Motor-control abnormalities resulting from an upper motor neuron lesion
Upper motor neuron syndrome (UMNS) is the motor control changes that can occur in skeletal muscle after an upper motor neuron lesion.
Following upper motor neuron lesions, affected muscles potentially have many features of altered performance including:
weakness (decreased ability for the muscle to generate force)
decreased motor control including decreased speed, accuracy and dexterity
altered muscle tone (hypotonia or hypertonia) – a decrease or increase in the baseline level of muscle activity
decreased endurance
exaggerated deep tendon reflexes including spasticity, and clonus (a series of involuntary rapid muscle contractions)
Such signs are collectively termed the "upper motor neuron syndrome". Affected muscles typically show multiple signs, with severity depending on the degree of damage and other factors that influence motor control. In neuroanatomical circles, it is often joked, for example, that hemisection of the cervical spinal cord leads to an "upper lower motor neuron syndrome and a lower upper motor neuron syndrome". The saying refers to lower motor neuron symptoms in the upper extremity (arm) and upper motor neurons symptoms in the lower extremity (leg).
Health professionals' understanding of impairments in muscles after an upper motor neuron lesion has progressed considerably in recent decades. However, a diagnosis of "spasticity" is still often used interchangeably with upper motor neuron syndrome, and it is not unusual to see patients labeled as spastic who demonstrate an array of UMN findings.[1]
Spasticity is an exaggerated stretch reflex, which means that a muscle has a reflex contraction when stretched, and that this contraction is stronger when the stretch is applied more quickly. The commonly quoted definition by Lance (1980) describes "a motor disorder, characterised by a velocity-dependent increase in tonic stretch reflexes with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neurone (UMN) syndrome".
Spasticity is a common feature of muscle performance after upper motor neuron lesions, but is generally of much less clinical significance than other features such as decreased strength, decreased control and decreased endurance. The confusion in the use of the terminology complicates assessment and treatment planning by health professionals, as many confuse the other findings of upper motor neuron syndrome and describe them as spasticity.[1] This confusion potentially leaves health professionals attempting to inhibit an exaggerated stretch reflex to improve muscle performance, potentially leaving more significant UMNS changes such as weakness unaddressed. Improved understanding of the multiple features of the upper motor neuron syndrome supports more rigorous assessment, and improved treatment planning.
^ abIvanhoe CB, Reistetter TA: Spasticity: The misunderstood part of the upper motor neuron syndrome. Am J Phys Med Rehabil 2004;83(suppl):S3–S9.
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