Hypertonia, or increased muscle tone, is a neurological symptom that results in muscle stiffness and resistance to movement. It is not a disease itself but a manifestation of damage to the central nervous system, which controls signals sent from the brain and spinal cord to the muscles. Damaged nerve pathways interrupt the precise regulation of muscle activity, leading to an abnormal increase in muscle tension. In adults, hypertonia can significantly impact mobility, posture, and daily function, making it difficult to move limbs freely.
Defining Hypertonia: Spasticity Versus Rigidity
Hypertonia presents in two distinct clinical forms: spasticity and rigidity. The differentiation between these two types is based on the underlying neurological mechanism and how the muscle responds to passive movement. Understanding this distinction is important because the two forms respond differently to treatment approaches.
Spasticity is defined as a velocity-dependent increase in muscle tone. This means the faster a joint is moved, the greater the resistance encountered in the muscle. Clinicians often observe the “clasp-knife” phenomenon, where an initial strong resistance to passive stretch suddenly gives way, similar to how a pocket knife blade closes. Spasticity usually affects specific muscle groups, such as the flexors in the upper limbs or the extensors in the lower limbs.
Rigidity, in contrast, is a non-velocity-dependent resistance to passive movement. The muscle resists movement uniformly throughout the entire range of motion, regardless of how quickly the limb is moved. This uniform resistance is sometimes described as “lead-pipe” rigidity. An alternative presentation, known as “cogwheel” rigidity, involves a tremor superimposed on the continuous resistance, creating a jerky, ratchet-like feel.
Primary Neurological Conditions That Cause Hypertonia
The root cause of hypertonia in adults is typically damage to the central nervous system, particularly the areas responsible for motor control. This damage interrupts the descending motor pathways, leading to disordered spinal reflexes and over-excitability in the muscle.
Spasticity most often results from an upper motor neuron lesion. Stroke is a frequent cause, as brain damage disrupts the signals responsible for regulating muscle tone. Traumatic Brain Injury (TBI) and Spinal Cord Injury (SCI) also commonly lead to spasticity by damaging the pathways. Multiple Sclerosis (MS), a chronic demyelinating disease, can also cause spasticity as the lesions affect motor pathways.
Rigidity, however, is typically associated with conditions that involve the basal ganglia, a deep region of the brain involved in movement regulation. Parkinson’s Disease (PD) is the most well-known cause of rigidity, where the loss of dopamine-producing neurons in the substantia nigra leads to characteristic muscle stiffness. Certain medication side effects or other neurodegenerative processes can also cause rigidity by affecting these same deep brain structures.
Current Approaches to Management and Treatment
The management of hypertonia involves a multidisciplinary approach focused on reducing stiffness, improving function, and minimizing pain. Treatment strategies are highly individualized and depend on the type of hypertonia, its severity, and the underlying cause. Pharmacological interventions are frequently used to help reduce muscle tone systemically or locally.
Oral medications, such as muscle relaxants like baclofen, diazepam, and dantrolene, work to decrease nerve excitability and reduce spasticity. Baclofen, for instance, acts by facilitating the effects of the neurotransmitter GABA, resulting in decreased neuronal firing and reduced reflexes. Dantrolene is unique because it works directly on the muscle fibers, blocking the release of calcium to reduce contraction strength.
For localized, severe hypertonia, injectable treatments offer a targeted approach. Botulinum toxin (BoNT-A) is injected directly into specific overactive muscles, where it blocks the signaling from the nerve to the muscle, causing temporary relaxation. The effects of these injections typically last for about three months, and the treatment is paired with physical therapy to maximize the functional gains.
Physical and occupational therapy are considered a foundation of hypertonia management, often used in conjunction with medication. Therapists employ stretching and range-of-motion exercises to maintain flexibility and combat stiffness. Orthotics and splinting may also be used to position limbs correctly and provide a prolonged, low-load stretch to the muscles.
Advanced interventions are reserved for severe cases that do not respond sufficiently to less invasive treatments. An Intrathecal Baclofen (ITB) pump can be surgically implanted to deliver baclofen directly into the spinal fluid, providing high concentrations of the drug to the central nervous system with fewer systemic side effects. In rare instances of severe, uncorrectable contracture, surgical procedures like selective neurotomy or tendon release may be considered to restore better limb position and ease hygiene.
Strategies for Preventing Secondary Physical Complications
Chronic, uncontrolled hypertonia can lead to secondary physical complications, including the development of contractures, which are permanent shortenings of the muscle and tendon tissues. Contractures severely limit joint mobility, making it difficult or impossible to fully straighten or bend the affected limb.
Aggressive daily stretching and positioning programs are essential to prevent this permanent shortening. Regular passive range-of-motion exercises, often performed by a caregiver or therapist, help to influence the extensibility of the soft tissues surrounding the joints. Positioning limbs to promote extension, through the use of specialized splints or orthotics, also opposes the muscle’s tendency to remain flexed.
Immobility and fixed postures caused by severe stiffness increase the risk of skin breakdown and pressure ulcers. Constant pressure on bony prominences, especially when combined with poor hygiene, can lead to tissue damage. Frequent repositioning, diligent skin checks, and proper limb positioning are necessary to relieve pressure and maintain skin integrity.
Chronic stiffness and involuntary spasms can be a significant source of pain. Maintaining a consistent stretching routine and adhering to prescribed pharmacological treatments help minimize muscle tightness and subsequent pain. Addressing these secondary complications is about preserving maximum possible function and preventing further decline in mobility.