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Stiff-person syndrome is a combination of symptoms which can include muscle stiffness, rigidity, spasm, muscle overactivity, specific contractions, seizure activity, and heightened response to stimuli resulting in exaggerated startle reflex triggered by noise, touch, or emotional distress. The classic pattern begins with stiffness and rigidity of the axial (skull, spine, and tailbone) muscles and can progress to the limbs. Stiff limb variants first affect primarily the leg muscles, and occasionally the arm muscles.

When discussing your symptoms with a doctor, it helps to understand the terminology used to describe them.

w A contracture is a shortening of a muscle or joint. Muscle contractures of the chest can lead to kyphosis (hunched back). Contractures of the lower spine can lead to hyperlordosis (exaggerated curve of the lower spine). Contractures of arms or legs occur in some cases.

w Muscle spasm is a sudden, involuntary contraction of the skeletal muscle. It is usually caused by acute trauma, but can also be triggered by hypocalcemia (low calcium), hypokalemia (low potassium), hyperthyroidism, hypothyroidism, chronic pain syndromes, or epilepsy. A large percentage of stiff-person patients have a concurrent seizure disorder and endocrine diseases. Muscle spasms cause pain and involuntary movement and distortion. The muscle contracts and becomes a hard knot. The blood vessels constrict, limiting the oxygen and chemicals that fuel the muscles.

w Spasticity is a state of continuously contracted muscle that causes stiffness, tightness, and can interfere with gait, movement, or speech. This is most often caused by damage to the portion of the brain or spinal cord that controls voluntary movement. It is a factor in diseases such as multiple sclerosis, cerebral palsy, brain trauma, head injury, and metabolic diseases. It can include hypertonicity, clonus, exaggerated deep tendon reflexes, muscle spasms, involuntary crossing of the legs, and fixed joints. It can be mild to completely disabling.

w Cramps, or tonic spasms, are caused by prolonged and strong muscle contractions with slow relaxation. Short-lived muscle cramps can be a sign of low electrolytes (magnesium, calcium, potassium).

w In tetany or tetanic contraction (tetanus) motor units have been stimulated by multiple impulses at a sufficiently high frequency. Each stimulus causes a twitch. If stimuli are delivered slowly enough, the tension in the muscle relaxes between successive twitches. If stimuli are delivered at high frequency, then the twitches run together, resulting in tetanic contraction. The contracting tension in the muscle remains in a steady state. The disease tetanus (lockjaw) takes its name from this. Tetanic contractions can be a symptom of hypoparathyroidism due to an underdeveloped parathyroid, or injury to the parathyroid and hypocalcemia (low calcium).

w Muscle contractures occur when, the muscle and its tendons shorten, resulting in reduced flexibility. Contractures can be caused by prolonged immobility, paralysis, muscle atrophy, muscle weakness, or spasticity. Loss of muscle tone inhibition causes a muscle to become hyperactive resulting in constant contraction.

w Muscle stiffness is difficulty in moving a muscle or joint. It may feel like weakness, fatigue, or fixed range of motion, but there is a difference between true stiffening and the unwillingness to move due to pain or injury. Stiffness can be caused by trigger points or tightly contracted muscle tissue. The muscle becomes resistant to lengthening. Chronic stiffness affects muscle contraction, coordination, and muscle activation, and can lead to limited range of motion, contractures, or bony deformities. It can be a factor in rheumatic diseases when attempting to move a joint after a period of rest. Stiffness is more severe and prolonged with increasing severity of joint inflammation. In the low back, morning stiffness that lasts longer than one hour may reflect spondylitis.

w Muscle fatigue is the loss of ability of the muscle to generate force and is not the same as true weakness, inability to move, and reluctance to move because of pain. Fatigue can be due to a systemic inflammatory disorder. It can be caused by barriers to or interference with the different stages of muscle contraction in two ways. There can be limitations in a nerve's ability to generate a sustained signal (neural fatigue). There can be a reduction in the ability of the muscle fiber to contract (metabolic fatigue) caused by shortage of fuel  (ATP, glycogen, creatine phosphate) within the muscle fiber or accumulation of metabolites within the muscle fiber, which interfere either with the release of calcium (Ca2+) or with the ability of calcium to stimulate muscle contraction.

w Neuromuscular fatigue can be central (a reduction in the neural drive or nerve-based motor command to working muscles that results in a decline in the force output) or peripheral (inability of the body to supply sufficient energy or metabolites to the contracting muscles to meet the increased energy demand).

w Muscle weakness (myasthenia) means the muscle struggles to generate force and cannot contract as opposed to being resistant to following the command due to pain or stiffness. True neuromuscular weakness means the force exerted by the muscles is less than would be expected. Perceived muscle weakness (non-neuromuscular weakness) means more effort than normal is required to exert a given amount of force but actual muscle strength is normal. It can be proximal (muscles closest to the body’s midline) or distal (muscles further out on the limbs).

w Hypertonicity is a state of abnormally increased muscle tone. It can involve muscle spasms, stiffening or straightening out of muscles, or shock-like contractions of specific or groups of muscle. It can be a factor of cerebral palsy, stroke, or spinal cord injury.

w Dystonic hypertonia refers to muscle resistance to passive stretching and the tendency of a limb to return to a fixed involuntary (abnormal) posture following movement. In spastic diplegia, GABA absorption becomes impaired by nerves damaged from the condition's upper motor neuron lesion, which leads to hypertonia of the muscles signaled by those nerves that can no longer absorb GABA.

w Rigidity is severe hypertonia. Muscle resistance occurs throughout the entire range of motion of the affected joint independent of velocity. It is associated with lesions of the basal ganglia. Individuals with rigidity present with stiffness, decreased range of motion, and loss of motor control. Rigidity is a fixture of PERM.

w Clonus, or clonic spasms, are a repetitive, quick succession of forceful contractions and relaxations of the affected muscle.

w Myoclonus is a sudden, involuntary twitch or jerk of a muscle, causing sudden contraction (positive myoclonus) or muscle relaxation (negative myoclonus). The jerks can occur alone, in sequence, and with or without a pattern. They can be infrequent or multiple times per minute. They can occur in response to a an external stimuli or during intentional movement. The twitching cannot be controlled. A hiccup is a benign example, as is the startle as you fall asleep at night. Widespread myoclonus which involves persistent, shock-like contractions that can spread from one muscle group to another are more severe and can affect your ability to eat, talk, or walk. This type of myoclonus is due to an underlying disorder of the brain or nerves and is a factor of PERM.

w Muscle pain is most frequently related to tension, overuse from exercise or physically-demanding work, or injury. The pain involves specific muscles and occurs during or just after the activity. This usually resolves on its own within a matter of days. Chronic muscle pain can be a sign of infection (such as the flu or meningitis). It is a factor in disorders of connective tissue such as lupus, lyme disease, polio, Rocky Mountain spotted fever, dermatomyositis, hyperthyroidism, hypothyroidism, fibromyalgia, polymyalgia rheumatica, and rhabdomyolysis (skeletal muscle tissue breakdown). Muscle pain can be a sign of electrolyte imbalances hypocalcemia (low calcium) and hypokalemia (low potassium). It can also be a side-effect of drugs such as ACE inhibitors for lowering blood pressure, cocaine, and statins for lowering cholesterol.

w Startle reflex to sudden or threatening stimuli is a brainstem reflectory reaction that serves to protect the back of the neck (whole-body startle) or the eye (eyeblink). It can be triggered by external stimuli or an individual's emotional state. Startle reflex is believed to be an orchestrated interaction of numerous transmitter systems in various brain areas The anterior cingulate cortex is the main area associated with emotional response and awareness. The amygdala triggers “fight or flight” response. The hippocampus forms memories of a stimulus and the emotions connected to it. Activation of the BNST (responsible for stress and anxiety responses) by certain hormones is thought to promote a startle response. Either or all of these areas of the brain could process the stimulus and decide to send chemical messengers to the muscles to respond.

Research suggests the role of glutamate receptors within the amygdala play an important role. A further hypothesis suggests startle amplitude is linked to CRH (corticotropin releasing hormone) and cortisol levels and a possible link with HPA (hypothalamus, pituitary, adrenal) axis activity.


What Is SPS?     Classic SPS     Progression & Stages     Variants     SPS + PERM     SPS + PNS     Causes