approach to providing ICF-based instruments for clinical practice and research (13). An example of such ICF-based measures is the Multiple Sclerosis Impact Profile (MSIP) (14). This measurement instrument has shown to be a feasible assessment tool in practice and psychometrically sound measures in research. To the best of our knowledge there is no broad ICF-based health measure covering all 600 NMDs. Therefore the aim of this study was to develop an ICF-based measure, the Neuromuscular Disease Impact Profile (NMDIP), with the intention of reflecting the prevalence and severity of a broad range of disabilities related to neuro­muscular diseases, using the ICF features such as ICF terminology and ICF qualifiers and to evaluate the psychometric properties of this new measurement instrument.

METHODS

Sample and procedures

A cross-sectional postal survey was conducted among patients diagnosed with a neuromuscular disease and registered at the Department of Neurology of the University Medical Center Groningen, the Nether­lands. Criteria for inclusion were: diagnosed with a neuromuscular disease, aged 18 years or older, and able to read and write in Dutch.

In total 1003 eligible patients were selected from the hospital patient record system with the aim of representing the 4 major neuromuscular disease (NMD) groups defined by Rowland: motor neurone disorders, muscle disorders, junction disorders and, peripheral nerve disorders (15). To prevent any inappropriate sending of the questionnaire, we cross-checked for deceased patients using the national population register.

Patients received information about the study and were invited to participate. Respondents completed the preliminary NMDIP, generic and domain-specific questionnaires, along with some demographic and disease-specific questions. Reminders were sent out after 2 weeks.

Ethical approval was obtained from the local ethics committee. Reference: METc 2009.310.

Preliminary Neuromuscular Disease Impact Profile

The preliminary NMDIP was developed as a disease-specific and ICF-based measure to assess disability among patients with an NMD. We used the 69 ICF categories of the NMD-Core Set (3). These categories are divided over the 4 ICF components. Selected categories were operationalized in order to estimate the patient’s objectified opinion (impairment in body functions, limitations in activities or restrictions in participation) of the incidence and severity of a disability, and to estimate the support from relevant environmental factors. Furthermore, ICF terminology for “disabilities” was applied, ICF item labels were used when formulating the subject of the question (e.g. “urination” functions instead of “bladder” functions), and ICF codes (e.g. b280 or p920) were documented for each question (14). Illustrative examples were annexed (using fourth-level ICF-items) to some questions to ensure an adequate response. To record the presence and severity of a problem in functioning, we applied response scales with scoring options specified for each ICF component, based on “qualifiers” proposed by the ICF (14).

The preliminary questionnaire was reviewed by patients, clinicians, nurse specialists, experts on the ICF and methodologists (n = 24) for clarity, comprehensiveness, redundancy and patient burden. A modified questionnaire was pre-tested in a random sample of 3 clinicians and 50 patients who were not involved in the first appraisal of the questionnaire. Unclear or ambiguous items and instructions were identified and some modifications of the questionnaire were made.

Finally, the preliminary NMDIP reflects an objectified view of the prevalence and severity of NMD-related disabilities and consists of 45 items representing the 4 ICF components.

Measurement instruments

For evaluating the psychometric properties of the NMDIP, 2 generic and 2 domain-specific measures were used.

The SF-36 is a broad and generic HRQoL measure that consists of 36 items divided over 8 domains (7). For each domain, item scores were transformed to a scale that ranges from 0 (worst health) to 100 (best health). In a previous study among Dutch multiple sclerosis patients the SF-36 domains showed satisfactory levels of internal consistency: Cronbach’s alpha ranged between 0.74 and 0.96 (14).

The World Health Organization Quality Of Life (abbreviation version) (WHOQOL-BREF) is a broad and generic measure of global QoL (16), and consists of 26 items divided over 4 domains. For each domain, item scores were transformed to a scale that ranges from 0 (worst health) to 20 (best health). In a previous study among Dutch multiple sclerosis patients the WHOQOL-BREF showed good levels of internal consistency: Cronbach’s alpha ranged between 0.80 and 0.81 (14).

The GARS is a domain-specific instrument to measure limitation, and consists of 18 items divided over 2 domains (9). A 4-category response format is used, ranging from 1 (no problem in performing without help) to 4 (impossible to perform). Scores are summed for each subscale. The GARS showed strong levels of internal consistency: Cronbach’s alpha ranged from 0.95 to 0.97 in a study in a Dutch sample of multiple sclerosis patients (14).

The IPAQ is a domain-specific questionnaire focusing on person-perceived participation and autonomy (10, 17). The instrument assesses 2 aspects of participation: perceived participation and the perceived problem. In this study we applied the perceived participation part that consists of 24 items divided over 5 domains. Items are scored on a 5-point rating scale with discrete responses, ranging from 1 (very good) to 5 (very poor). Scores are summed for each domain. In a previous study among Dutch multiple sclerosis patients, the IPAQ showed good levels of internal consistency: Cronbach’s alpha ranged from 0.86 to 0.94 (14).

Item reduction

Exploratory factor analysis with oblique rotation (Geomin) (20, 23) was used to examine whether the domains measured by the NMDIP represent the 4 ICF components. To improve the content validity the prevalence of each item was examined before entering items in the factor analysis. Items with a low prevalence (≤ 20%) were excluded from further analysis (18, 19). Factor analyses were performed using Mplus 6 software (20). Given the categorical nature of the variables, methods based on polychoric correlations and the robust-weighted least squares estimators (21) were used. Goodness-of-fit of the underlying factorial structure was measured by the root mean squared error of approximation (RMSEA, adequate if below 0.06) and the standardized root mean square residual (SRMR, adequate if below 0.08), the Comparative Fit Index and the Tucker-Lewis Index. For the latter 2 indices, it is recommended that values should be greater than 0.95 (22, 23). Items with factor loadings ≥ 0.40 were selected for scale construction.

Items that could not meet the criteria of scalability were taken into consideration for use as a single indicator.

Missing items

The maximum number of missing items allowed to be replaced by the mean scale score was determined by a sufficient Cronbach’s alpha in relation to the number of scale items (19, 24).

Psychometric evaluation

The distribution of scale scores was evaluated by calculating the median, mean and standard deviation and observed score range. Proportion of patients with worst and best possible scores (floor and ceiling effect) were calculated. Proportions ≤ 20% were considered acceptable (18).

Internal consistency was examined with Cronbach’s alpha (25) and the mean inter-item correlation coefficient (MICC) for each scale (26, 27). Alpha was considered sufficient if ≥ 0.70 (28, 29), and MICC if ≥ 0.30 (26).

To test whether NMDIP scales measure physical, psychological, social, and environmental domains of functioning, as they purport to measure, convergent and discriminant validity were assessed by examining the extent to which correlation values between NMDIP scales and concurrent measures were consistent with hypotheses (30, 31). Regarding convergent validity, we hypothesized that the NMDIP scales would have a strong correlation (≥ 0.70) (32) with scales that cover the same domain in concurrent measures. For example, NMDIP scales for physical functions should correlate highly with the SF-36 “Physical Functioning scale”. To support discriminant validity, we hypothesized that the NMDIP scales would correlate weakly (< 0.40) with scales measuring different domains in NMDIP or concurrent measures. For example, NMDIP scales for physical function would correlate weakly with mental or emotional scales of the SF-36.

Regarding known-groups validity (30, 31), we hypothesized that the NMDIP scales should be able to discriminate between subgroups of respondents known to differ on relevant clinical characteristics. The level of limitations due to a neuromuscular disease was used to create relevant subgroups of respondents. Therefore, the generic question “Extent of limitations” was used. Respondents were asked to answer the question “To what extent are you limited due to a neuromuscular disease?” on an 11-point scale with a score range from 0 (not limited at all) to 10 (severely limited). Next, respondents were divided into 2 groups: those with a “lower extent of limitations” (score 1–4), and those with a “higher extent of limitations” (score 5–10).

Statistical analysis

Patient characteristics were analysed using descriptive statistics. Spearman’s correlation coefficient (rho) was used to examine convergent and discriminant validity. Known-groups validity was assessed using the independent Mann-Whitney U test.

To estimate the magnitude of the difference in scores between subgroups of respondents, the non-parametric effect size (coefficient r) for unrelated samples was calculated for statistically significant group differences (alpha = 0.05) (33). Coefficient r is calculated by dividing the z statistic (obtained from the Mann-Whitney U test) by the root of the sample size (n). To interpret the non-parametric effect sizes using coefficient r. Cohen suggested the following thresholds for interpretation: an r of < 0.10 indicates a trivial effect, an r of ≥ 0.10 to < 0.24 a small effect, an r of ≥ 0.24 to < 0.37 a moderate effect, and an r ≥ 0.37 a large effect. An r ≥ 0.10 reflects a clinically relevant difference between groups (33, 34). IBM SPSS statistics version 20 was used.

RESULTS

Patient characteristics

In total 702 participants (70% response rate) completed the questionnaires. Demographics and disease-specific characteristics are described in Table I. Mean age was 59 years (SD 16, range 19–92 years), while slightly more than half of the patients were younger than 65 years. Mean number of “years since diagnosis” was 12 (SD 11, range 0–65 years). Approximately 30% of the patients were retired due to a neuromuscular disease. The motor neurone disorder subgroup was a relatively small sample compared with the other neuromuscular disease subgroups according to Rowland’s classification (15).

Non-respondents did not differ from respondents in terms of gender, but were significantly younger (mean 53, SD 19 years) than respondents (mean 59, SD 16 years).

Table I. Sample characteristics (n = 702)

Variable

Total sample

Gender, n (%)

Female

350 (50)

Male

352 (50)

Age, years, mean (SD) [range]

59 (16) [19–92]

Years since diagnosis, mean (SD) [range]

12 (11) [0–65]

Relationship status, n (%)

Relationship (married/partnership)

498 (71)

Single (unmarried/widowed/divorced)

186 (27)

Educational level, n (%)

Primary school/vocational training

235 (33)

Secondary school/vocational training

270 (38)

Higher education/vocational training

161 (23)

University

28 (4)

Employment status (more answers possible), n (%)

Enrolled in a training or study course

36 (5)

Employment (part-time or full-time)

173 (25)

Voluntary work (part-time or full-time)

42 (6)

(Partially) retired due to NMD

213 (30)

Housewife/househusband

171 (24)

Retired due to age

244 (35)

NMD category, n (%)

Motor neurone disorder

43 (6)

Muscle disorder

154 (22)

Junction disorder

234 (33)

Peripheral nerve disorder

271 (39)

NMD: neuromuscular disease; SD: standard deviation.

Content validity

Nine of the original 45 items showed a low prevalence (≤ 20%) and were not entered in the factor analysis. These items were from the component “Activities” (“a350 Conversation”, “a360 Using communication devices and techniques”, and “a465 Moving around using equipment”), the component “Participation” (p510–p540 items concerning “Personal care”, “p360 Communication devices and techniques, “p630 Eating and drinking”, “p610 Acquiring a place to live” and “p850 Remunerative employment”), and from the component “Environmental Factors” (“e340 Personal care providers and personal assistants”).

Item reduction

The EFA models showed a (very) good fit for the 5-factor model for the Body Functions component, the 2-factor model for the Activities component, and the 1-factor model for the Participation component. Comparative Fit Index and Tucker-Lewis Index values were above 0.95, SRMR values were below 0.08 as recommended. The RMSEA values were below 0.06 for the Body Functions component and Participation component, while the value for the Activities component was slightly higher (0.069), but still acceptable. For the Environmental Factors component no satisfying fit was found. Factor analysis reduced the remaining 36 items of the initial NMDIP to 32 items:

• Five factors within the Body Functions component included 15 items (Table II). Interpretation of item content, using the ICF first- and second-level category labels led to the following scale labels: “Muscle Functions” (MuF), “Movement Functions” (MoF), “Swallowing and Speech Functions” (SSF), “Excretion and Reproductive Functions” (ERF), and “Mental Functions and Pain” (MFP).

Table II. Factor analysis with Body Functions component categories (n = 702)

Factor 1

Factor 2

Factor 3

Factor 4

Factor 5

Muscle Functions (MuF)

Impairment in…

b730 Muscle power functions

0.612

0.473

0.032

0.015

–0.054

b740 Muscle endurance functions

0.677

0.329

–0.003

–0.020

0.195

Movement Functions (MoF)

Impairment in…

b760 Control of voluntary movements functions

0.009

0.777

–0.100

0.023

–0.038

b765 Involuntary movements functions

–0.315

0.849

0.106

–0.022

0.044

b780 Sensation related to muscles and movement functions

0.045

0.590

–0.041

0.053

0.198

Swallowing and speech functions (SSF)

Impairment in…

b320 Speech functions

0.024

0.144

0.842

0.008

–0.013

b5105 Swallowing functions

0.246

–0.035

0.687

0.053

0.054

Excretion and reproductive Functions (ERF)

Impairment in…

b525 Defecation functions

0.022

–0.030

–0.002

0.656

0.168

b620 Urination functions

–0.052

0.028

0.036

0.785

–0.032

b640 Sexual functions

0.046

0.281

0.026

0.426

0.043

Mental Functions and Pain (MFP)

Impairment in…

b134 Sleep functions

–0.028

–0.008

–0.055

0.091

0.680

b1300 Fatigue

0.460

0.019

0.062

0.011

0.593

b152 Emotional functions

–0.020

–0.014

0.100

–0.148

0.725

b160 Thought functions

–0.230

0.040

0.259

0.023

0.539

b280 Sensation of pain

0.078

0.267

–0.238

0.040

0.527

Bold figures are sufficient factorloadings selected for scale construction.

• Two factors within the Activities component included 14 items (Table III). These factors were given the labels, “Activities of Moving around”(AMA) and “Self-care and Domestic Activities” (SDA). One factor within the Participation component included 3 items (Table IV). This factor was labelled “Participation in Life Situations” (PLS).

Table III. Factor analysis with Activities component categories (n = 702)

Factor 1

Factor 2

Activities of Moving around (AMA)

Limitations in…

a410 Changing body position

0.860

0.005

a415 Maintaining body position

0.639

0.270

a420 Transferring oneself

0.711

0.251

a450 Walking

0.952

–0.035

a470 Using transportation

0.589

0.330

a920 Recreation and leisure

0.411

0.354

Self-care and Domestic Activities (SDA)

Limitations in…

a440 Fine hand use

–0.003

0.758

a445 Hand and arm use

–0.102

0.894

a510 Washing oneself

0.272

0.709

a520 Caring for body parts

–0.007

0.904

a530 Toileting

0.335

0.585

a540 Dressing

0.200

0.754

a630 Preparing meals

0.057

0.863

a640 Doing housework

0.398

0.458

Bold figures are sufficient factorloadings selected for scale construction.

Table IV. Factor analysis with Participation components (n = 702)

Factor 1

Participation in Life Situations (PLS)

Restriction in…

p460/p470 Mobility

0.690

p740-p760 Relationships

0.719

p910/p920 Recreation and leisure

0.903

Bold figures are sufficient factorloadings selected for scale construction.

Finally, the Body Functions component items “Seeing functions” and the Environmental Factor component items “Immediate family”, “Social security services”, and “Health services” with a sufficient prevalence and clinical relevance were added to the questionnaire as single items.

Scale features are shown in Table V. Internal consistency for 7 NMDIP scales for the total sample was good and moderate for 1 scale. Cronbach’s alpha ranged from 0.63 to 0.92 and mean inter-item correlation coefficient ranged from 0.38 to 0.77. Four scales showed a high floor effect.

Table V. Scale features of the Neuromuscular Disease Impact Profile (NMDIP) scales (n = 702)

Cases

n

Items

k

Possible

score range

Observed score range

Floor effect

%

Ceiling effect

%

Median

Mean

SD

Alpha

micc

NMDIP scales

Muscle Functions

658

2

0–8

0–8

8

3

4

3.4

1.9

0.87

0.77

Movement Functions

594

3

0–12

0–12

18

0

2

2.4

2.1

0.72

0.47

Swallowing and Speech Functions

669

2

0–8

0–6

59

1

0

0.7

1.1

0.69

0.53

Excretion and Reproductive Functions

509

3

0–12

0–12

27

0

1

1.8

3.1

0.63

0.38

Mental Functions and Pain

597

5

0–20

0–16

7

0

4

1.8

1.8

0.80

0.59

Activities of Moving Around

702

6

0–18

0–18

18

2

4

5.1

4.8

0.90

0.62

Self-care and Domestic Activities

701

8

0–24

0–24

28

1

2

4.7

6.0

0.92

0.59

Participation in Life Situations

695

3

0–12

0–12

49

0

1

1.7

2.4

0.72

0.47

NMDIP single item

Seeing functions

666

1

0–4

0–4

0

0.68

0.88

Immediate family

697

1

0–2

0–2

0

0.51

0.73

Social security services

268

1

0–2

0–2

1

0.74

0.77

Health services

693

1

0–2

0–2

0

0.66

0.75

MICC: mean inter-item correlation coefficient; SD: standard deviation.

The NMDIP scales within the NMD groups showed acceptable to good internal consistency. (Table VI). For some scales the Cronbach’s alpha was weak, but this was compensated by a sufficient mean inter-item correlation coefficient, except for the “Swallowing and Speech Functions” scale in the peripheral nerve disorder group and the “Excretion and Reproductive Functions” scale in the muscle disorder group.

Table VI. Internal consistency (Cronbach’s alpha) and the mean inter-item correlation coefficient (MICC) of the Neuromuscular Disease Impact Profile (NMDIP) scales per neuromuscular diseases group

Motor neurone disorders

(n = 43)

Muscle disorders

(n = 154)

Junction disorders

(n = 234)

Peripheral disorders

(n = 271)

Alpha

MICC

Alpha

MICC

Alpha

MICC

Alpha

MICC

Muscle functions

0.86

0.77

0.79

0.67

0.87

0.78

0.86

0.76

Movement functions

0.72

0.48

0.72

0.48

0.67

0.42

0.67

0.41

Swallowing and speech functions

0.82

0.72

0.74

0.59

0.63

0.47

0.42

0.27

Excretion and reproductive functions

0.71

0.47

0.54

0.30

0.63

0.36

0.68

0.45

Mental functions and pain

0.69

0.34

0.77

0.40

0.73

0.36

0.72

0.33

Activities of moving around

0.90

0.62

0.90

0.60

0.85

0.50

0.90

0.59

Self-care and domestic activities

0.95

0.69

0.93

0.63

0.83

0.43

0.88

0.47

Participation in life situations

0.80

0.57

0.74

0.49

0.67

0.40

0.67

0.41

The final version of the NMDIP consists of 36 items divided over 8 scales and 4 single items (see Appendix I).

Psychometric testing

Table VII provides evidence of convergent and discriminant validity for the NMDIP scales, reflecting the impact of a neuromuscular disease on physical, psychological, and social aspects of functioning. The predictions were consistent with the direction, magnitude, and patterns of correlations.

Table VII. Results of convergent and divergent validity analyses of the Neuromuscular Disease Impact Profile (NMDIP) scales (n = 702)

NMDIP

Body functions

Activities

Participation

MuF

MoF

SSF

ERF

MFP

AMA

SDA

PLS

Alpha

NMDIP

Muscle Functions (MuF)

1

0.87

Movement Functions (MoF)

0.57

1

0.72

Swallowing and Speech Functions (SSF)

0.26

0.23

1

0.69

Excretion and Reproductive Functions (ERF)

0.35

0.43

0.31

1

0.63

Mental Functions and Pain (MFP)

0.49

0.59

0.34

0.48

1

0.80

Activities of Moving around (AMA)

0.72

0.58

0.23

0.41

0.48

1

0.90

Self-care and Domestic Activities (SDA)

0.63

0.52

0.31

0.45

0.51

0.79

1

0.92

Participation in Life Situations (PLS)

0.54

0.46

0.26

0.38

0.53

0.68

0.61

1

0.72

SF-36

Physical functioning

–0.71

–0.52

–0.23

–0.40

–0.47

–0.85

0.77

–0.61

0.94

Role physical

–0.43

–0.42

–0.26

–0.37

–0.52

–0.43

–0.45

–0.40

0.88

Bodily pain

–0.38

–0.51

–0.17

–0.32

–0.65

–0.42

–0.40

–0.38

0.91

General health

–0.49

–0.49

–0.32

–0.44

–0.60

–0.49

–0.48

–0.46

0.78

Mental health

–0.21

–0.33

–0.18

–0.18

0.53

–0.21

–0.25

–0.32

0.83

Role emotional

–0.18

–0.30

–0.20

–0.21

–0.39

–0.23

–0.26

–0.25

0.87

Social functioning

–0.44

–0.46

–0.29

–0.40

–0.63

–0.46

–0.46

–0.51

0.77

Vitality

–0.41

–0.42

–0.34

–0.33

–0.68

–0.35

–0.39

–0.39

0.81

WHOQOL-BREF

Physical health and autonomy

–0.29

–0.30

–0.22

–0.29

–0.49

–0.29

–0.29

–0.33

0.84

Psychological health

–0.25

–0.33

–0.25

–0.23

–0.43

–0.29

–0.30

–0.35

0.80

Social relation

–0.23

–0.22

–0.14

–0.40

–0.37

–0.26

–0.25

–0.33

0.60

Environment

–0.34

–0.39

–0.24

–0.27

–0.51

–0.41

–0.39

–0.46

0.82

GARS

Activities of daily living

0.64

0.54

0.26

0.45

0.44

0.81

0.81

0.62

0.95

Instrumental activities of daily living

0.67

0.49

0.30

0.44

0.49

0.78

0.84

0.62

0.93

IPAQ

Autonomy indoors

0.54

0.52

0.25

0.43

0.50

0.66

0.65

0.56

0.94

Family role

0.53

0.52

0.30

0.41

0.59

0.61

0.61

0.53

0.92

Autonomy outdoors

0.57

0.50

0.34

0.48

0.63

0.68

0.66

0.66

0.84

Social relations

0.34

0.35

0.32

0.41

0.53

0.41

0.45

0.48

0.85

NMDIP: n = 484–658; SF-36: n = 654–657; WHOQOL-BREF: n = 628–649; GARS: n = 655–658; IPAQ: n = 654–657.

Bold correlations = expected convergent correlations. Italic correlations = expected discriminant correlations.

MuF: Muscle functions; MoF: Movement functions; SSF: Swallowing and Speech functions; ERF: Excretion and Reproductive functions; MFP: Mental functions and Pain; AMA: “Activities of Moving around”; SDA: “Self-care and Domestic Activities”; PLS: Participation in life situations; GARS: Groningen Activity Restriction Scale; IPAQ: Impact on Participation and Autonomy Questionnaire.

Convergent validity was supported by a strong correlation between the NMDIP “Muscle Functions” scale and the SF-36 “Physical Functioning” scale, and a strong correlation between the NMDIP “Lower Extremity Activities” and “Upper Extremity Activities” scales, and the GARS “Activities of Daily Living” and “Instrumental Activities of Daily Living” scales. The NMDIP “Participation in Life Situations” scale was moderately correlated with the IPAQ participation scales.

Discriminant validity was supported by weak correlation values found, for example, between the NMDIP “Mental Functions and Pain” scale and the SF-36 “Physical Functioning” scale, and the GARS “Activities of Daily Living” and “Instrumental Activities of Daily Living” scales. Similarly, the NMDIP “Muscle Functions” scale correlated weakly with the SF-36 “Mental Health and “Role Emotional” scales, and the NMDIP “Participation in Life Situations” scale correlated weakly with the SF-36 “Mental Health” scale.

Evidence of known-groups validity was obtained for all NMDIP scales by statistically significant group differences and clinically relevant effect sizes (Table VIII). Patients classified as having a higher extent of limitation had statistically significant higher scores on all NMDIP scales compared with those classified as having a lower extent of limitation. Effect sizes were moderate for 2 scales, and large for 6 scales.

Table VIII. Results of known-groups validity analyses of the Neuromuscular Disease Impact Profile scales (n = 702)

Low (1–4) vs high (5–10)

Extent of limitations

n

Low

Mean Rank

High

Mean Rank

p-value

(Z-statistic)

Effect size

Muscle functions (MuF)

640

197.84

390.25

0.000 (–12.973)

0.51

Movement functions (MoF)

577

201.52

342.52

0.000 (–9.994)

0.42

Swallowing and speech functions (SSF)

651

273.86

359.14

0.000 (–6.445)

0.25

Excretion and reproductive functions (ERF)

495

198.34

279.74

0.000 (–6.374)

0.29

Mental functions and pain (MFP)

581

201.45

346.38

0.000 (–10.170)

0.42

Activities of moving around (AMA)

683

210.67

422.72

0.000 (–13.704)

0.52

Self-care and domestic activities (SDA)

682

227.58

411.25

0.000 (–11.956)

0.46

Participation in life situations (PLS)

677

239.81

400.46

0.000 (–11.110)

0.43

DISCUSSION

The objective of this study was to develop a psychometrically sound ICF-based measure for estimating the prevalence and severity of a broad range of disabilities related to neuromuscular diseases using ICF features such as ICF terminology and ICF qualifiers.

The results provide evidence to support the validity and reliability of the final version of the Neuromuscular Disease Impact Profile (NMDIP) as an instrument to measure the prevalence and severity of a broad spectrum of consequences of a neuromuscular disease including disabilities in Body Functions, Activities and Participation, and lack of support from Environmental Factors. The NMDIP can be used as a clinical and research instrument for the assessment of the impact of a neuromuscular disease.

The original 45 items in the preliminary NMDIP could be reduced to 36 items: 32 items covering 8 domains representing 3 ICF-components, and 4 clinically relevant items (1 Body Functions item and 3 Environmental Factors items), which were applied as single items in the questionnaire (see final version in Appendix I).

Although the NMDIP used the same items as the initial MSIP (14), results of the factor analysis showed some differences compared with the final MSIP scales. For example, the MSIP “Muscle and Movement functions” 4-item scale is represented in the NMDIP in 2 separate and recognizable scales “Muscle functions” with 2 items and “Movement functions”, also with 2 items. Furthermore, the 3-item “Mental functions” MSIP scale appeared in the NMDIP as a 5-item version: new scale items were Fatigue and Pain. This can be explained by the fact that pain and fatigue are the direct result of (using) weakened muscles, which is a common symptom in neuromuscular diseases. Unlike neuromuscular diseases fatigue in multiple sclerosis is most likely related to the process of inflammation, while pain originates from spasticity and/or neuropathy. Furthermore, scale construction also identified a construct that was not present in the MSIP: “Swallowing and Speech Functions”. This can be explained by the fact that some myopathies and the myasthenia’s tend to affect bulbar musculature. Finally, analysis showed no consistent factor for the Environmental Factors items.

Reliability of the NMDIP scales for the total sample was sufficient for 2 scales and good for 6 scales. The internal consistency of the scales per separate NMD group was generally sufficient, except for the “Swallowing and Speech Functions” scale in the peripheral nerve disorder group and the “Excretion and Reproductive Functions” scale in the muscle disorder group. Some caution is advised in the interpretation of the results of these scales. Convergent and discriminant validity analysis indicated that the NMDIP measures the impact on physical, mental, and social functioning for people with a neuromuscular disease.

The correlation between the NMDIP “Participation in Life Situations” scale and the SF-36 “Physical Functioning” scale was unexpectedly higher. It is likely that the activity-related participation items in the NMDIP scale are responsible for this moderate correlation.

Known-groups validity was supported for the 8 NMDIP scales. Scales discriminated sufficiently between groups of patients with a neuromuscular disease that differed in extent of limitations.

An important strength of this study is the large and broad group of participating patients with a neuromuscular disease, and the sound conceptual basis used in developing the NMDIP (3, 35).

A possible limitation of this study is the small sample size of the motor neurone disorder group, compared with the sample size of the 3 other NMD groups. However, in our opinion the disabilities in this group are sufficiently represented in the NMDIP because the basis of the NMDIP, the NMD ICF-Core set, covers all items of the disease-specific Amyotrophic Lateral Sclerosis Assessment Questionnaire-40 (8). Another limitation could be the high floor effect of some scales that might affect the reliability of these scales (36). However, these floor effects match the course of the slowly progressive nature of most NMDs. This means that some disabilities appear years after onset, such as speech and swallowing functions or upper extremity activities.

Further research should focus on psychometric evaluation concerning stability and sensitivity to change of the NMDIP scales, and validation across other populations of neuromuscular disease patients in other cultures. It would also be interesting to examine the differences in prevalence and severity of disabilities between the 4 major NMD groups, as defined by Rowland (15). Finally, it would be interesting to investigate the impact of the broad range of NMDIP-related disabilities on HRQoL of patients with neuromuscular diseases.

We considered the possibility of undertaking group invariance testing; however, the sub-groups are relatively small and will affect the test of Differential Item Functioning. We therefore suggest further examination of the factor structure in a new sufficient sample.

In clinical practice, especially in multidisciplinary rehabilitation teams, the NMDIP may contribute to better understanding the patients’ health problems when used as an assessment tool. Although positive results were found in the feasibility studies with the preliminary NMDIP and the MSIP, it is advisable to combine this application with research; for example, in order to investigate the effects on the healthcare plan when using the NMDIP.

ACKNOWLEDGEMENTS

The authors wish to thank the patients with a neuromuscular disease who participated in this study and were so kind as to share personal information about the consequences of their disease, and for taking the time to complete the questionnaires. We also wish to thank the students who sent out the questionnaires and entered the data from the questionnaires: Kyra van der Beek, Hanna Bosman, Annelies, Carolien and Marieke Verschure, and Ronald Brans. We finally thank Berrie Middel (PhD) for methodological support and Josué Almansa Ortiz (PhD) for statistical support.

The authors declare that they have no conflicts of interest regarding this manuscript.

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Appendix I. Neuromuscular Disease Impact Profile (NMDIP). (Final 36-item version)

NMDIP

Body functioning questions

Scale

Response options

0 = no, not at all

1 = yes, I have a slight impairment

2 = yes, I have a moderate impairment

3 = yes, I have a severe impairment

4 = yes, I have a complete impairment

MuF b1

Do you face loss of your muscle power functions? (b730)

MuF b2

Do you face loss of muscle endurance functions? (b740)

MoF b3

Do you face loss of control of voluntary movements? (b760)

MoF b4

Do you face involuntary movements? (e.g. tremors or tics) (b765)

MoF b5

Do you face muscle stiffness or muscle spasm? (b7800/b7801)

SSF b6

Do you face impairment in your speech functions? (b320)

SSF b7

Do you face impairment in your swallowing functions? (b5105)

ERF b8

Do you face impairment in your defecation functions? (e.g. changes in frequency, constipation, incontinence) (b525)

ERF b9

Do you face impairment in your urination functions? (e.g. frequency of urination, incontinence, difficulties with urination) (b620)

ERF b10

Do you face limitations in sexual functions? (b640)

14706.png

Appendix I. Contd.

NMDIP

Body functioning questions

MFP b11

Do you face impairment in your sleep functions? (e.g. onset of sleep, the

maintenance of sleep or the quality of sleep) (b134)

MFP b12

Do you experience fatigue? (b1300/b455)

MFP b13

Do you face changes in your emotional functions? (e.g. fear, depression,

happiness) (b152)

MFP b14

Do you face changes in your thought functions? (e.g. the ability to think

logically, the ability to memorize, the ability to concentrate) (b160)

MFP b15

Do you experience sensation pain? (b280)

single b16

Do you face impairment in your seeing functions? (With eyeglasses on or item lenses in) (b210)

NMDIP

Activities questions

Scale

Response options

0 = No

1 = Yes, but assistance devices and/or adaptations are not necessary

2 = Yes, and assistance devices and/or adaptations are necessary

3 = Yes, and assistance devices and/or adaptations and another person’s help are necessary

AMA a1

Do you face limitations in changing your body position? (e.g. moving from lying down to standing up or from standing to sitting) (a410)

AMA a2

Do you face limitations in maintaining your body position? (e.g. maintaining

kneeling, standing, and sitting postures) (a415)

AMA a3

Do you face limitations in transferring yourself? (e.g. moving from a chair into bed; from a wheelchair into a car) (a420)

AMA a4

Do you face limitations in walking? (a450)

AMA a5

Do you face limitations in using transportation? (a470)

AMA a6

Do you face limitations in activities you would like to undertake for recreation and leisure? (a920)

SDA a7

Do you face limitations in your fine hand use? (e.g. picking up small objects;

manipulating a keyboard) (a440)

SDA a8

Do you face limitations in your arm(s) and hand(s) use? (e.g. pulling or pushing objects; turning or twisting knobs or handles; reaching for kitchen cupboard) (a445)

SDA a9

Do you face limitations in washing yourself? (a510)

SDA a10

Do you face limitations in caring for body parts? (e.g. brushing teeth,

clipping your nails, combing your hair, shaving) (a520)

SDA a11

Do you face limitations in toileting? (a530)

SDA a12

Do you face limitations in dressing yourself? (a540)

SDA a13

Do you face limitations in preparing meals? (a630)

SDA a14

Do you face limitations in doing housework? (a640)

NMDIP

Participation questions

Scale

Response options

0 = no

1 = Yes, as a consequence I have some trouble with ….

2 = Yes, as a consequence I have trouble with..

3 = Yes, as a consequence I have a lot of trouble with …

4 = Yes, as a consequence …. is (nearly) impossible

PLS p1

Are there obstacles in your environment that complicate your participation in community, recreation, and leisure? (e.g. accessibility of clubs or associations) (p910/p920)

PLS p2

Are there obstacles in your environment that complicate the maintenance of your relationships with your closest family, friends, or relatives? (e.g. the travel distance, the attitude of others) (p740–p760)

PLS p3

Are there obstacles in your environment that complicate your mobility inside or outside your home? (e.g. thresholds; curbs; absence of elevators) (p460/470)

NMDIP

Environmental factors questions

Scale

Response options

0 = Yes, very supportive;

1 = Yes, somewhat supportive;

2 = No, not supportive

Single

Item e1

Is your relationship with your immediate family supportive for you? (e.g. partner, children, parents, brothers, sisters) (e310)

Single

Item e2

Are the social security services supportive for you? (e.g. income support) (e570)

Single

Item e3

Are the health services supportive for you? (e.g. medical and nursing care) (e580)

MuF: Muscle functions; MoF: Movement functions; SSF: Swallowing and Speech functions; ERF: Excretion and Reproductive functions; MFP: Mental functions and Pain; AMA: Activities of moving around; SDA: Self-care and domestic activities; PLS: Participation in life situations.



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