Stability and relative validity of the. Neuromuscular Disease Impact Profile (NMDIP)

Transcription

1 Bos et al. BMC Neurology (2017) 17:87 DOI /s RESEARCH ARTICLE Ope Access Stability ad relative validity of the Neuromuscular Disease Impact Profile (NMDIP) Isaäc Bos 1*, Ja B.M. Kuks 1, Josué Almasa 2, Hubertus P.H. Kremer 1 ad Klaske Wyia 1,2 Abstract Backgroud: The aim of this study was to examie the stability ad relative validity (RV) of the Neuromuscular Disease Impact Profile (NMDIP) usig criterio-related groups. I a previous study the NMDIP-scales showed good iteral cosistecy, coverget ad discrimiat validity. Kow-groups aalysis showed that the NMDIP discrimiates betwee categories of extet of limitatios. Methods: A cross-sectioal postal survey study was performed o patiets diagosed with a NMD ad registered at the Departmet of Neurology, Uiversity Medical Ceter Groige, the Netherlads. Participats were asked to complete the prelimiary NMDIP, the Medical Outcome study Short Form Questioaire (SF-36), the World Health Orgaizatio Quality Of Life-abbreviatio versio (WHOQOL-bref), ad two geeric domai specific measures: the Groige Activity Restrictio Scale (GARS) ad the Impact o Participatio ad Autoomy Questioaire (IPAQ). The variables Extet of Limitatios ad Quality of Life were used to create criterio-related groups. Stability over time was tested usig the Wilcoxo Siged Rak Test for paired samples ad the itraclass correlatio coefficiets for repeated measures. RV was examied by comparig the ability of NMDIP with geeric multidimesioal health impact measures, ad domai specific measures i discrimiatig betwee criterio-related subgroups usig the Kruskal-Wallis H-test. Results: Respose rate was 70% ( = 702). The NMDIP-scales showed sufficiet stability over time, ad satisfactory or strog RV. I geeral, the NMDIP scales performed as well as or better tha the cocurret measuremet istrumets. Coclusios: The NMDIP proved to be a valid ad reliable disease-targeted measure with a broad scope o physical, psychological ad social fuctioig. Keywords: Neuromuscular Disease Impact Profile, Relative validity, Test-retest reliability, Stability, Criterio validity Backgroud Neuromuscular Diseases (NMDs) may be caused by a abormality of the aterior hor cells, sesory gaglio cells (euroopathy), the peripheral erves (europathy), euromuscular juctios (myastheia), or muscle (myopathy). Commo symptoms ad sigs of NMD iclude muscle weakess, impairmet i muscle edurace, ivolutary muscle activity (stiffess, myotoia, cramps, ad fasciculatios), sesory loss, autoomic dysfuctio ad impairmet i cotrol of volutary movemets. Sesatios of * Correspodece: i.bos@umcg.l 1 Departmet of Neurology, Uiversity Medical Ceter Groige, Uiversity of Groige, P.O. Box RB Groige, The Netherlads Full list of author iformatio is available at the ed of the article pai ad fatigue are commo cosequeces of muscle ad ervepathology[1,2].easytoapplynmd-specificreliable ad validated self-report assessmet tools are essetial for obtaiig isight ito the prevalece ad severity of the broad rage of patiet perceived health-related problems i NMDs. This is importat for research ad for cliical practise as well, i order to arrow the gap betwee the cliicia s ad patiet s view o the actual health situatio ad to help to tailor care plas to the patiet s eed ad prefereces [3]. We therefore developed the disabilityseverity Neuromuscular Disease Impact Profile (NMDIP) based o the ICF-Core set for NMDs, a set of categories selected from the Iteratioal Classificatio of Fuctioig, Disability ad Health (ICF) [2, 4]. The Author(s) Ope Access This article is distributed uder the terms of the Creative Commos Attributio 4.0 Iteratioal Licese ( which permits urestricted use, distributio, ad reproductio i ay medium, provided you give appropriate credit to the origial author(s) ad the source, provide a lik to the Creative Commos licese, ad idicate if chages were made. The Creative Commos Public Domai Dedicatio waiver ( applies to the data made available i this article, uless otherwise stated.

2 Bos et al. BMC Neurology (2017) 17:87 Page 2 of 8 The NMDIP cosists of 36 items that cover all ICFcompoets ad are divided ito eight scales ad four sigle items. The NMDIP-scales showed moderate to good Crobach s alpha ad mea iter-item correlatio coefficiets. Coverget ad discrimiat validity aalysis idicated that the NMDIP measures the impact of euromuscular disease o physical, psychological ad social fuctioig. The NMDIP discrimiates betwee groups of patiets who differ i Extet of limitatios. The four sigle items represet the Evirometal Factors compoet (three items) ad oe Body Fuctios item (Seeig fuctio) [4]. The objective of this study was to further examie the psychometric properties of the NMDIP ad to build o previous studies o this measuremet istrumet [2, 4]. We examied its stability over time by assessig the test-retest reliability of the NMDIP-scales. We furthermore compared the ability of the NMDIP scales to discrimiate betwee criterio related subgroups with this ability of four established cocurret measuremet istrumets, by assessig the Relative Validity (RV) [5, 6]. The RV coefficiet idicates how much more or less valid each outcome measure is related to the best outcome measure. Method Sample ad procedure A cross sectioal study, usig a postal survey, was admiistered to patiets diagosed with a NMD who were registered at the Departmet of Neurology of the Uiversity Medical Ceter Groige, Uiversity of Groige, the Netherlads. Iclusio criteria for this study were: diagosis with a NMD ad represetig oe of Rowlad s NMD classificatio groups: motor-euro disorders, muscle disorders, juctio disorders ad peripheral erve disorders [7]; beig aged 18 or older; beig able to read ad write i Dutch; ad beig able to give iformed coset. No exclusio criteria were formulated. A total of 1003 eligible patiets were selected from the hospital patiet records system. To avoid iappropriately sedig the questioaires, we crosschecked for deceased patiets usig the atioal populatio register. Patiets received iformatio about the study ad were ivited to participate. Respodets completed demographic ad disease specific questios, the NMDIP, two criterio variables to measure the Extet of Limitatios ad Quality of Life. Also,cocurret measures were completed: two geeric multidimesioal health impact measures (the Medical Outcome study Short Form Questioaire (SF-36) [8], ad the World Health Orgaizatio Quality Of Life-abbreviatio versio (WHOQOL-bref) [9], ad two geeric domai specific measures the Groige Activity Restrictio Scale (GARS) [10] ad the Impact o Participatio ad Autoomy Questioaire (IPAQ) [11]. To assess stability over time, the NMDIP was admiistered o two occasios to patiets who agreed to fill i the questioaire twice. We, arbitrary, selected a time frame from eight to 10 weeks to be sure that patiets could ot remember their aswers o the first questioaire, ad the likelihood of chages i the health situatio was miimal. Measuremet istrumets The NMDIP icludes 36 items ad cosists of eight scales ad four additioal items. The 36 items were divided over the four ICF compoets. For the Body Fuctios compoet items ad for the Participatio compoet items scorig optios raged from 0 (o disability) to 4 (complete disability); for the Activities compoet items scorig optios raged from 0 (o disability) to 3 (complete disability); ad for the Evirometal Factors compoet items scorig optios raged from 0 (o support) to 2 (full support) [4]. Item scores were summed ito a scale with higher scores idicatig more disability. To evaluate the RV, we used the Physical Fuctioig costruct as represeted by the Activities of Movig aroud ad Self-care ad Domestic Activities scales, the Psychological Fuctioig costruct as represeted by the Metal Fuctios ad Pai scale, ad the Social Fuctioig costruct as represeted by the Participatio i Life Situatios scale. These scales were selected because items i these scales are closely associated with the scales i the cocurret measures. The SF-36 was selected as a well-kow reliable ad valid geeric multidimesioal health-impact measure used for NMD [12, 13]. The SF-36 [8] comprises 36 items with eight fuctioal dimesios. Three scales were used to examie the RV: Physical Fuctioig, Metal Health ad Social Fuctioig. Item scores were coded, summed ad trasformed to a score of 0 (worst health) to 100 (best health) for each scale. The overall Crobach s alpha for these scales was0.79iastudyofamyotrophiclateralsclerosispatiets [14]. I our previous study the Crobach s alpha for the selected scales raged from 0.77 to 0.94 [4]. The WHOQOL-bref [9] was selected as a geeric measuremet istrumet for a broad evaluatio of quality of life. It cosists of 28 items i four costructs ad two separate questios. Three scales were used to examie the RV: Physical Health ad Autoomy, Psychological Health, ad Social Relatios. Item scores from each scale were coded, summed ad trasformed to a score of 0 (worst health) to 20 (best health). The Crobach s alpha raged from 0.63 to 0.81 i a study of Multiple Sclerosis patiets [15]. I our previous study the Crobach s alpha for the selected scales raged from 0.60 to 0.84 [4]. The GARS [10] is a domai specific geeric measuremet istrumet for assessig disability i Activities of daily livig (ADL) ad Istrumetal activities of daily

3 Bos et al. BMC Neurology (2017) 17:87 Page 3 of 8 livig (IADL). It cosists of eleve ADL items ad seve IADL items. A four-category respose format was used, ad raged from 1 (o problem i performig without help) to 4 (impossible to perform). The scores were summed for each subscale. The Crobach s alpha raged from 0.95 to 0.97 i a study of Multiple Sclerosis patiets [15]. I our previous study the Crobach s alpha raged from 0.93 to 0.95 [4]. The IPAQ [11, 16] is a domai specific geeric measuremet istrumet for assessig participatio. It cosists of fiftee items focusig o perso-perceived participatio ad autoomy. The istrumet assesses two aspects of participatio: perceived participatio ad the perceived problems with participatio. I this study the perceived participatio aspect was used sice this costruct is closely associated with the Participatio i Life Situatios costruct i the NMDIP questioaire. The sub-domais were Autoomy Idoors, Family Role, Autoomy Outdoors, ad Social Relatios. The respose optios raged from 1 (very good) to 5 (very poor). Scores were summed for each domai. The Crobach s alpha raged from 0.86 to 0.94 i a study of Multiple Sclerosis patiets [15]. I our previous study the Crobach s alpharagedfrom 0.84 to 0.94 [4]. Criterio variables Two questios were selected as criterio variables: Extet of limitatios ad Quality of life. To evaluate the Extet of Limitatios respodets were asked to aswer the questio: To what extet are you limited due to your NMD? Resposes were o a te-poit scale ragig from 1 (ot limited at all) to 10 (completely limited). Respodets were classified ito oe of four groups: A with a very low extet of limitatio (score 1 2), B with a moderate extet of limitatio (score 3 5), C with a high extet of limitatio (score 6 8) ad, D with a very high extet of limitatio (score 9 10). The secod criterio variable for evaluatio of quality of life was oe of the two sigle items adapted from the WHOQOL-bref. Respodets were asked: How would you rate your quality of life?. Respose optios were: 1 = very poor, 2 = poor, 3 = either poor or good, 4 = good ad 5 = very good. Respodets were classified ito three groups: A very poor or poor quality of life, B either poor or good, ad C good or very good quality of life. Aalysis Descriptive statistics were used to characterize the total sample ad the test-retest sample. Differeces betwee both samples were examied usig the differece i proportios test, the two-sample t-test, ad if data are ot ormally distributed a o-parametric test for idepedet samples were used. Test-retest reliability or stability over time was examied usig the Wilcoxo Siged Test ad the oe-way radom itraclass correlatio coefficiets (ICCs) [17]. Relative Validity was examied i several steps. First, the Chi-square was computed for each scale by calculatig the Kruskal-Wallis H-test. Secod, the RV of each scale was computed by dividig each H- statisticbytheh-statisticforthescalewiththehighest H-statistic, ad multiplied by oe hudred. The resultig RV-estimate idicates the extet to which a scale or costruct is able to discrimiate betwee two groups compared to the measure with the highest H- statistic [18, 19]. Fially, the cliical relevace of the differeces betwee respodet subgroups, ad the oparametric effect size (coefficiet r) for urelated samples, was calculated for statistically sigificat group differeces (α = 0.05) with post hoc tests (Boferroi correctio) [20]. Effect sizes where estimated through coefficiet r, which was calculated by dividig the z-statistic (obtaied from the Ma-Whitey U test)bytherootofthesamplesize().toiterpret this oparametric effect sizes (coefficiet r), Cohe suggested the followig thresholds: a r of <0.10 idicates a trivial effect, a r of 0.10 to <0.24 a small effect, a r of 0.24 to <0.37 a moderate effect, ad a r 0.37 a large effect. A r 0.10 reflects a cliically relevat differece betwee groups [20, 21]. IBM SPSS statistics versio 22 was used. Results A total of 702 participats (70% respose rate) completed the questioaires. Of the 202 patiets who agreed to complete the NMDIP twice 185 participats (92% respose rate) actually retured the questioaire. The o-respodets from the 1003 eligible patiets did ot differ from respodets i terms of geder, but o-respoders were sigificatly youger tha respodets (p-value < ot i table). The total sample ( = 702) ad the test-retest sample ( = 185) differed i Age, Years sice diagosis. Participats i the total sample were older ad were diagosed more recetly with a NMD compared to the test-retest sample. Also a sigificat larger proportio of respodets i the total sample was Retired due to age compared to test-retest sample (p-value = 0.007) (Table 1). Fially the NMD category distributio differed sigificatly betwee the samples with less patiets with Motor-euro disorders ad Muscle disorders ad more patiets with Peripheral erve disorders i the total sample compared to the test-retest sample.

4 Bos et al. BMC Neurology (2017) 17:87 Page 4 of 8 Table 1 Patiet characteristics of total sample ad test-retest sample Variable Total sample N = 702 Test-retest sample p-value N = 185 Geder (%) Female 350 (50) 105 (57) 0.095^ Male 352 (50) 80 (43) 0.095^ Age ## Media 61 (21) 57 (18) Rage Year sice diagosis ## Media 7 (11) 10 (14) Rage Extet of limitatios ## Media 5 (4) 6 (4) Rage Quality of life (WHOQOL-bref) ## Media 4 (1) 4 (1) Rage Relatioship status (%) Relatioship (married/partership) 515 (73) 135 (76) 0.910^ Sigle (umarried/widowed/divorced) 186 (27) 45 (24) 0.549^ Educatioal level (%) Primary school/vocatioal traiig 235 (33) 57 (31) 0.492^ Secodary school/vocatioal traiig 270 (38) 81 (44) 0.188^ Higher educatio /vocatioal traiig 161 (23) 37 (20) 0.394^ Uiversity 28 (4) 8 (4) 0.837^ Employmet status (more aswers possible) (%) Followig a traiig or study 36 (5) 12 (7) 0.468^ Employmet (part-time or full time) 173 (25) 43 (23) 0.693^ Volutary work (part-time or full time) 42 (6) 15 (8) 0.294^ (Partially) retired due to NMD 213 (30) 67 (36) 0.126^ Housewife/househusbad 171 (24) 55 (30) 0.136^ Retired due to age 244 (35) 45 (24) 0.007^ NMD category (%) Motor euro disorder (MND) 43 (6) 20 (11) 0.027^ Muscle disorder (MD) 154 (22) 69 (37) <0.001^ Juctio disorder (JD) 234 (33) 66 (36) 0.549^ Peripheral erve disorder (PND) 271 (39) 30 (16) <0.001^ ^Differece i proportios test, ## Ma-Whitey U test. Iterquartile rage = Q3-Q1 Test-retest reliability Wilcoxo Siged Rak Test (Table 2) showed o sigificat score differeces betwee time poits for most of the NMDIP scales, idicatig stability over time, except for the Metal Fuctios ad Pai scale. However this differece was ot cliically relevat (ES 0.18, ot show i table). The ICC of all scales showed sufficiet agreemet ad raged from 0.79 to 0.97, idicatig good stability over time. Criterio-related relative validity Media scores of patiets with a low Extet of limitatio (Table 3) or very poor or poor Quality of life level

5 Bos et al. BMC Neurology (2017) 17:87 Page 5 of 8 Table 2 Test-retest reliability for the NMDIP scales ( = 185) Compariso of scores at measuremet 0 ad 1 Itraclass correlatio Cases Media Cases Media Z-statistic p-value * (oe way (N) (N) 0 1 radom) Muscle Fuctios (2) (2) Movemet Fuctios (3) (2) Excretio ad Reproductive Fuctios (3) (3) Swallowig ad Speech Fuctio (2) (2) Metal Fuctios ad Pai (4) (3) Activities of Movig aroud (6) (6) Self-care ad Domestic Activities (9) (7) Participatio i Life Situatios (2) (2) * Wilcoxo Siged Rak Test, 2-tailed. Iterquartile rage = Q3-Q1 (Table 4) were sigificatly differet i the hypothesized directio whe compared to the ext higher group mea. Extet of limitatios About 16% ( = 110) of the respodets reported low extet of limitatios ( A) due to NMD, while 36% ( = 250) reported a moderate extet of limitatio ( B), ad 39% ( = 270) reported a high extet of limitatio ( C). About 8% ( = 58) of the respodets reported a very high extet of limitatios ( D). Comparisos of the RV coefficiets, as summarized i Table 3, revealed that the NMDIP Activities of Movig aroud scale ad SF-36 Physical Fuctioig scale were the most valid i discrimiatig betwee groups with a icreasig extet of limitatio. We the examied the performace of the NMDIPscales i idicatig the differeces betwee extreme groups (A-D) ad subgroups (A-B, B-C, C-D) regardig the physical-, psychological- ad social fuctioig costructs, as they relate to similar costructs i the cocurret measuremet istrumets. Regardig physical fuctioig, we foud that both NMDIP activity scales tured out to be the most sesitive (followed by the Muscle Fuctios scale) for measurig differeces betwee extreme groups ad subgroups. However, the performace of the cocurret SF-36 Physical fuctioig scale ad both GARS scales were almost idetical. Regardig the psychological fuctioig costruct we foud that the NMDIP Metal Fuctios ad Pai scale was the best performig scale compared to the SF-36 Metal Health scale ad the WHOQOL-bref Psychological Health scale, showig the highest extreme group ad subgroup differeces. Regardig the social fuctioig costruct the NMDIP Participatio i Life Situatios scale performed better tha the SF-36 Social Fuctioig ad the WHOQOL-bref Social Relatios scales, ad roughly as well as the same as the comparable costructs i the domai-specific IPAQ. I summary, the NMDIP scales performed sufficiet to good i discrimiatig betwee (sub) groups with a icreasig extet of limitatios compared to similar costructs i cocurret measures regardig physical fuctioig, psychological fuctioig ad social fuctioig costructs. Quality of life Eight percet ( = 53) of the respodets reported poor or very poor quality of life ( A), while 25% ( = 175) experieced their quality of life as either poor or good ( B) ad 67% ( = 474) reported a good or very good quality of life ( C). Comparisos of the RV-coefficiets, as summarized i Table 4, revealed that the SF-36 Psychological Health scale ad IPAQ Autoomy outdoors scales were the most valid i discrimiatig betwee groups with differeces i quality of life. The Metal Fuctios ad Pai NMDIP scale was the third most valid scale. Whe examiig the performace of the NMDIPscales i idicatig the differeces betwee extreme ad subgroups for quality of life, we foud about the same extreme group differeces for the physical fuctioig scales for all cocurret costructs with moderate Effect Sizes (ESs). The same goes for the subgroup differeces, although the NMDIP Metal Fuctios ad Pai scale, ad the WHOQOL-bref Psychological Health scale performed slightly better tha the SF-36 Metal Health scale. Fially, whe examiig the social fuctioig scales we foud that the comparable NMDIP Participatio i Life Situatios scale performed about as well as the SF-36 Social Fuctioig scaleadtheipaq scales with a moderate to large ESs for extreme group differeces. The NMDIP Participatio i Life Situatios scale also performed better compared to the social fuctioig costruct of the WHOQOL-bref, the Social Relatios scale. The same goes for the subgroup differeces.

6 Bos et al. BMC Neurology (2017) 17:87 Page 6 of 8 Table 3 Relative validity of the NMDIP, domai specific ad geeric measuremet istrumets compared, usig subgroups of extet of limitatios ( = 702) A ( = 110) low extet of limitatio Media B ( = 250) moderate extet of limitatio Media C ( = 270) high extet of limitatios Media D ( = 58) very high extet of limitatios Media Kruskal- Wallis H A-B a B-C a C-D a Chi Square RV b es es es es NMDIP Muscle Fuctios 97 2 (2) (2) (2) 56 6 (3) Movemet Fuctios 92 0 (1) (2) (2) 49 5 (4) Excretio ad Reproductive Fuctios 82 0 (1) (2) (3) 43 3 (3) Swallowig ad Speech Fuctio (0) (1) (1) 55 1 (2) Metal Fuctios ad Pai 95 2 (2) (3) (4) (5) Activities of Movig aroud (2) (4) (7) (9) Self-care ad Domestic Activities (1) (3) (8) (14) Participatio i Life Situatios (0) (1) (4) 55 4 (4) SF-36 Physical Fuctioig (6) (8) (7) (3) Metal Health (3) (5) (6) (7) Social Fuctioig (1) (2) (3) (4) WHOQOL-bref Physical Health ad Autoomy (1) (0) (0) 57 3 (1) Psychological Health (0) (0.5) (1) 57 3 (0) Social Relatios (1) (0) (1) 55 4 (1) GARS Activities of Daily Livig (1) (6) (10) (17) Istrumetal Activities of Daily Livig (2) (7) (9) (5) IPAQ Autoomy Idoors (3) (6) (6) (11) Family Role (7) (7) (7) (9) Autoomy Outdoors (4) (3) (5) (4) Social Relatios (6) (4) (5) (4) mdip, gars, ad ipaq: higher scores = more uable to perform activity; sf-36 ad WHOQOL-bref scales: higher scores = better quality of life ad more able to perform activity. Iterquartile rage = Q3-Q1. - = ot statistically sigificat. Boferroi correctio a = (p-value 0.05/4). RV = relative validity, b = score idicatig the relative validity with score 100 related to the highest H-statistic. ES = effect size A-D a I summary, the NMDIP scales performed well i discrimiatig betwee subgroups with differeces i quality of life compared to similar costructs i cocurret measures cocerig the physical fuctioig, psychological fuctioig ad social fuctioig costructs. Discussio I this study the NMDIP, that was developed to reflect theprevaleceadseverityofabroadrageofnmdrelated disabilities [4], showed stability ad performed well i the criterio-related subgroups of NMDpatiets who differed i the extet of limitatio ad quality of life. The results of the test-retest reliability aalysis were sufficiet idicatig stability i the eight NMDIP scales. Although the results showed a differece for Metal Fuctios ad Pai scale while the effect size was trivial, the itraclass correlatio showed sufficiet agreemet for all NMDIP scales betwee the two measuremet momets. I geeral, the NMDIP scales performed well i discrimiatig betwee relevat subgroups with icreasig extet of limitatio. This was the case for costructs evaluatig physical, psychological, ad social fuctioig. The NMDIP scales showed satisfactory relative validity ad moderate to strog ESs idicatig the stregth

7 Bos et al. BMC Neurology (2017) 17:87 Page 7 of 8 Table 4 Relative validity of the NMDIP, domai specific ad geeric measuremet istrumets compared, usig subgroups of Quality of Life ( = 702) A ( = 53) Very poor or poor QoL Media B ( = 175) Neither poor or good QoL Media C ( = 474) Good or very good QoL Media Kruskal-Wallis H Chi Square RV b A-B a B-C a es es es NMDIP Muscle Fuctios 52 5 (2) (2) (2) Movemet Fuctios 46 4 (3) (2) (3) Excretio ad Reproductive Fuctios 36 3 (5) (2) (2) Swallowig ad Speech Fuctio 49 2 (3) (1) (1) Metal Fuctios ad Pai 45 8 (4) (4) (4) Activities of Movig aroud (8) (6) (5) Self-care ad Domestic Activities (13) (8) (4) Participatio i Life Situatios (5) (4) (2) SF-36 Physical Fuctioig (4) (8) (10) Metal Health (6) (6) (4) Social Fuctioig 53 5 (3) (2) (2) WHOQOL-bref Physical Health ad Autoomy 53 3 (5) (0) (0) Psychological Health 53 3 (0) (1) (0) Social Relatios 50 3 (1) (1) (0) GARS Activities of Daily Livig (16) (10) (7) Istrumetal Activities of Daily Livig (12) (9) (9) IPAQ Autoomy Idoors (9) (5) (7) Family Role (8) (8) (8) Autoomy Outdoors (4) (5) (5) Social Relatios (6) (3) (5) mdip, gars, IPAQ scales: higher scores = lower quality of life, sf-36, ad WHOQOL-bref scales: higher scores = higher quality of life. Iterquartile rage = Q3- Q1. - = ot statistically sigificat. Boferroi correctio a = 0.02 (p-value 0.05/3). RV relative validity. b = score idicatig the relative validity with score 100 related to the highest H-statistic. ES effect size A-C a of the differeces betwee subgroups. The NMDIP showed satisfactory performace i discrimiatig betwee relevat subgroups with decreasig Quality of Life. This was the case for costructs evaluatig physical, psychological ad social fuctioig. Stregth of this study is the iclusio of a large populatio of patiets diagosed with a NMD. Some potetial study limitatios should be metioed. First, RV was examied as criterio-related validity value i this study. Because of the absece of a widely accepted criterio measure we chose to use self-report measures, which tured out to be a useful method. Secodly, the (relatively) small group sizes for very high extet of limitatios ( D) ad very poor or poor quality of life ( A) might have a egative impact o detectig group differeces, though the differece betwee these subgroups ad the adjacet groups showed sufficiet ESs. The results i this study permit us to recommed that researchers cosider Relative Validity as a useful method to select a valid ad with cautio a sesitive measure, especially whe data from logitudial studies or itervetio studies are lackig. At the same time, we wat to stress that RV is ot a substitute for the sesitivity-tochage test. The fidigs i this study caot be geeralized to logitudial studies. We recommed further research to evaluate the sesitivity to chage of the NMDIP scales. Furthermore geeric health measures have some disadvatages agaist disease-specific health measures i addressig topics of a particular relevace to patiets with a

8 Bos et al. BMC Neurology (2017) 17:87 Page 8 of 8 specific disease. Therefore it is recommeded that the idividual items i a scale be examied to estimate the suitability of the scale for a particular patiet populatio [13]. Coclusios The results i this study cofirmed the stability of the NMDIP over time, ad showed good relative validity compared to geeric QOL ad domai-specific measures. I combiatio with the fidigs i our previous study [4], the NMDIP proved to be a valid ad reliable diseasetargeted measure with a broad scope o physical, psychological ad social fuctioig. Further research should examie the resposiveess of the NMDIP scales. Abbreviatios ADL: Activities of daily livig; GARS: Groige Activity Restrictio Scale; IADL: Istrumetal activities of daily livig; ICC: Itraclass correlatio coefficiet; IPAQ: Impact o Participatio ad Autoomy Questioaire; NMD: Neuromuscular Diseases; NMDIP: Neuromuscular Disease Impact Profile; RV: relative validity; SF-36: Medical Outcome study Short Form Questioaire; WHOQOL-bref: World Health Orgaizatio Quality Of Lifeabbreviatio versio Ackowledgemets We wish to thak the patiets who participated i this study ad were so kid to share persoal iformatio about the cosequeces of their disease by takig the time to fill i the questioaires. We also wish to thak the studets who assisted i data collectio: Roald Bras, Kyra va der Beek, Haa Bosma, Aelies Verschure, Carolie Verschure, ad Marieke Verschure. Fudig Not Applicable. Availability of data ad materials The data that support the fidigs of this study are available from the correspodig author upo reasoable request. Authors cotributios IB cotributed to the study desig, coordiated the data collectio, ad drafted the mauscript. JBMK supervised the study ad helped draftig the mauscript. JA supervised the statistical aalyses ad helped draftig the mauscript. HPHK supervised the study ad helped draftig the mauscript. KW iitiated, coordiated ad supervised the study ad draftig of the mauscript. All authors read ad approved the fial mauscript. Competig iterests The authors declare that they have o competig iterests. Coset for publicatio Not applicable. Ethics approval ad coset to participate Ethical approval was obtaied from the local ethics committee, the Medical Ethical Committee of the Uiversity Medical Ceter Groige. Referece: METc Iformed coset was obtaied from all participats. Publisher s Note Spriger Nature remais eutral with regard to jurisdictioal claims i published maps ad istitutioal affiliatios. Received: 7 October 2016 Accepted: 26 April 2017 Refereces 1. 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