Spasticity is a feature of altered skeletal muscle performance in muscle tone involving hypertonia; it is also referred to as an unusual "tightness", stiffness, and/or "pull" of muscles. The word spasm comes from the Greek word, σπασμός (spasmos), meaning to pull or drag.
Clinically spasticity is defined as velocity dependent resistance to stretch, where a lack of inhibition results in excessive contraction of the muscles, ultimately leading to hyperflexia (overly flexed joints). It mostly occurs in disorders of the central nervous system (CNS) impacting the upper motor neuron in the form of a lesion, such as spastic diplegia, but it can also present in various types of multiple sclerosis, where it occurs as a symptom of the progressively-worsening attacks on myelin sheaths and is thus unrelated to the types of spasticity present in neuromuscular cerebral palsy rooted spasticity disorders.
The clinical underpinnings of two of the most common spasticity conditions, spastic diplegia and multiple sclerosis, can be described as follows: in spastic diplegia, the upper motor neuron lesion arises often as a result of neonatal asphyxia, while in conditions like multiple sclerosis, spasticity is thought by some to be as a result of the autoimmune destruction of the myelin sheaths around nerve endings — which in turn can mimic the gamma amino butyric acid deficiencies present in the damaged nerves of spastic diplegics, leading to roughly the same presentation of spasticity, but which clinically is fundamentally different from the latter.
Spasticity Pathophysiology
The cause of spasticity is not really known, but there are several theories. In general, spasticity develops when an imbalance occurs in the excitatory and inhibitory input to α motor neurons caused by damage to the spinal cord and/or central nervous system. The damage causes a change in the balance of signals between the nervous system and the muscles, leading to increased excitability in the muscles.
One factor that is thought to be related to spasticity is the stretch reflex. This reflex is important in coordinating normal movements in which muscles are contracted and relaxed and in keeping the muscle from stretching too far. Although the end result of spasticity is problems with the muscles, spasticity is actually caused by an injury to a part of the central nervous system (the brain or spinal cord) that controls voluntary movements. The damage causes a change in the balance of signals between the nervous system and the muscles. This imbalance leads to increased activity (excitability) in the muscles. Receptors in the muscles receive messages from the nervous system, which sense the amount of stretch in the muscle and sends that signal to the brain. The brain responds by sending a message back to reverse the stretch by contracting or shortening.
Overall, a defining feature of spasticity is that the increased resistance to passive stretch is velocity-dependent. Lance (1980) describes it this way: “...a motor disorder, characterised by a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neurone (UMN) syndrome."
Spasticity is found in conditions where the brain and/or spinal cord are damaged or fail to develop normally; these include cerebral palsy, multiple sclerosis, spinal cord injury and acquired brain injury including stroke. Damage to the CNS as a result of stroke or spinal cord injury, alter the of peripheral nerves in the affected region. This change in input to bodily structures tends to favor excitation and therefore increase nerve excitability. CNS damage also causes nerve cell membranes to rest in a more state. The combination of decreased inhibition and an increased depolarized state of cell membranes, decreases action potential threshold for nerve signal conduction, and thus increases activity of structures innervated by the affected nerves (spasticity). Muscles affected in this way have many other potential features of altered performance in addition to spasticity, including muscle weakness; decreased movement control; clonus (a series of involuntary rapid muscle contractions often symptomatic of muscle over-exertion and/or muscle fatigue); exaggerated deep tendon reflexes; and decreased endurance.
Spasticity and Clonus
Clonus tends to co-exist with spasticity in many cases of stroke and spinal cord injury likely due to their common physiological origins. Some consider clonus as simply an extended outcome of spasticity. Although closely linked, clonus is not seen in all patients with spasticity. Clonus tends to not be present with spasticity in patients with significantly increased muscle tone, as the muscles are constantly active and therefore not engaging in the characteristic on/off cycle of clonus. Clonus results due to an increased motor neuron excitation (decreased action potential threshold) and is common in muscles with long conduction delays, such as the long reflex tracts found in distal muscle groups. Clonus is commonly seen in the ankle but may exist in other distal structures as well, such as the knee or spine.
Read more about Spasticity: Historical Progression of Spasticity Hypotheses, Presentation and Assessment, Treatment, Prognosis