Muscle tone describes the continuous, passive partial contraction of muscles, which helps maintain posture and allows for movement. This tension is the resting state of a muscle, regulated by signals traveling between the brain, spinal cord, and muscle fibers. Increased muscle tone, medically termed hypertonia, occurs when this neurological balance is disrupted, leading to abnormal stiffness and resistance to movement. Hypertonia is a symptom of an underlying neurological issue affecting the central nervous system.
What Hypertonia Means
Hypertonia describes an abnormal increase in muscle tension, manifesting as sustained stiffness in the limbs and trunk. This pathological tension results from an imbalance in the nervous system’s control over muscle contraction and relaxation. Specifically, it involves a loss of inhibitory signals that normally regulate muscle activity, leading to hyperactivity of the motor neurons.
This excessive tension causes noticeable resistance when a joint is moved passively. The sustained stiffness makes it difficult to initiate or control voluntary movements, limiting the range of motion and interfering with daily activities. If left unaddressed, the persistent shortening of muscles and tendons can lead to fixed joint deformities known as contractures.
Understanding Spasticity and Rigidity
Hypertonia presents primarily as spasticity or rigidity, which are the two most common clinical presentations. Distinguishing between them is important because it guides the diagnostic and therapeutic approach.
Spasticity is characterized by a velocity-dependent resistance to passive stretching; the faster a limb is moved, the greater the resistance felt. This resistance often gives way suddenly after an initial strong catch, known as the “clasp-knife” response. Spasticity arises from damage to the upper motor neurons, such as those in the corticospinal tract, leading to hyperexcitability of the stretch reflexes.
In contrast, rigidity is non-velocity-dependent, presenting as a constant resistance to passive movement regardless of speed. The muscle resistance is uniform throughout the entire range of motion, affecting both flexor and extensor muscles equally. Rigidity can be described as “lead-pipe” when the resistance is smooth and sustained, or “cogwheel” if interrupted by small, jerking movements, often due to an underlying tremor. This type of hypertonia is primarily associated with dysfunction in the basal ganglia, deep brain structures involved in motor control.
Neurological Sources of Increased Tone
The location of damage within the central nervous system dictates whether hypertonia manifests as spasticity or rigidity. Since spasticity is linked to upper motor neuron lesions, it is a common symptom following a stroke or traumatic brain injury. Conditions involving progressive damage, such as Multiple Sclerosis, frequently cause spasticity. Cerebral Palsy, caused by damage to the developing brain, also features spasticity as its most common motor presentation.
Rigidity, due to its link with basal ganglia dysfunction, is a hallmark feature of Parkinson’s Disease. The disruption of neurotransmitter pathways within these deep brain nuclei results in sustained muscle stiffness. Spinal cord injuries can also result in spasticity below the level of the injury, as the pathways that normally inhibit spinal reflexes are interrupted.
Current Management Strategies
Addressing hypertonia requires a multidisciplinary approach focused on improving function, reducing discomfort, and preventing secondary complications like contractures. Management is generally palliative, aiming to alleviate symptoms and maximize independence rather than providing a cure for the underlying neurological damage.
Physical and Occupational Therapy
Physical and occupational therapy form the foundation of management, utilizing stretching exercises and functional training to maintain muscle length and joint mobility. Therapists also employ bracing and splinting to position limbs correctly and provide support against the abnormal muscle pull.
Pharmacological Treatments
Pharmacological treatments are used in conjunction with therapy to reduce muscle tone. Oral medications, such as baclofen or tizanidine, act as muscle relaxants to dampen the overactive nerve signals causing stiffness. For localized spasticity, botulinum toxin injections temporarily block the chemical signals that tell a specific muscle to contract. These injections provide several months of relief and are often timed to coincide with intensive physical therapy.
Surgical Interventions
For severe, widespread hypertonia that does not respond to conservative measures, surgical interventions may be considered. Options include the placement of an intrathecal baclofen pump, which delivers medication directly into the fluid surrounding the spinal cord for a more targeted effect. Another procedure is selective dorsal rhizotomy, where specific sensory nerve roots are cut to reduce the exaggerated reflexes contributing to spasticity.