Hypertonia is caused by upper motor neuron lesions which may result from injury, disease, or conditions that involve damage to the central nervous system. The lack of or decrease in upper motor neuron function leads to loss of inhibition with resultant hyperactivity of lower motor neurons. Different patterns of muscle weakness or hyperactivity can occur based on the location of the lesion, causing a multitude of neurological symptoms, including spasticity, rigidity, or dystonia.

Spastic hypertonia involves uncontrollable muscle spasms, stiffening or straightening out of muscles, shock-like contractions of all or part of a group of muscles, and abnormal muscle tone. It is seen in disorders such as cerebral palsy, stroke, and spinal cord injury. Rigidity is a severe state of hypertonia where muscle resistance occurs throughout the entire range of motion of the affected joint independent of velocity. It is frequently associated with lesions of the basal ganglia. Individuals with rigidity present with stiffness, decreased range of motion and loss of motor control. Dystonic hypertonia refers to muscle resistance to passive stretching (in which a therapist gently stretches the inactive contracted muscle to a comfortable length at very low speeds of movement) and a tendency of a limb to return to a fixed involuntary (and sometimes abnormal) posture following movement.

Clonus (i.e. involuntary, rhythmic, muscular contractions and relaxations) 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.

Upper motor neuron syndrome (UMNS) is the motor control changes that can occur in skeletal muscle after an upper motor neuron lesion.

Following upper motor neuron lesions, affected muscles potentially have many features of altered performance including:

- weakness (decreased ability for the muscle to generate force)

- decreased motor control including decreased speed, accuracy and dexterity

- altered muscle tone (hypotonia or hypertonia) – a decrease or increase in the baseline level of muscle activity

- decreased endurance

- exaggerated deep tendon reflexes including spasticity, and clonus (a series of involuntary rapid muscle contractions)

Such signs are collectively termed the "upper motor neuron syndrome". Affected muscles typically show multiple signs, with severity depending on the degree of damage and other factors that influence motor control. In neuroanatomical circles, it is often joked, for example, that hemisection of the cervical spinal cord leads to an "upper lower motor neuron syndrome and a lower upper motor neuron syndrome". The saying refers to lower motor neuron symptoms in the upper extremity (arm) and upper motor neurons symptoms in the lower extremity (leg).

The upper motor neuron syndrome signs are seen in conditions where motor areas in the brain and/or spinal cord are damaged or fail to develop normally. These include spinal cord injury, cerebral palsy, multiple sclerosis and acquired brain injury including stroke. The impact of impairment of muscles for an individual is problems with movement, and posture, which often affects their function.

Health professionals' understanding of impairments in muscles after an upper motor neuron lesion has progressed considerably in recent decades. However, a diagnosis of "spasticity" is still often used interchangeably with upper motor neuron syndrome, and it is not unusual to see patients labeled as spastic who demonstrate an array of UMN findings.

Spasticity is an exaggerated stretch reflex, which means that a muscle has a reflex contraction when stretched, and that this contraction is stronger when the stretch is applied more quickly. The commonly quoted definition by Lance (1980) describes "a motor disorder, characterised by a velocity-dependent increase in tonic stretch reflexes with exaggerated tendon jerks, resulting from hyper-excitability of the stretch reflex as one component of the upper motor neurone (UMN) syndrome".

Spasticity is a common feature of muscle performance after upper motor neuron lesions, but is generally of much less clinical significance than other features such as decreased strength, decreased control and decreased endurance. The confusion in the use of the terminology complicates assessment and treatment planning by health professionals, as many confuse the other findings of upper motor neuron syndrome and describe them as spasticity. This confusion potentially leaves health professionals attempting to inhibit an exaggerated stretch reflex to improve muscle performance, potentially leaving more significant UMNS changes such as weakness unaddressed. Improved understanding of the multiple features of the upper motor neuron syndrome supports more rigorous assessment, and improved treatment planning.

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 is assessed by feeling the resistance of the muscle to passive lengthening in its most relaxed state. A spastic muscle will have immediately noticeable, often quite forceful, increased resistance to passive stretch when moved with speed and/or while attempting to be stretched out, as compared to the non-spastic muscles in the same person's body (if any exist). As there are many features of the upper motor neuron syndrome, there are likely to be multiple other changes in affected musculature and surrounding bones, such as progressive misalignments of bone structure around the spastic muscles (leading for example to the scissor gait in spastic diplegia). Also, following an upper motor neuron lesion, there may be multiple muscles affected, to varying degrees, depending on the location and severity of the upper motor neuron damage. The result for the affected individual, is that they may have any degree of impairment, ranging from a mild to a severe movement disorder. A relatively mild movement disorder may contribute to a loss of dexterity in an arm, or difficulty with high level mobility such as running or walking on stairs. A severe movement disorder may result in marked loss of function with minimal or no volitional muscle activation. There are several scales used to measure spasticity, such as the King's hypertonicity scale, the Tardieu, and the modified Ashworth. Of these three, only the King's hypertonicity scale measures a range of muscle changes from the UMN lesion, including active muscle performance as well as passive response to stretch.

Assessment of a movement disorder featuring spasticity may involve several health professionals depending on the affected individual's situation, and the severity of their condition. This may include physical therapists, physicians (including neurologists and rehabilitation physicians), orthotists and occupational therapists. Assessment is needed of the affected individual's goals, their function, and any symptoms that may be related to the movement disorder, such as pain. A thorough assessment will include analysis of posture, active movement, muscle strength, movement control and coordination, and endurance, as well as spasticity (response of the muscle to stretch). Spastic muscles typically demonstrate a loss of selective movement, including a loss of eccentric control (decreased ability to actively lengthen). While multiple muscles in a limb are usually affected in the upper motor neuron syndrome, there is usually an imbalance of activity, such that there is a stronger pull in one direction, such as into elbow flexion. Decreasing the degree of this imbalance is a common focus of muscle strengthening programs. Spastic movement disorders also typically feature a loss of stabilisation of an affected limb or the head from the trunk, so a thorough assessment requires this to be analysed as well.

Secondary effects are likely to impact on assessment of spastic muscles. If a muscle has impaired function following an upper motor neuron lesion, other changes such as increased muscle stiffness are likely to affect the feeling of resistance to passive stretch. Other secondary changes such as loss of muscle fibres following acquired muscle weakness are likely to compound the weakness arising from the upper motor neuron lesion. In severely affected spastic muscles, there may be marked secondary changes, such as muscle contracture, particularly if management has been delayed or absent.

Hypertonia is a term sometimes used synonymously with spasticity and rigidity in the literature surrounding damage to the central nervous system, namely upper motor neuron lesions. Impaired ability of damaged motor neurons to regulate descending pathways gives rise to disordered spinal reflexes, increased excitability of muscle spindles, and decreased synaptic inhibition. These consequences result in abnormally increased muscle tone of symptomatic muscles. Some authors suggest that the current definition for spasticity, the velocity-dependent over-activity of the stretch reflex, is not sufficient as it fails to take into account patients exhibiting increased muscle tone in the absence of stretch reflex over-activity. They instead suggest that "reversible hypertonia" is more appropriate and represents a treatable condition that is responsive to various therapy modalities like drug and/or physical therapy.

Symptoms associated with central nervous systems disorders are classified into positive and negative categories. Positive symptoms include those that increase muscle activity through hyper-excitability of the stretch reflex (i.e., rigidity and spasticity) where negative symptoms include those of insufficient muscle activity (i.e. weakness) and reduced motor function. Often the two classifications are thought to be separate entities of a disorder; however, some authors propose that they may be closely related.

Spastic quadriplegia can be detected by the abnormal development of motor skills in children. Symptoms can present themselves as early as three months but are generally seen before the child reaches two years of age. Some warning signs include: a child of more than two months who has stiff legs that scissor and is unable to control his or her head, and a child of more than twelve months who has not developed the ability to crawl or stand.

Spastic quadriplegia also presents a range of symptoms that affect the musculature. Many experience contractures, which are defined as joints that cannot be stretched or moved. Clonus is another symptom that is characterized by alternating, rapid muscle contraction and relaxation. This presents itself as tremors and scissoring of the limbs. Distonia, or lasting muscle contractions and tightness, is also often experienced by those affected by spastic quadriplegia. These involuntary muscle contractions may affect the development of structural muscle around the hip and lead to hip dysplasia and dislocation, making it difficult to sit. The combination of these symptoms often makes it difficult for the patients to walk as well. Although the arms and legs of patients are often stiff, the neck is usually limp due to the lack of voluntary muscle control. Some adults have issues with sexual organs such as the ones that control the sphincter (anus) as well and bladder control. These can sometimes be treated with training and stimulation even if the problems have presented for years, some issues can be corrected in many cases with nutrition modification in 90 percent of cases, especially B12. Stimulation of the muscles involved can treat some forms of nerve damage, depending on what the issue is. Sexual issues can be difficult for those with this, and sexual acts and stimulation can correct most of the sexual issues.

ADCP is often characterized by slow, uncontrolled movements of the extremities and trunk. Small, rapid, random and repetitive, uncontrolled movements known as chorea may also occur. Involuntary movements often increase during periods of emotional stress or excitement and disappear when the patient is sleeping or distracted. Patients experience difficulty in maintaining posture and balance when sitting, standing, and walking due to these involuntary movements and fluctuations in muscle tone. Coordinated activities such as reaching and grasping may also be challenging. Muscles of the face and tongue can be affected, causing involuntary facial grimaces, expressions, and drooling. Speech and language disorders, known as dysarthria, are common in athetoid CP patients. In addition, ADCP patients may have trouble eating. Hearing loss is a common co-occurring condition, and visual disabilities can be associated with Athetoid Cerebral Palsy. Squinting and uncontrollable eye movements may be initial signs and symptoms. Children with these disabilities rely heavily on visual stimulation, especially those who are also affected by sensory deafness.

Cognitive impairment occur in 30% of cases.

Epilepsy occur in 25% of cases.

Assessment of motor control may involve several health professionals depending on the affected individual's situation, and the severity of their condition. This may include physical therapists, physicians (including neurologists and psychiatrists ) and rehabilitation physicians, orthotists, occupational therapists, and speech-language pathologists. Assessment is needed of the affected individual's goals, their function, and any symptoms that may be related to the movement disorder, such as pain. A thorough assessment then uses a clinical reasoning approach to determine why difficulties are occurring. Elements of assessment will include analysis of posture, active movement, muscle strength, movement control and coordination, and endurance, as well as muscle tone and spasticity. Impaired muscles typically demonstrate a loss of selective movement, including a loss of eccentric control (decreased ability to actively lengthen); this decreased active lengthening of a muscle is a key factor that limits motor control. While multiple muscles in a limb are usually affected in the Upper Motor Neuron Syndrome, there is usually an imbalance of muscle activity (muscle tone), such that there is a stronger pull on one side of a joint, such as into elbow flexion. Decreasing the degree of this imbalance is a common focus of muscle strengthening programs. Impaired motor control also typically features a loss of stabilisation of an affected limb or the head from the trunk, so a thorough assessment requires this to be analysed as well, and exercise to improve proximal stability may be indicated.

Secondary effects are likely to impact on assessment of impaired muscles. If muscle tone is assessed with passive muscle lengthening, increased muscle stiffness may affect the feeling of resistance to passive stretch, in addition to neurological resistance to stretch. Other secondary changes such as loss of muscle fibres following acquired muscle weakness are likely to compound the weakness arising from the upper motor neuron lesion. In severely affected muscles, there may be marked secondary changes, such as muscle contracture, particularly if management has been delayed or absent.

Spastic hemiplegia is a neuromuscular condition of spasticity that results in the muscles on one side of the body being in a constant state of contraction. It is the "one-sided version" of spastic diplegia. It falls under the mobility impairment umbrella of cerebral palsy. About 20–30% of people with cerebral palsy have spastic hemiplegia. Due to brain or nerve damage, the brain is constantly sending action potentials to the neuromuscular junctions on the affected side of the body. Similar to strokes, damage on the left side of the brain affects the right side of the body and damage on the right side of the brain affects the left side of the body.

The affected side of the body is rigid, weak and has low functional abilities. In most cases, the upper extremity is much more affected than the lower extremity. This could be due to preference of hand usage during early development. If both arms are affected, the condition is referred to as double hemiplegia. Some patients with spastic hemiplegia only suffer minor impairments, where in severe cases one side of the body could be completely paralyzed. The severity of spastic hemiplegia is dependent upon the degree of the brain or nerve damage.

People with hemiparesis often have difficulties maintaining their balance due to limb weaknesses leading to an inability to properly shift body weight. This makes performing everyday activities such as dressing, eating, grabbing objects, or using the bathroom more difficult. Hemiparesis with origin in the lower section of the brain creates a condition known as ataxia, a loss of both gross and fine motor skills, often manifesting as staggering and stumbling. Pure Motor Hemiparesis, a form of hemiparesis characterized by sided weakness in the leg, arm, and face, is the most commonly diagnosed form of hemiparesis.

Spastic quadriplegia, also known as spastic tetraplegia, is a subset of spastic cerebral palsy that affects all four limbs (both arms and legs).

Compared to quadriplegia, spastic tetraplegia is defined by spasticity of the limbs as opposed to strict paralysis. It is distinguishable from other forms of cerebral palsy in that those afflicted with the condition display stiff, jerky movements stemming from hypertonia of the muscles.

Spastic quadriplegia, while affecting all four limbs more or less equally, can still present parts of the body as stiffer than others, such as one arm being tighter than another arm, and so forth. Spastic triplegia, meanwhile, involves three limbs (such as one arm and two legs, or one leg and two arms, etc.); spastic diplegia affects two limbs (commonly just the legs), spastic hemiplegia affects one or another entire side of the body (left or right); and spastic monoplegia involves a single limb.

Signs and symptoms of pseudobulbar palsy include:

- Slow and indistinct speech

- Dysphagia (difficulty in swallowing)

- Small, stiff and spastic tongue

- Brisk jaw jerk

- Dysarthria

- Labile affect

- Gag reflex may be normal, exaggerated or absent

- Examination may reveal upper motor neuron lesion of the limbs

Symptoms vary according to the kind of dystonia involved. In most cases, dystonia tends to lead to abnormal posturing, in particular on movement. Many sufferers have continuous pain, cramping, and relentless muscle spasms due to involuntary muscle movements. Other motor symptoms are possible including lip smacking.

Early symptoms may include loss of precision muscle coordination (sometimes first manifested in declining penmanship, frequent small injuries to the hands, and dropped items), cramping pain with sustained use, and trembling. Significant muscle pain and cramping may result from very minor exertions like holding a book and turning pages. It may become difficult to find a comfortable position for arms and legs with even the minor exertions associated with holding arms crossed causing significant pain similar to restless leg syndrome. Affected persons may notice trembling in the diaphragm while breathing, or the need to place hands in pockets, under legs while sitting or under pillows while sleeping to keep them still and to reduce pain. Trembling in the jaw may be felt and heard while lying down, and the constant movement to avoid pain may result in the grinding and wearing down of teeth, or symptoms similar to temporomandibular joint disorder. The voice may crack frequently or become harsh, triggering frequent throat clearing. Swallowing can become difficult and accompanied by painful cramping.

Electrical sensors (EMG) inserted into affected muscle groups, while painful, can provide a definitive diagnosis by showing pulsating nerve signals being transmitted to the muscles even when they are at rest. The brain appears to signal portions of fibers within the affected muscle groups at a firing speed of about 10 Hz causing them to pulsate, tremble and contort. When called upon to perform an intentional activity, the muscles fatigue very quickly and some portions of the muscle groups do not respond (causing weakness) while other portions over-respond or become rigid (causing micro-tears under load). The symptoms worsen significantly with use, especially in the case of focal dystonia, and a "mirror effect" is often observed in other body parts: Use of the right hand may cause pain and cramping in that hand as well as in the other hand and legs that were not being used. Stress, anxiety, lack of sleep, sustained use and cold temperatures can worsen symptoms.

Direct symptoms may be accompanied by secondary effects of the continuous muscle and brain activity, including disturbed sleep patterns, exhaustion, mood swings, mental stress, difficulty concentrating, blurred vision, digestive problems, and short temper. People with dystonia may also become depressed and find great difficulty adapting their activities and livelihood to a progressing disability. Side-effects from treatment and medications can also present challenges in normal activities.

In some cases, symptoms may progress and then plateau for years, or stop progressing entirely. The progression may be delayed by treatment or adaptive lifestyle changes, while forced continued use may make symptoms progress more rapidly. In others, the symptoms may progress to total disability, making some of the more risky forms of treatment worth considering. In some cases with patients who already have dystonia, a subsequent tramatic injury or the effects of general anethesia during an unrelated surgery can cause the symptoms to progress rapidly.

An accurate diagnosis may be difficult because of the way the disorder manifests itself. Sufferers may be diagnosed as having similar and perhaps related disorders including Parkinson's disease, essential tremor, carpal tunnel syndrome, TMD, Tourette's syndrome, conversion disorder or other neuromuscular movement disorders. It has been found that the prevalence of dystonia is high in individuals with Huntington's disease, where the most common clinical presentations are internal shoulder rotation, sustained fist clenching, knee flexion, and foot inversion. Risk factors for increased dystonia in patients with Huntington's disease include long disease duration and use of antidopaminergic medication.

Onset of PLS usually occurs spontaneously after age 50 and progresses gradually over a number of years, or even decades. The disorder usually begins in the legs, but it may start in the tongue or the hands. Symptoms may include difficulty with balance, weakness and stiffness in the legs, and clumsiness. Other common symptoms are spasticity (involuntary muscle contraction due to the stretching of muscle, which depends on the velocity of the stretch) in the hands, feet, or legs, foot dragging, and speech and swallowing problems due to involvement of the facial muscles. Breathing may also become compromised in the later stages of the disease, causing those patients who develop ventilatory failure to require noninvasive ventilatory support. Hyperreflexia is another key feature of PLS as seen in patients presenting with the Babinski's sign. Some people present with emotional lability and bladder urgency, and occasionally people with PLS experience mild cognitive changes detectable on neuropsychological testing, particularly on measures of executive function.

PLS is not considered hereditary when onset is in adulthood; however, juvenile primary lateral sclerosis (JPLS) has been linked to a mutation in the ALS2 gene which encodes the cell-signalling protein alsin.

The issue of whether PLS exists as a different entity from ALS is not clear, as some patients initially diagnosed as having PLS ultimately develop lower motor neuron signs.

There are no specific tests for the diagnosis of PLS. Therefore, the diagnosis occurs as the result of eliminating other possible causes of the symptoms and by an extended observation period.

Changes in muscle performance can be broadly described as the upper motor neuron syndrome. These changes vary depending on the site and the extent of the lesion, and may include:

- Muscle weakness. A pattern of weakness in the extensors (upper limbs) or flexors (lower limbs), is known as 'pyramidal weakness'

- Decreased control of active movement, particularly slowness

- Spasticity, a velocity-dependent change in muscle tone

- Clasp-knife response where initial higher resistance to movement is followed by a lesser resistance

- Babinski sign is present, where the big toe is raised (extended) rather than curled downwards (flexed) upon appropriate stimulation of the sole of the foot. The presence of the Babinski sign is an abnormal response in adulthood. Normally, during the plantar reflex, it causes plantar flexion and the adduction of the toes. In Babinski's sign, there is dorsiflexion of the big toe and abduction of the other toes. Physiologically, it is normally present in infants from birth to 12 months. The presence of the Babinski sign after 12 months is the sign of a non-specific upper motor neuron lesion.

- Increased deep tendon reflex (DTR)

- Pronator drift

People with the spastic/spasticity type of CP are hypertonic—i.e., they present with very stiff and tight muscle groups, far greater than typical humans—and have what is essentially a neuromuscular mobility impairment (rather than hypotonia or paralysis) which stems from an upper motor neuron lesion in the brain. The corticospinal tract or the motor cortex may be secondarily affected.

Spastic muscles are continuously contracting, or "tight", because the corresponding nerves permanently over-fire the command to tighten. This is caused by their inability to properly absorb GABA, or gamma amino butyric acid. The tightness, in addition to restricting movement, also acts as an overwhelming opposing force to neighbouring muscles and joints, eventually leaving the entire skeleton deformed compared to normal skeletal, bone, and joint structure in people without spasticity. Abnormal postures are usually associated with the antigravity muscles, which are extensors in the leg and the flexors in the arm. Deformities of joints develop which may become joint contractures, or "fixed contractures", with time.

Changes in spasticity and corresponding postures may also occur with other brain activity, such as excitement, fear or anxiety, or even pain, which increase muscle tension.

A person with spastic CP will commonly show, in addition to higher muscle tone, persistent primitive reflexes, greater stretch reflexes, plantar reflex, and ankle clonus.

A third of people with cerebral palsy have seizures - this is most common in spastic CP.

Primary lateral sclerosis (PLS) usually presents with gradual-onset, progressive, lower-extremity stiffness and pain due to muscle spasticity. Onset is often asymmetrical. Although the muscles do not appear to atrophy as in ALS (at least initially), the disabling aspect of PLS is muscle spasticity and cramping, and intense pain when those muscles are stretched, resulting in joint immobility. A normal walking stride may become a tiny step shuffle with related instability and falling.

Depending on the type of hemiparesis diagnosed, different bodily functions can be affected. Some effects are expected (e.g., partial paralysis of a limb on the affected side). Other impairments, though, can at first seem completely non-related to the limb weakness but are, in fact, a direct result of the damage to the affected side of the brain.

Although the most obvious symptom is impairment to the limbs, functioning is also impaired in the torso. This can mean a loss or impairment in controlling bowel and bladder, sexual function, digestion, breathing and other autonomic functions. Furthermore, sensation is usually impaired in affected areas. This can manifest as numbness, reduced sensation or burning neuropathic pain.

Secondarily, because of their depressed functioning and immobility, people with tetraplegia are often more vulnerable to pressure sores, osteoporosis and fractures, frozen joints, spasticity, respiratory complications and infections, autonomic dysreflexia, deep vein thrombosis, and cardiovascular disease.

Severity depends on both the level at which the spinal cord is injured and the extent of the injury.

An individual with an injury at C1 (the highest cervical vertebra, at the base of the skull) will probably lose function from the neck down and be ventilator-dependent. An individual with a C7 injury may lose function from the chest down but still retain use of the arms and much of the hands.

The extent of the injury is also important. A complete severing of the spinal cord will result in complete loss of function from that vertebra down. A partial severing or even bruising of the spinal cord results in varying degrees of mixed function and paralysis. A common misconception with tetraplegia is that the victim cannot move legs, arms or any of the major function; this is often not the case. Some individuals with tetraplegia can walk and use their hands, as though they did not have a spinal cord injury, while others may use wheelchairs and they can still have function of their arms and mild finger movement; again, that varies on the spinal cord damage.

It is common to have movement in limbs, such as the ability to move the arms but not the hands or to be able to use the fingers but not to the same extent, as before the injury. Furthermore, the deficit in the limbs may not be the same on both sides of the body; either left or right side may be more affected, depending on the location of the lesion on the spinal cord.

Symptoms depend on the type of HSP inherited. The main feature of the disease is progressive spasticity in the lower limbs due to pyramidal tract dysfunction. This also results in brisk reflexes, extensor plantar reflexes, muscle weakness, and variable bladder disturbances. Furthermore, among the core symptoms of HSP are also included abnormal gait and difficulty in walking, decreased vibratory sense at the ankles, and paresthesia.

Initial symptoms are typically difficulty with balance, stubbing the toe or stumbling. Symptoms of HSP may begin at any age, from infancy to older than 60 years. If symptoms begin during the teenage years or later, then spastic gait disturbance usually progresses over many years. Canes, walkers, and wheelchairs may eventually be required, although some people never require assistance devices.

More specifically, patients with the autosomal dominant pure form of HSP reveal normal facial and extraocular movement. Although jaw jerk may be brisk in older subjects, there is no speech disturbance or difficulty of swallowing. Upper extremity muscle tone and strength are normal. In the lower extremities, muscle tone is increased at the hamstrings, quadriceps and ankles. Weakness is most notable at the iliopsoas, tibialis anterior, and to a lesser extent, hamstring muscles.

In the complex form of the disorder, additional symptoms are present. These include: peripheral neuropathy, amyotrophy, ataxia, mental retardation, ichthyosis, epilepsy, optic neuropathy, dementia, deafness, or problems with speech, swallowing or breathing.

Anita Harding classified the HSP in a pure and complicated form. Pure HSP presents with spasticity in the lower limbs, associated with neurogenic bladder disturbance as well as lack of vibration sensitivity (pallhypesthesia). On the other hand, HSP is classified as complex when lower limb spasticity is combined with any additional neurological symptom.

This classification is subjective and patients with complex HSPs are sometimes diagnosed as having cerebellar ataxia with spasticity, mental retardation (with spasticity), or leukodystrophy. Some of the genes listed below have been described in other diseases than HSP before. Therefore, some key genes overlap with other disease groups.

The condition of paralysis affecting four limbs is alternately termed "tetraplegia" or "quadriplegia". Quadriplegia combines the Latin root "quadra", for "four", with the Greek root πληγία "plegia", for "paralysis". Tetraplegia uses the Greek root τετρα "tetra" for "four". Quadriplegia is the common term in North America; tetraplegia is more commonly used in Europe.

Pseudobulbar palsy is a medical condition characterized by the inability to control facial movements (such as chewing and speaking) and caused by a variety of neurological disorders. Patients experience difficulty chewing and swallowing, have increased reflexes and spasticity in tongue and the bulbar region, and demonstrate slurred speech (which is often the initial presentation of the disorder), sometimes also demonstrating uncontrolled emotional outbursts.

The condition is usually caused by the damage (bilateral degeneration) to the neurons of the brain stem, specifically to the corticobulbar tract (upper motor neuron tract to cranial nerve motor nuclei).

The upper motor neuron lesion in the brain impairs the ability of some nerve receptors in the spine to properly receive gamma amino butyric acid (GABA). That leads to hypertonia in the muscles signaled by those damaged nerves. The limbs and body areas in which hypertonia manifests can be any or even all of them, depending which specific nerve groupings within the spine are rendered unable to receive GABA. Thus, spastic CP is often designated by body topography.

An upper motor neuron lesion (also known as pyramidal insufficiency) occurs in the neural pathway above the anterior horn cell of the spinal cord or motor nuclei of the cranial nerves. Conversely, a lower motor neuron lesion affects nerve fibers traveling from the anterior horn of the spinal cord or the cranial motor nuclei to the relevant muscle(s).

Upper motor neuron lesions occur in the brain or the spinal cord as the result of stroke, multiple sclerosis, traumatic brain injury and cerebral palsy.

Facial diplegia refers to people with paralysis of both sides of their face. Bilateral occurs when the onset of the second side occurs within one month of the onset of the first side. Facial diplegia occurs in 50% of patients with Guillain–Barré syndrome. Facioscapulohumeral muscular dystrophy (FSHD) is the second most common adult-onset muscular dystrophy with facial weakness being a distinct feature of FSHD in over 90% of cases.