Dominic Stone
Muscle tone is defined as the resistance to passive stretch, and it is primarily regulated by the central nervous system. The assessment of muscle tone is critical to several movement disorders, such as Parkinson's disease and dystonia. The Modified Ashworth Scale (MAS) is the most universally accepted clinical tool used to measure the increase of muscle tone. MAS assigns a grade of spasticity from a 0-4 ordinal scale. The reliability of muscle tone assessments is weaker among younger patients. Recent studies have also been conducted to assess muscle tone by machine learning using surface electromyography.
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What You'll Learn
Modified Ashworth Scale
The Modified Ashworth Scale (MAS) is a universally accepted clinical tool used to measure the increase in muscle tone and is the current standard for assessing extremity spasticity. It is a revised version of the original Ashworth Scale, which was a 5-point numerical scale designed to assess the effectiveness of antispasticity drugs on spasticity in people with multiple sclerosis. The Modified Ashworth Scale adds a 1+ scoring category to indicate resistance through less than half of the movement, with scores ranging from 0-4.
The Modified Ashworth Scale is used to assess the resistance experienced during the passive range of motion of a patient's limb. The test is performed by extending the patient's limb from a position of maximal possible flexion to maximal possible extension (the point at which the first soft resistance is met). The scale is then assessed while moving from extension to flexion. The grade of spasticity is assigned by moving a joint/muscle through a high-velocity quick stretch.
The Modified Ashworth Scale has been utilised in various populations, including stroke, spinal cord injury, multiple sclerosis, cerebral palsy, traumatic brain injury, paediatric hypertonia, and central nervous system lesions. It is widely used in research and clinical practice due to its quick and easy procedure. However, it has been criticised for its poor inter and intra-rater reliability, specifically its inability to differentiate between the various factors contributing to resistance to passive stretch.
Despite the criticism, the Modified Ashworth Scale remains a valuable tool for assessing spasticity and muscle tone. It is essential in several movement disorders, such as Parkinson's disease and dystonia, where distinguishing between subtypes of increased muscle tone is critical. Validity and reliability studies have been conducted to ensure the accuracy and consistency of the Modified Ashworth Scale when used by different assessors and across multiple testing occasions.
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Muscle tone in children
Muscle tone is the term for the resting length of muscles in the body or the length of a muscle before contraction. In low muscle tone, also known as hypotonia, the resting length of the muscles is greater than average, causing hyperextension at the joints, or what some refer to as "double-jointedness". On the other hand, in high muscle tone, also called hypertonic, the resting length of the muscle is less than average, causing the limbs to feel rigid and inflexible.
In children, muscle tone differences are most evident and a child with low muscle tone may have decreased head control and sitting balance. They may also take longer to crawl, and as they grow older, they will present with decreased endurance for playtime, sports, or school, or may sit with poor posture. Children with low muscle tone may also have delayed coordination with activities such as throwing, catching, running, skipping, and galloping. They may also tire easily and have decreased endurance during play, sports, and school.
Clinically significant high muscle tone in children is most commonly caused by in-utero stroke, brain bleeds, or cerebral palsy. High muscle tone in children will be identified within the first 18 months of life, and the child may seem stiff or rigid in all limbs or on one side of the body. Children with high muscle tone may have delayed gross and fine motor skill development and may have difficulty relaxing their muscles. They may also maintain a fisted hand beyond 6 months of age and present with very stiff legs that appear to move like scissors when standing or attempting movement.
For children with low muscle tone, physical and occupational therapy can help strengthen muscle groups and decrease the impact of low muscle tone. Some exercises and activities for children with low muscle tone include bouncing on a therapy ball, animal walks, and tug of war. Hand exercises are also important, as writing and drawing are often more difficult for kids with low muscle tone.
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Rigidity, dystonia, and spasticity
Rigidity is a type of hypertonia where it is difficult to move the muscles through their range of motion, regardless of the speed with which the limb is moved. This type of posturing usually means there has been severe damage to the brain. Decorticate and decerebrate rigidity are a form of spastic dystonia. Decorticate posture is a sign of damage to the nerve pathway in the midbrain, which is between the brain and spinal cord.
Dystonia is a type of muscle tone that can occur in children with cerebral palsy. Dystonia can be distinguished from spasticity in neonatal rabbits subjected to prenatal hypoxia/ischemia as models for cerebral palsy. Dystonia is also one of the main challenges in the assessment of muscle tone in movement disorders such as Parkinson's disease.
Spasticity affects a multitude of patients following some inciting event. It occurs in 20 to 30% of post-stroke patients and can also occur in cerebral palsy, multiple sclerosis, brain injury, trauma, and spinal cord injury. Spasticity is considered to be a form of sustained efferent muscular hyperactivity, dependent on the continuous supraspinal drive to the alpha motor neuron.
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Muscle tone and positioning
Muscle tone is defined as the resistance to passive stretch, but this definition is often criticised for its ambiguity. Some suggest it is related to a state of preparation for movement. It is primarily regulated by the central nervous system, and individuals with neurological disorders may lose the ability to control normal tone and can exhibit abnormalities.
The assessment of muscle tone is critical to several movement disorders, such as Parkinson's disease and dystonia. The main challenge is to distinguish among the main subtypes of increased muscle tone: rigidity, dystonia, and spasticity. Spasticity is defined as a velocity-dependent increase in muscle stretch reflexes associated with increased muscle tone as a component of upper motor neuron syndrome.
The Modified Ashworth Scale (MAS) is a universally accepted clinical tool used to measure the increase in muscle tone. MAS assigns a grade of spasticity from a 0-4 ordinal scale. The grade is assigned by moving a joint/muscle through a high-velocity quick stretch. The MAS is the current standard for clinical assessment of extremity spasticity and is the most commonly used tool to evaluate the efficacy of pharmacologic and rehabilitation interventions for the treatment and management of spasticity.
Despite the type of muscle tone, optimal positioning is crucial for feeding and swallowing. The proper feeding position includes neutral alignment of the head and neck, midline orientation, a symmetrical trunk position, 90-degree pelvic/femoral alignment, and symmetrical arm position with neutral shoulders.
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Muscle tone abnormalities
Increased muscle tone, or hypertonia, can manifest as spasticity, rigidity, or dystonia. Spasticity is characterised by tense and resistant muscles, with involuntary muscle contractions. Rigidity refers to stiffness and resistance to passive stretch, while dystonia is a movement disorder characterised by abnormal postures and sustained muscle contractions. Paratonia, another form of increased muscle tone, is described as increased muscle tone at rest, resulting in involuntary resistance to passive movement.
Decreased muscle tone, or hypotonia, is often associated with LMN disease. It can cause difficulty in maintaining a stable posture, impaired self-feeding skills, compromised rib cage expansion, and poor oesophageal motility. Hypotonia can also impact respiratory function and airway management, requiring careful positioning during feeding and swallowing.
The assessment of muscle tone is crucial for diagnosing and managing these conditions. Evaluations may include physical and occupational therapy assessments, gait analysis, nerve conduction studies, and imaging techniques such as Magnetic Resonance Imaging (MRI). The Modified Ashworth Scale (MAS) is a widely accepted tool for grading muscle spasticity and increased muscle tone. It assesses the resistance during passive range of motion and is used to evaluate the effectiveness of treatments for spasticity.
In children, muscle tone abnormalities may be observed in neurodevelopmental disorders, early brain injuries, genetic conditions, and developmental coordination disorders. Assessments such as the ATNA, NNNS, Premie-Neuro, HINE, and NSMDA can help evaluate resting and active muscle tone in infants and children, aiding in the identification and management of these conditions.
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Frequently asked questions
The Modified Ashworth Scale (MAS) is a clinical tool used to measure the increase of muscle tone and is the most universally accepted scale for this purpose. It is a revised version of the original Ashworth Scale, which was designed to assess the effectiveness of antispasticity drugs on patients with multiple sclerosis. The MAS grades spasticity from 1-4, with 1 being a slight increase in muscle tone and 4 being a considerable increase.
The MAS works by moving a joint or muscle through a high-velocity quick stretch. The degree of resistance is determined to be less than normal (hypotonic), normal, or more than normal (hypertonic).
The MAS has been criticised for its poor inter and intra-rater reliability. The reliability of muscle tone assessments is also weaker among younger patients.
The ATNA at term, NNNS, Premie-Neuro, HINE, and NSMDA are some of the methods used to assess resting and active muscle tone in infants and children.
