This gait pattern is reminiscent of a marionette. Hypertonia in the legs, hips and pelvis means these areas become flexed to various degrees, giving the appearance of crouching, while tight adductors produce extreme adduction, presented by knees and thighs hitting, or sometimes even crossing, in a scissors-like movement while the opposing muscles, the abductors, become comparatively weak from lack of use. Most common in patients with spastic cerebral palsy, the individual is often also forced to walk on tiptoe unless the plantarflexor muscles are released by an orthaepedic surgical procedure.

These features are most typical with the scissors gait and usually result in some form and to some degree regardless of the mildness or severity of the spastic CP condition:

- rigidity and excessive adduction of the leg in swing

- plantar flexion of the ankle

- flexion at the knee

- adduction and internal rotation at the hip

- progressive contractures of all spastic muscles

- complicated assisting movements of the upper limbs when walking.

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 extensor Babinski reflex is usually absent. Muscle paresis/paralysis, hypotonia/atonia, and hyporeflexia/areflexia are usually seen immediately following an insult. Muscle wasting, fasciculations and fibrillations are typically signs of end-stage muscle denervation and are seen over a longer time period. Another feature is the segmentation of symptoms – only muscles innervated by the damaged nerves will be symptomatic.

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.

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.

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.

Scissor gait is a form of gait abnormality primarily associated with spastic cerebral palsy. That condition and others like it are associated with an upper motor neuron lesion.

Persons suffering from peripheral neuropathy experience numbness and tingling in their hands and feet. This can cause ambulation impairment, such as trouble climbing stairs or maintaining balance. Gait abnormality is also common in persons with nervous system problems such as cauda equina syndrome, multiple sclerosis, Parkinson's disease, Alzheimer's disease, myasthenia gravis, normal pressure hydrocephalus, and Charcot–Marie–Tooth disease. Research has shown that neurological gait abnormalities are associated with an increased risk of falls in older adults.

Orthopedic corrective treatments may also manifest into gait abnormality, such as lower extremity amputation, post-fracture, and arthroplasty (joint replacement). Difficulty in ambulation that results from chemotherapy is generally temporary in nature, though recovery times of six months to a year are common. Likewise, difficulty in walking due to arthritis or joint pains (antalgic gait) sometimes resolves spontaneously once the pain is gone. Hemiplegic persons have circumduction gait and those with cerebral palsy often have scissoring gait.

Abasia (from Greek: "a-", without and "basis", step) is the inability to walk owing to impairment in motor coordination.

The term covers a spectrum of medical disorders such as:

- choreic abasia: caused by chorea of the legs

- paralytic abasia: caused by paralysis of the leg muscles

- spastic abasia: caused by spastic stiffening of the leg muscles

- trembling abasia: caused by trembling of the legs

Abasia is frequently accompanied by astasis, an inability to stand, see Astasia-abasia.

Gait abnormality is a deviation from normal walking (gait). Watching a patient walk is the most important part of the neurological examination. Normal gait requires that many systems, including strength, sensation and coordination, function in an integrated fashion. Many common problems in the nervous system and musculoskeletal system will show up in the way a person walks.

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 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.

In the past, HSP has been classified as early onset beginning in early childhood or later onset in adulthood. The age of onsets has two points of maximum at age 2 and around age 40. New findings propose that an earlier onset leads to a longer disease duration without loss of ambulation or the need for the use of a wheelchair. This was also described earlier, that later onset forms evolve more rapidly.

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.

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.

Individuals with spastic diplegia are very tight and stiff and must work very hard to successfully resist and "push through" the extra tightness they perpetually experience. Other than this, however, these individuals are almost always normal in every significant clinical sense. When they are younger, spastic diplegic individuals typically undergo gait analysis so that their clinicians can determine the best assistive devices for them, if any are necessary, such as a walker or crutches. The main difference between spastic diplegia and a normal gait pattern is its signature "scissor gait"—a style that some able-bodied people might tend to confuse with the effects of drunkenness, multiple sclerosis, or another nerve disease. The degree of spasticity in spastic diplegia (and, for that matter, other types of spastic CP) varies widely from person to person. No two people with spastic diplegia are exactly alike. Balance problems and/or stiffness in gait can range from barely noticeable all the way to misalignments so pronounced that the person needs crutches (typically forearm crutches/lofstrand crutches) or a cane / walking stick to assist in ambulation. Less often, spasticity is severe enough to compel the person to use a wheelchair. In general, however, lower-extremity spasticity in spastic diplegia is rarely so great as to totally prevent ambulation—most people with the condition can walk, and can do so with at least a basic amount of overall stability. Regardless, it should be noted that from case to case, steeply varying degrees of imbalance, potential tripping over uneven terrain while walking, or needing to hold on to various surfaces or walls in certain circumstances to keep upright, are typically ever-present potential issues and are much more common occurrences amongst those with spastic diplegia than among those with a normal or near-normal gait pattern. Among some of the people with spastic diplegia who choose to be ambulatory on either an exclusive or predominant basis, one of the seemingly common lifestyle choices is for the person to ambulate within his or her home without an assistive device, and then to use the assistive device, if any, once outdoors. Others may use no assistive device in any "indoor" situation at all, while always using one when outdoors. Above the hips, persons with spastic diplegia typically retain normal or near-normal muscle tone and range of motion, though some lesser spasticity may also affect the upper body, such as the trunk and arms, depending on the severity of the condition in the individual (the spasticity condition affecting the whole body equally, rather than just the legs, is spastic quadriplegia, a slightly different classification). In addition, because leg tightness often leads to instability in ambulation, extra muscle tension usually develops in the shoulders, chest, and arms due to compensatory stabilisation movements, regardless of the fact that the upper body itself is not directly affected by the condition.

A lower motor neuron lesion is a lesion which affects nerve fibers traveling from the ventral horn or anterior grey column of the spinal cord to the relevant muscle(s) – the lower motor neuron.

One major characteristic used to identify a lower motor neuron lesion is flaccid paralysis – paralysis accompanied by loss of muscle tone. This is in contrast to an upper motor neuron lesion, which often presents with spastic paralysis – paralysis accompanied by severe hypertonia.

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.

Spastic diplegia's particular type of brain damage inhibits the proper development of upper motor neuron function, impacting the motor cortex, the basal ganglia and the corticospinal tract. Nerve receptors in the spine leading to affected muscles become unable to properly absorb gamma amino butyric acid (GABA), the amino acid that regulates muscle tone in humans. Without GABA absorption to those particular nerve rootlets (usually centred, in this case, around the sectors L1-S1 and L2-S2), affected nerves (here, the ones controlling the legs) perpetually fire the message for their corresponding muscles to permanently, rigidly contract, and the muscles become permanently hypertonic (spastic).

The abnormally high muscle tone that results creates lifelong difficulty with all voluntary and passive movement in the legs, and in general creates stress over time—depending on the severity of the condition in the individual, the constant spasticity ultimately produces pain, muscle/joint breakdown including tendinitis and arthritis, premature physical exhaustion (i.e., becoming physically exhausted even when you internally know that you have more energy than you are able to use), contractures, spasms, and progressively worse deformities/mis-alignments of bone structure around areas of the tightened musculature as the person's years progress. Severe arthritis, tendinitis, and similar breakdown can start as early as the spastic diplegic person's mid-20s (as a comparison, typical people with normal muscle tone are not at risk of arthritis, tendinitis, and similar breakdown until well into their 50s or 60s, if even then).

No type of CP is officially a progressive condition, and indeed spastic diplegia does not clinically "get worse" given the nerves, damaged permanently at birth, neither recover nor degrade. This aspect is clinically significant because other neuromuscular conditions with similar surface characteristics in their presentations, like most forms of multiple sclerosis, indeed do degrade the body over time and do involve actual progressive worsening of the condition, including the spasticity often seen in MS. However, spastic diplegia is indeed a chronic condition; the symptoms themselves cause compounded effects on the body that are typically just as stressful on the human body as a progressive condition is. Despite this reality and the fact that muscle tightness is the symptom of spastic diplegia and not the cause, symptoms rather than cause are typically seen as the primary area of focus for treatment, especially surgical treatment, except when a selective dorsal rhizotomy is brought into consideration, or when an oral baclofen regimen is attempted.

Unlike any other condition that may present with similar effects, spastic diplegia is entirely congenital in origin—that is, it is almost always acquired shortly before or during a baby's birth process. Things like exposure to toxins, traumatic brain injury, encephalitis, meningitis, drowning, or suffocation do not tend to lead to spastic diplegia in particular or even cerebral palsy generally. Overall, the most common cause of spastic diplegia is Periventricular leukomalacia, more commonly known as neonatal asphyxia or infant hypoxia—a sudden in-womb shortage of oxygen-delivery through the umbilical cord. This sudden lack of oxygen is also almost always combined with premature birth, a phenomenon that, even by itself, would inherently risk the infant developing some type of CP. On the other hand, the presence of certain maternal infections during pregnancy such as congenital rubella syndrome can also lead to spastic diplegia, since such infections can have similar end results to infant hypoxia.

Myopathic gait (or waddling gait) is a form of gait abnormality.

The "waddling" is due to the weakness of the proximal muscles of the pelvic girdle.

The patient uses circumduction to compensate for gluteal weakness.

Conditions associated with a myopathic gait include pregnancy, congenital hip dysplasia, muscular dystrophies and spinal muscular atrophy

Due to impaired balance, patients suffering from ataxic cerebral palsy often walk with their feet unusually far apart (a wide gait). In addition, the low muscle tone caused by ataxic cerebral palsy causes people suffering from the disease to appear very unsteady, as their body is constantly trying to counterbalance itself. Infants with the disease often take a significantly longer amount of time to be able to walk without support, and over 50% of all children with ataxic cerebral palsy experience some form of a learning disability or speech impediment.

The condition, whether resulting from cerebellar malformation or injury, results in incomplete cerebral development and no two people are affected in the same way. In general, cerebral palsy is a physical impairment that affects posture and the development of movement. Ataxic cerebral palsy in particular, is manifested in the performance of movements with abnormal force, rhythm, and accuracy. Patients have hypotonia (decreased muscle tone), signs of ataxia (loss of full control of bodily movement), impaired balance and coordination, intention tremors, and a wide-based gait (in walking patients).

Cerebral development normally occurs in the first two years of life when the infant is acquiring new motor and adaptive skills, consequently signs and symptoms of ataxic cerebral palsy begin to manifest during this time period. Typically patients fail to reach motor milestones and show a qualitative difference in motor development. During the neonatal period (first 28 days of life), children are noted to be lethargic, relatively immobile, and floppy. Moreover, hypotonia is greatest during this period, even though muscle tone increases with age, it never reaches normal levels. The limbs show weakness, incoordination in voluntary movement, dysdiadochokinesis (in inability to perform rapidly alternating movements), and titubation.

Ataxia is a motor disorder that affects the spinal cord, brain and brainstem. Symptoms of ataxia include tremors, lack of coordination, loss of balance, instability, inaccuracy, clumsiness, gait problems, speech problems, and involuntary eye movements. Medication is the main treatment of ataxia. Some of these medicines include selegiline, amantadine, entacapone, dopamine agonists, and anticholinergics (“Movement Disorders”).

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.

Onset usually occurs within the first two decades of life, commonly in the teenage years or the twenties. Life expectancy is normal. High arch of the foot (pes cavus) is common. Patients also have trouble controlling their hands, due to muscle loss on the thumb side of the index finger and palm below the thumb. It is rare for a person with this disorder to lose the ability to walk, though changes in gait may occur later in life.

Frequency of this disorder is unknown.