Spastic quadriplegia is generally caused by brain damage or disruptions in normal brain development preceding birth. According to the National Institutes of Health, there are four types of brain damage that can cause spastic quadriplegia. These include, damage to the white matter (periventricular leukomalacia), abnormal brain development (cerebral dysgenesis), bleeding in the brain (intracranial hemorrhage), and brain damage due to lack of oxygen (hypoxic-ischemic encephalopathy or intrapartum asphyxia).

The white matter of the brain is especially vulnerable between the 26th and 34th weeks of maturation, and damage to the white matter can interfere with the brain’s ability to transmit signals to the rest of the body. Spastic quadriplegia can be caused by a condition known as periventricular leukomalacia which results in the formation of lesions and holes in the white matter of the brain.

Prior to the 26th week of maturation, the fetal brain is particularly susceptible to various toxins whose effects can ultimately hinder normal development. Exposure of the brain to infectious agents is especially dangerous because they can trigger immune responses that activate cytokines and lead to inflammation of the brain. Some infections that have been linked to the development of spastic quadriplegia include meningitis, herpes, rubella, and encephalitis. A difference in blood types between the mother and the fetus can also initiate a problematic immune response and cause brain damage. Severe jaundice, can also lead to brain damage and spastic quadriplegia due to a buildup of bilirubin in the blood.

Bleeding in the brain caused by fetal strokes, blood clots, weak and malformed blood vessels, or high maternal blood pressure may also lead to brain damage causing spastic quadriplegia. Maternal infection, most specifically pelvic inflammatory disease, has been shown to increase the risk of fetal stroke.

Hypoxia, lack of oxygen to the brain, can also cause damage in the cerebral motor cortex and other brain regions. This lack of oxygen can be the result of placenta malfunction, womb rupture, umbilical cord damage, low maternal blood pressure or asphyxia during labor and delivery.

Children who experienced many complications during birth, such as, prematurity, insufficient oxygen, low birthweight, aspiration, head injury, or bleeding in the brain have a greater risk of developing spastic quadriplegia. Children whose mothers were ill during the pregnancy or did not receive adequate nutrition are also more likely to develop the disease.

In the industrialized world, the incidence of overall cerebral palsy, which includes but is not limited to spastic diplegia, is about 2 per 1000 live births. Thus far, there is no known study recording the incidence of CP in the overall nonindustrialized world. Therefore, it is safe to assume that not all spastic CP individuals are known to science and medicine, especially in areas of the world where healthcare systems are less advanced. Many such individuals may simply live out their lives in their local communities without any medical or orthopedic oversight at all, or with extremely minimal such treatment, so that they are never able to be incorporated into any empirical data that orthopedic surgeons or neurosurgeons might seek to collect. It is shocking to note that—as with people with physical disability overall—some may even find themselves in situations of institutionalization, and thus barely see the outside world at all.

From what "is" known, the incidence of spastic diplegia is higher in males than in females; the Surveillance of Cerebral Palsy in Europe (SCPE), for example, reports a M:F ratio of 1.33:1. Variances in reported rates of incidence across different geographical areas in industrialized countries are thought to be caused primarily by discrepancies in the criteria used for inclusion and exclusion.

When such discrepancies are taken into account in comparing two or more registers of patients with cerebral palsy and also the extent to which children with mild cerebral palsy are included, the incidence rates still converge toward the average rate of 2:1000.

In the United States, approximately 10,000 infants and babies are born with CP each year, and 1200–1500 are diagnosed at preschool age when symptoms become more obvious. It is interesting to note that those with extremely mild spastic CP may not even be aware of their condition until much later in life: Internet chat forums have recorded men and women as old as 30 who were diagnosed only recently with their spastic CP.

Overall, advances in care of pregnant mothers and their babies has not resulted in a noticeable decrease in CP; in fact, because medical advances in areas related to the care of premature babies has resulted in a greater survival rate in recent years, it is actually "more" likely for infants with cerebral palsy to be born into the world now than it would have been in the past. Only the introduction of quality medical care to locations with less-than-adequate medical care has shown any decreases in the incidences of CP; the rest either have shown no change or have actually shown an increase. The incidence of CP increases with premature or very low-weight babies regardless of the quality of care.

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.

Doublecortin positive cells, similar to stem cells, are extremely adaptable and, when extracted from a brain, cultured and then re-injected in a lesioned area of the same brain, they can help repair and rebuild it. The treatment using them would take some time to be available for general public use, as it has to clear regulations and trials.

Triplegia is a medical condition characterized by the paralysis of three limbs (Triplegia Muscle Anatomy) . A person with triplegia can be referred to as triplegic. While there is no typical pattern of involvement, it is usually associated with paralysis of both legs and one arm — but can also involve both arms and one leg. Triplegia can sometimes by considered a combination of hemiplegia (paralysis of arm and leg of one side of the body) overlaying diplegia (paralysis of both legs), or as quadriplegia (paralysis of four limbs) with less involvement in one extremity.

The condition is commonly associated with cerebral palsy, although conditions such as stroke can also lead to it. Triplegia has also been found to be due to an increase in intracranial pressure associated with hydrocephalus resulting from traumatic brain injury.

A similar condition is triparesis, in which the patient suffers from paresis in three limbs, meaning that the limbs are very weak, but not completely paralyzed.

In a case reported only due to its rarity, triplegia was reported following a tonsillectomy (surgical removal of the tonsils). An eight-year-old male patient was sent to Willard Parker Hospital on August 12, 1929 and had been diagnosed with poliomyelitis. After an unrelated, and routine, tonsillectomy there was complete flaccid paralysis and loss of feeling in both the legs, right arm, and muscles in the trunk.

Triparesis is a medical condition, similar to triplegia, but the major difference between the two is primarily that triplegia is total loss of function in three limbs, and triparesis denotes weakening of three limbs.

Doublecortin (DCX or XLIS) mutations are responsible for X-linked disorders. While LIS1 mutations tend to cause severe malformations in the posterior brain, DCX mutations focus much of their destruction on anterior malformations and are linked to lissencephaly in males and subcortical band heterotopias in females. Women with DCX mutations tend to have an anteriorly-predominant subcortical band heterotopia and pachygyria.

Spastic

For details on the most common form of cerebral palsy, see spastic diplegia.

Spastic cerebral palsy is by far the most common type, occurring in 70% to 80% of all cases. Moreover, spastic CP accompanies one of the other types in 30% of all cases. People with this type are hypertonic and have a neuromuscular condition stemming from damage to the corticospinal tract or the motor cortex that affects the nervous system's ability to receive gamma amino butyric acid in the area(s) affected by the disability. Spastic CP is further classified by topography dependent on the region of the body affected; these include:

Spastic hemiplegia (one side being affected). Generally, injury to muscle-nerves controlled by the brain's left side will cause a right body deficit, and vice versa. Typically, people that have spastic hemiplegia are the most ambulatory, although they generally have dynamic equinus on the affected side and are primarily prescribed ankle-foot orthoses to prevent said equinus.[11]

Spastic diplegia (the lower extremities are affected with little to no upper-body spasticity). The most common form of the spastic forms. Most people with spastic diplegia are fully ambulatory and have a scissors gait. Flexed knees and hips to varying degrees are common. Hip problems, dislocations, and in three-quarters of spastic diplegics, also strabismus (crossed eyes), can be present as well. In addition, these individuals are often nearsighted. The intelligence of a person with spastic diplegia is unaffected by the condition.

Spastic tetraplegia (all four limbs affected equally). People with spastic quadriplegia are the least likely to be able to walk, or if they can, to want to walk, because their muscles are too tight and it is too much effort to do so. Some children with quadriplegia also have hemiparetic tremors, an uncontrollable shaking that affects the limbs on one side of the body and impairs normal movement.

Occasionally, terms such as monoplegia, paraplegia, triplegia, and pentaplegia may also be used to refer to specific manifestations of the spasticity.

Because pachygyria is a structural defect no treatments are currently available other than symptomatic treatments, especially for associated seizures. Another common treatment is a gastrostomy (insertion of a feeding tube) to reduce possible poor nutrition and repeated aspiration pneumonia.