Muscle force and other factors for falls in myotonic dystrophy type 1: studies of persons with a rare disease

Background

Myotonic dystrophy type 1 (DM1) is a dominantly inherited neuromuscular disorder with a slow progressive muscular weakness. It is a rare disease, the prevalence in Europe varies between 3-15/100,000, but it differs greatly from region to region in the world [1]. The highest known prevalence is found in the northeast region of the Quebec Province in Canada, 189/100,000, and it is more common than other muscular dystrophies [2]. Functional disturbances in different organs (heart, eyes, stomach and brain) are frequent, as DM1 is also a multisystem disorder [3]. The needs for care, support and exercise are great. Accidental falling and activity restrictions due to fear of falls seem to be a common problem, as well as respiratory failure as a consequence of muscle weakness in the diaphragm and other chest muscles.

Presently, there is no cure for patients with DM1, but the research is ongoing. The Neuromuscular Centre, Sahlgrenska University Hospital, in Sweden, is a centre where, on a yearly basis, a team consisting of a physiotherapist, occupational therapist, nurse, neurologist and social worker follow these persons, in the region [1–9] Västra Götaland about 240 adult individuals. For persons with DM1, the reliability of measurement methods, the natural course of the muscle force decrease and functional balance impairment and the factors behind functional balance skills and falls have not been investigated.

Purpose

The aims were, in adults with DM1, to investigate factors of importance for functional balance skills and falls, and to investigate the natural course of leg muscle force and functional balance impairments with reliable functional balance measurement methods.

Methods

All studies were quantitative with continuous and categorical data. All genetically proven 20- to 60-year-old persons with DM1 in the western part of Sweden, able to perform Timed Up & Go (TUG), were invited to participate [8].

In the first study, we evaluated test–retest stability in static and dynamic balance tests and gait, with three assessment occasions spaced one week apart, in 10 patients. They were tested at three sessions one week apart to examine any systematic difference/learning effect. In the second study, which was a cross-sectional study, 51 patients were assessed for muscle strength, gait and functional balance together with self-reported balance confidence, walking ability and falls. A multivariate analysis of factors of importance for functional balance impairment was performed. Of these 51 patients, 43 were further analysed in a third five-year prospective study for changes in muscle force, gait and functional balance together with self-reported balance confidence, walking ability and falls.

The isometric muscle force was measured with a handheld gauge meter. The TUG was performed in a self-selected safe pace. 10-meter walk (10MW) was performed in a long corridor, with standing start and flying finish in both self-selected (10mCOM) and maximal speed (10mMAX). Step test (STEP) was performed as described by Hill [7]. Number of falls was asked for by recall. The Activities-specific Balance Confidence (ABC) scale measured fear of falls [9].

Results

The functional tests (TUG, STEP & 10mMAX) showed reliable repeatability, ICC_2.1 0.83–0.94, and satisfying (low) variability [6]. Factors of importance for falls, in the cross-sectional study, showed to be the isometric ankle dorsiflexor force (r_s −0.38, p < .007) and a low balance confidence assessed on the ABC scale (r_s −0.52, p < .001). In the results of the univariate ordinal regression analysis, an augmented risk of falling with 49% at a decrease of 10 units in ABC score was shown. The multivariate analysis detected that a decrease of 10 N in ankle dorsiflexor force would infer 15% increased fall risk and an increase in time difference with one second between comfortable and maximum walking speed, measured over 10 metres, would increase the fall risk with 42% [4].

All functional tests showed not only statistically significant deteriorations but also clinical important changes after five years (p< .001) in the 43 persons, 18 men/25 women, who were followed during this time. A force decrease was shown in the four examined leg muscles together (mean change −8 to −15 N, p =.017 to < .001). There was an unexpected difference between sexes, the men had a worse natural course, the force in all their muscle groups had deteriorated and they had all fallen the year prior to the five-year assessment. Also, this change was visible on the ABC scale where the men scored −10 units (p < .05), but for the whole group, the change was −7 units (p < .024). The women only showed a statistically significant decrease in the hip flexors (−7%, p < .001) and 60% had fallen at least once the previous year.

Discussion

The balance confidence was shown to be a measure reflecting the risks for frequent falls. The confidence is likely to diminish after experiencing repeated falls, and the men seemed to be aware of this augmented fall risk, as they had a statistically significant decrease in the ABC score.

Conclusions

Dynamic functional balance tests were more consistent than static tests and should be preferred in persons with myotonic dystrophy type 1. Both isometric leg muscle force and functional balance measures deteriorated significantly after five years, but the course was worse in the men. An increasing proportion of the persons with DM1 avoided activities due to fear of falling.

Implications

This patient group should be

• regularly followed to prevent falling

• asked about falls

• encouraged to wear orthoses, use walking aids and (if needed):

• provided with electric wheelchair for continued participation in different activities.

Ethics approval

The study was approved by the Regional Ethical Review Board in Gothenburg, Sweden, dnr 248-06.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The study was funded by Norrbacka-Eugenia Foundation; Promobilia Foundation; the Local Research and Development Board for Gothenburg/Bohuslän; Muskelfonden; and Greta and Einar Asker Foundation.