Original article
The Hammersmith Functional Motor Scale for Children with Spinal Muscular Atrophy: a Scale to Test Ability and Monitor Progress in Children with Limited Ambulation

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Abstract

A functional motor scale was devised for use in children with spinal muscular atrophy type 2 and type 3, in particular those with limited mobility, to give objective information on motor ability and clinical progression. The scale, which has 20 scored activities, was designed to be self-explanatory, quick, easy to use, reproducible and reliable. In this paper we describe the development of the scale, reporting the criteria used to choose the items to be included, their application in a normal cohort and in a cohort of children with SMA and how we arrived to a final version of the scale in which the items are arranged in order of difficulty.

The analysis of 120 assessments over 2 years from 51 children with type 2 and type 3 SMA also led to a more accurate profile of functional achievements in both type 2 and 3 SMA and to a more detailed sub-classification of type 2 SMA.

Introduction

SMA is a neuromuscular condition due to the degeneration of the anterior horn cells, inherited in an autosomal recessive way.1 It is the second most common neuromuscular disease of childhood affecting approximately 1:10,000–12,000 children. There is a wide range of severity, the most severe form being Werdnig–Hoffman disease (or SMA type 1), in which affected children are unable to sit unsupported. The mildest form is Kugelberg–Welander disease (SMA type 3) in which independent ambulation is achieved but mild to moderate proximal muscle weakness may be experienced, causing difficulty with stairs and rising from the floor. Children with the intermediate form (SMA type 2) are, in contrast, able to sit but not to walk independently.2., 3.

Prognosis is dependent on severity and almost invariably children with Werdnig–Hoffman disease will die of respiratory complications by the age of 2 years. Long term survival is expected for children with the type 2 and type 3 and currently available medical treatment is directed at secondary complications such as respiratory problems or management of deformities.

Although the weakness itself does not progress in SMA, several studies have documented a deterioration in functional abilities due to increased body size without an increase in power to cope with these extra demands. As a result a significant proportion of children with SMA type 3 can lose the ability to walk independently.4., 5.

There are numerous functional motor scales that can be used for patients with neuromuscular disorders, but these cannot always be used in non-ambulant children or in those with limited ambulation.

In our department, functional motor scoring was based almost exclusively on the scale developed by Scott, Hyde et al published in 19826 devised for boys with Duchenne Muscular Dystrophy. The scale helps to monitor disease progression and gives an indication of when boys with Duchenne Muscular Dystrophy are about to lose independent ambulation. The scale is also useful when assessing children with other neuromuscular disorders who have deteriorating ambulation but it is not a reliable indicator of change in ability for many of the weaker non-ambulant children. There are also other scales, such as the Vignos scale7 which determines levels of mobility and others assessing ability in function and ‘activities of daily living’,8., 9., 10. but although they all undoubtedly have their uses, none has been shown to be sufficiently sensitive when assessing functional change in weaker, non-ambulant children. In contrast, a new scale has been recently proposed for assessing function in patients with neuromuscular disorders with more severe functional impairment but this was designed to be used in non-ambulatory patients only.11

The purpose of our study was to develop a useful and reliable scale, and to use it in clinical practice. The objectives used when developing the scale were that it should be easy to use and of rapid execution (for the therapist and the child), suitable for very young children but relevant to adolescents and if necessary, adults, self-explanatory, reproducible and reliable.

The aims of the scale were to:

  • (i)

    Evaluate and illustrate the motor ability of children with SMA with limited ambulation;

  • (ii)

    monitor the progression of function;

  • (iii)

    provide a tool for an accurate classification of SMA and in particular to allow a graded scale that takes into account the significant clinical variability of children with this disorder.

In this paper we will describe the development of the scale, reporting the criteria used to choose the items to be included, their application in a normal cohort and in a cohort of children with SMA and how we arrived to a final version of the scale in which the items are arranged in order of difficulty.

Section snippets

Choice of items

The intention of the scale was that it included both upper and lower limb activities as well as trunk and head control. It was not possible to add many specific upper limb tasks without using additional standardised equipment or making the instructions to the assessor more complicated. It was felt that the activities such as rolling fully prone/supine, propping on arms and crawling, all gave indirect but useful indicators of upper limb performance.

The activities included in the assessment were,

Discussion

In this paper we describe the use of a simple clinical scale to assess functional ability in children with SMA with limited mobility. The scale does not use any equipment and can, therefore, be used in any clinical setting. It is also self-explanatory, indicating precisely how it is scored. While this scale has been primarily devised for non-ambulant children with SMA, it can be used for any child with neuromuscular disease causing a similar level of functional impairment.

All the activities

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