Paralysis is most often caused by damage to the nervous system, especially the spinal cord. Major causes are stroke, trauma, poliomyelitis, amyotrophic lateral sclerosis (ALS), botulism, spina bifida, multiple sclerosis, and Guillain-Barré syndrome. Temporary paralysis occurs during REM sleep, and dysregulation of this system can lead to episodes of waking paralysis. Drugs that interfere with nerve function, such as curare, can also cause paralysis. Many causes of this are varied, and could also be unknown.
Paralysis may be localized, or generalized, or it may follow a certain pattern.
Most paralyses caused by nervous system damage (i.e.spinal cord injuries) are constant in nature; however, there are forms of periodic paralysis, including sleep paralysis, which are caused by other factors.
Spinal cord injuries cause myelopathy or damage to nerve roots or myelinated fiber tracts that carry signals to and from the brain. [1][2] Depending on its classification and severity, this type of traumatic injury could also damage the gray matter in the central part of the cord, causing segmental losses of interneurons and motorneurons. Spinal cord injury can occur from many causes, including:
Trauma such as automobile crashes, falls, gunshots, diving accidents, war injuries, etc.
Tumor such as meningiomas, ependymomas, astrocytomas, and metastatic cancer.
Ischemia resulting from occlusion of spinal blood vessels, including dissecting aortic aneurysms, emboli, arteriosclerosis.
Developmental disorders, such as spina bifida, meningomyolcoele, and other.
Neurodegenerative diseases, such as Friedreich's ataxia, spinocerebellar ataxia, etc.
Demyelinative diseases, such as Multiple Sclerosis.
Transverse myelitis, resulting from stroke, inflammation, or other causes.
Vascular malformations, such as arteriovenous malformation (AVM), dural arteriovenous fistula (AVF), spinal hemangioma, cavernous angioma and aneurysm.
Paraplegia[1] is an impairment in motor and/or sensory function of the lower extremities. It is usually the result of spinal cord injury or a congenital condition such as spina bifida which affects the neural elements of the spinal canal. The area of the spinal canal which is affected in paraplegia is either the thoracic, lumbar, or sacral regions. If the arms are also affected by paralysis, quadriplegia is the proper terminology. If only one limb is affected the correct term is monoplegia.
Quadriplegia, also known as tetraplegia, is paralysis caused by illness or injury to a human that results in the partial or total loss of use of all of their limbs and torso; paraplegia is similar but does not affect the arms. The loss is usually sensory and motor, which means both sensation and control are lost.
Ptosis (πτωσις "falling, a fall") refers to droopiness of a body part. Specifically, it can refer to:
Ptosis (eyelid) (the most common usage of the term)
Ptosis (breasts)
Enteroptosis (intestine)
Gastroptosis (stomach)
Nephroptosis or renal ptosis (kidney)
Visceroptosis (internal organs)
Sleep paralysis is paralysis associated with sleep which may occur in normal subjects or be associated with narcolepsy, cataplexy, and hypnagogic hallucinations. The pathophysiology of this condition is closely related to the normal hypotonia that occur during REM sleep.When considered to be a disease, isolated sleep paralysis is classified as MeSH D020188.Some evidence suggests that it can also, in some cases, be a symptom of migraine. 
Hemiparesis is weakness on one side of the body. Contrast with Hemiplegia, which is total paralysis of the arm, leg, and trunk on the same side of the body. Hemiparesis is generally caused by lesions of the corticospinal tract, which runs down from the cortical neurons of the frontal lobe to the motor neurons of the spinal cord (see the second paragraph of Amyotrophic lateral sclerosis) and is responsible for the movements of the muscles of the body and its limbs.
On its way the tract passes through several parts of the brain stem; namely the midbrain, pons and medulla, respectively. The tract crosses to the opposite side (or decussates) on the lowest portion of the medulla (forming an anatomical structure named as the pyramids) and goes down along the opposite side of the spinal cord to meet the contralateral motor neurons. For this reason, one side of the brain controls the muscle movements of the opposite side of the body, and thus the disruption of the right corticospinal tract on brain stem or upper brain structures causes a hemiparesis on the left side of the body and vice versa. On the other hand, the lesions of the tract on the spinal cord lead to a hemiparesis on the same side of the body. The facial muscles are also controlled by the same tract.
The tract activates the facial nuclei (see ganglion) and the facial nerve emerging from these nuclei activate the facial muscles during voluntary facial muscle contraction. Since the facial nuclei are located in the pons above the decussation, the lesions of the tract on the pons or upper structures give rise to a hemiparesis on the opposite side of the body and a paresis on the same side of the face and that is called a crossed hemiparesis. If the patient's face is not involved, this is highly suggestive of a lesion of the tract on lower parts of the brain stem or spinal cord. Since the spinal cord is a very small structure, it is very unusual for only one side to be affected by a lesion and usually both tracts are affected. Therefore, the spinal cord lesions usually present with the paralysis of both arms and legs (quadriparesis) or both legs (paraparesis).
Hemiplegia is similar to hemiparesis, but hemiparesis is considered less severe.
Cerebral palsy (CP) (also cerebral pares) is an umbrella term encompassing a group of non-progressive,[1][2] non-contagious motor conditions that cause physical disability in human development.[3]
Cerebral refers to the cerebrum, which is the affected area of the brain (although the disorder most likely involves connections between the cortex and other parts of the brain such as the cerebellum), and palsy refers to disorder of movement. CP is caused by damage to the motor control centers of the developing brain and can occur during pregnancy (about 75 percent), during childbirth (about 5 percent) or after birth (about 15 percent) up to about age three.[4][5] Further research is needed on adults with CP as the current literature is highly focused on the pediatric patient.
Cerebral palsy describes a group of permanent disorders of the development of movement and posture, causing activity limitation, that are attributed to nonprogressive disturbances that occurred in the developing fetal or infant brain. The motor disorders of cerebral palsy are often accompanied by disturbances of sensation, perception, cognition, communication, and behaviour, by epilepsy, and by secondary musculoskeletal problems.
In another study, the incidence in six countries surveyed was 2.12–2.45 per 1,000 live births,indicating a slight rise in recent years. Improvements in neonatal nursing have helped reduce the number of babies who develop cerebral palsy, but the survival of babies with very low birth weights has increased, and these babies are more likely to have cerebral palsy.
