Spasticity

Definition

Spasticity is a velocity-dependent increase in resistance to passive movement of a limb. This means that if you try to move the child's arm or leg, resistance increases as the speed of the movement is increased. In some cases, the rapid increase in resistance leads to a "catch" as the limb is moved, with a subsequent "release" of the resistance once the limb stops moving. Spasticity is one symptom of the "upper motor neuron syndrome," a condition caused by damage to portions of the brain or spinal cord controlling movement.

Spasticity does not necessarily interfere with the child's attempts at voluntary movement. It is essentially a property of passive movement. Therefore, spasticity is triggered by the interaction between the child and the environment.

Impairment in voluntary movement is more often caused by symptoms associated with spasticity or the upper motor neuron syndrome (such as weakness, dystonia, or ataxia), rather than by the spasticity itself. In some cases, the increased tone due to spasticity is helpful to maintain the legs straight and thereby to support the child's weight against gravity.

However, when spasticity is very severe, it may be present even when the child is not moving. This may lead to a rigidly fixed joint. The terms "spasticity" and "increased tone" may, in some cases, be used interchangeably.

Spasticity in children is most commonly due to cerebral palsy (CP), and there are sets of spasticity syndromes or patterns that are well recognized. These include...

Spastic diplegia (both legs involved greater than arms)
Hemiplegia (involves an arm and a leg on the same side of the body)
Double hemiplegia (both arms involved, more than legs)
Tetraplegia (all four limbs involved, usually severely)
Some clinicians distinguish between "plegia" (meaning complete paralysis) and "paresis" (meaning weakness)

Examination

In order to examine a child with spasticity, the involved muscles must be stretched at varying speeds. For example, if the muscles of the upper leg are involved, then, during the examination, the knee is flexed and then extended at slow, intermediate, and fast speeds. Evaluating these stretches helps in determining if the resistance varies with velocity or if "catch" occurs as the velocity becomes fast enough and the spasticity has time to activate. Often there is also a "clasp-knife" release, in which there is a sudden reduction in resistance following the catch.

In children with mild or moderate spasticity, there is no increased resistance to slow movements. Therefore, if the evaluator perceives a continuous "waxy" or "lead-pipe" feeling is more suggestive of dystonia or parkinsonian rigidity. Since spasticity is thought to be caused by a failure in communication between the brain and spinal cord, it is also important to check for other evidence of such failure.

In particular, the involved muscles may be weak, with primitive withdrawal reflexes. These reflexes include the "triple-flexion" response of the leg, or Babinski sign (a toe moves upward when the bottom of the foot is stroked firmly with a metal object).

The mechanism in the body that produces spasticity is probably an increase in the stretch reflex; therefore, it is important to look for signs of increased stretch reflex or clonus (multiple jerks from a single tendon tap) throughout the body. The location of spasticity, weakness, and increased reflexes may be a clue to the location of the interruption of communication.

In spasticity due to brain injury, including cerebral palsy, it is helpful to determine the overall distribution of weakness in order to classify the syndrome appropriately as diplegia, hemiplegia, etc.

As with other childhood movement disorders, spasticity may be only one of several symptoms that are simultaneously present. It is therefore important to examine for dystonia and rigidity contributing to limb stiffness, as well as other possible disorders.

Mechanism

Spasticity is caused by an increase in the reaction of the stretch reflex. Normally, this reflex is activated whenever a muscle is rapidly stretched. This reflex causes the muscle to contract in order to resist the force that is stretching it.

However, normal movement often requires turning off this reflex. The brain normally sends inhibitory signals through the corticospinal tract to the cells of the spinal cord. The corticospinal tract refers to nerves fibers that project from the brain's cortex into the nerves of the spine. These signals may reduce the stretch reflex. If there is damage to the corticospinal tract or the cells in the brain that are its origin, the stretch reflex may be disinhibited. Over time, an uncontrolled stretch reflex may gradually increase in strength with a decreased threshold for triggering; this continues until even small stretches of the muscle lead to an excessive complete contraction of the muscle.

At its worst, the muscle contracts even at rest, and the limb becomes impossible to move. Longstanding spasticity is further complicated by shortening of the muscles and tendons; even complete relaxation of the muscle does not allow full movement of the affected joint.

Spasticity is not usually present immediately after an injury to the brain or spinal cord. It develops gradually over the following weeks or months, and then may stabilize.

Etiology

Direct Damage to Motor Cortex:
In common practice, direct damage to the motor cortex is most often due to an episode or prolonged occurrence of low oxygen [hypoxia] or low blood flow [ischemia] or both. Hypoxia (e.g., prenatal utero-placental insufficiency and perinatal asphyxia, near-drowning, apparent life-threatening events, suffocation, etc.), trauma, tumor, stroke, brain malformations, viral encephalitis. "Cerebral palsy" means any weakness due to brain injury.

Injury to Corticospinal Tract in the Brain:
Periventricular leukomalacia (a form of cerebral palsy, probably caused by hypoxic-ischemic injury in premature newborns), tumor, stroke, multiple sclerosis (MS)

Injury to Corticospinal Tract in the Spinal Cord:
Spinal cord traumatic injury, spinal tumor, epidural abscess, spina bifida (and tethered cord), multiple sclerosis (transverse myelitis), syringomyelia, spinal ischemia, venous thrombosis

Degenerative Disease:
Ceroid lipofuscinosis, Tay-Sachs disease, Rett syndrome, tropical spastic paraparesis, spinocerebellar ataxias (including Machado-Joseph disease), Rasmussen's encephalitis, hemiatrophy-hemiseizure syndrome, sialidosis, Pelizaeus-Merzbacher syndrome.

Workup

Spasticity usually becomes worse over the first several months following an injury; however, it typically remains stable after that time. This is also true in newborns, in which spasticity may not become evident until the age of 6 months or more. If there is worsening spasticity beyond this point, the suspicion for...

Degenerative disease may prompt a metabolic workup
A tethered cord or syringomyelia may prompt MRI imaging of the spinal cord
A tumor may prompt MRI imaging of the brain

When spasticity is stable, it usually represents a fixed old injury to the brain or spinal cord. Further workup and evaluation are often unnecessary. The location of the injury may usually be determined from the neurological examination. If a diagnosis is needed, then a brain or spinal MRI often reveals the cause. If multiple sclerosis is suspected, then a MRI of the brain needs to be performed, including the injection of contrast dye to determine which, if any, lesions are currently active and which are likely to be old lesions.

Treatment

There is much research investigating possible repair of damaged cells in the brain or spinal cord; however, there is no effective treatment that reverses the injury. (See the more complete description of treatments.)

Physical therapy is essential in order to stretch the muscles, maintain mobility about joints, and reduce the potential for joint injury and freezing. In a very stiff muscle, serial casting may be used to gradually stretch out the involved muscles. In severe cases, surgical procedures may be needed to cut the muscle or lengthen the tendons.

Medications
Medications to treat spasticity are aimed at reducing symptoms by decreasing the strength of the stretch reflex. Medications to reduce spasticity operate by different mechanisms, some of which involve receptors in the spinal cord, while other involve receptors in the brain.

Baclofen is an activator of the inhibitory GABA type B receptors. These receptors are present in the spinal cord and modulate the stretch reflex. Often, doses of baclofen as high as 100 mg per day or more are required for optimal effect.

Tizanidine (Zanaflex®) operates in the brain on a different receptor, and it is often helpful in spasticity as well.

Diazepam (Valium®) and other benzodiazepines are often used; they are more effective at the GABA type A receptors. Some clinicians use dantrolene, which directly weakens the muscles. Since dopa-responsive dystonia (DRD) may produce a syndrome very similar to spastic diplegia, all children with unexplained spasticity should receive a trial of L-dopa therapy.

Chemodenervation
Botulinum toxin is an effective treatment for children with spasticity as it causes a temporary weakening of spastic muscles. It is also possible that the toxin decreases the signals coming from the muscle stretch receptors, thereby directly reducing the signals that result in spasticity. Botulinum toxin is most effective when only a small number of muscles are involved; injections usually need to be repeated every 3 to 6 months. Alternatives to botulinum toxin include injections of phenol or ethanol onto the nerves supplying the muscles. Larger and more powerful muscles are more often the target for these treatments. These treatments are also temporary, and carry a risk for pain in some nerves.

Intrathecal Baclofen
Some children benefit from using a pump to inject baclofen directly into the spinal cord. This allows a high local concentration of the medication to be achieved, without the same degree of side effects as oral medications. The pump is inserted surgically below the skin in the abdomen and refilled by injecting the medication through a sealed, sterile port. A tube from the pump is inserted into the space surrounding the spinal cord. The baclofen pump may have serious complications, including meningitis and withdrawal reactions. Therefore, it must be used with caution.

Surgery
Dorsal rhizotomy has been used for many years for the treatment of spasticity. Improvements in this surgical technique have led to better results. During this procedure, the sensory nerves from the muscles are cut. It is hoped that this will interrupt the reflex loops that are causing the spasticity. The procedure is not always effective; some disappointing results may be due to surgery on children whose primary problem is more likely to be dystonia. This procedure is not reversible, and, in some cases, may make children worse by loosening muscles to the point where children can no longer stand.

Treatment Summary
Any treatment of spasticity needs to take into account that the spasticity itself is only part of the problem. Because the connection from the brain to the spinal cord is damaged, the muscles are also weak in the sense that the child has reduced voluntary control. In some cases, the spasticity actually improves overall function. For example, a spastic knee that extends involuntarily may allow a child to stand. If medication or surgery is used to reduce the spasticity, children may no longer able to stand.

Source: We Move. Visit their website at www.wemove.org/kidsmove/

Kids Move is WE MOVE's Web site devoted to pediatric movement disorders. Healthcare professionals and parents may access up-to-date information about the recognition, assessment, treatment, and avenues of support that are available for individuals concerned with childhood movement disorders