The brain is divided into two hemispheres, separated by a bundle of fibers called the corpus callosum. Generally speaking, the right side of the brain controls muscles and other functions on the left side of the body, while the left side of the brain controls much of the right side of the body. Thus, hemiplegia often indicates a problem with one side of the brain.

However, this is not the only potential cause. For example, an incomplete spinal cord injury (SCI) in the cervical spinal cord can also cause hemiplegia. These incomplete SCIs are a common cause of hemiplegia in adults (as well as other forms of partial paralysis), and may result from vehicular accidents, acts of violence, slip and fall accidents, or other events which stress the musculoskeletal system.

Though the arms, legs, and possibly torso are the regions of the body most obviously affected by hemiplegia, in most cases of hemiplegia these body regions are actually perfectly healthy. Instead, the problem resides in the brain, which is unable to produce, send, or interpret signals due to disease or trauma-related damage.

Less frequently, hemiplegia results from damage to one side of the spinal cord, but these sorts of injuries more typically produce global difficulties, not just paralysis on one side of the body.

Common causes of hemiplegia (and other forms of trauma-induced paralysis) include:

Stroke

Strokes and transient ischemic attacks (better known as TIA or mini-strokes). Healthline cites strokes as “one of the most common causes of hemiparesis,” which can develop into hemiplegia as symptoms worsen.

Brain and Nervous System Infections

Infections, particularly encephalitis and meningitis. Some serious infections, particularly sepsis and abscesses in the neck, may spread to the brain if left untreated. It can cause hemiplegia as well as affect a person’s ability to think and even cause death.

Brain Trauma

This includes traumatic brain injuries to one side of the brain. These TBIs may be caused by car accidents, falls, acts of violence, and other factors.

Genetic Disorders/Congenital Diseases

Congenital abnormalities, including cerebral palsy and neonatal-onset multi-inflammatory disease. Genetic disorders can impact the development of the nervous system and lead to conditions like hemiplegia, paraplegia, or even quadriplegia.

Brain Tumors

Cancerous growths in the brain can apply pressure to different parts of the brain required for motor control. Left unchecked, brain tumors can grow and make symptoms worse over time, or even affect things other than muscle function.

Brain Lesions

Lesions in the brain can cause hemiplegia or hemiparesis. This is because these lesions can impede function on one side of the brain. Lesions are areas of tissue in the brain that have been damaged by injury or disease. Sometimes, these get confused with brain tumors, but lesions are a bit different.

What makes a lesion different from a tumor is that it typically will not keep growing on its own (unless repeated injuries occur at the site of the lesion because of strokes or infections).

Brain Diseases

Brain diseases include conditions that cause demyelination of the brain, such as multiple sclerosis and some other autoimmune diseases. There are many brain diseases that may cause brain lesions or other issues that lead to hemiplegia or hemiparesis.

Psychological Conditions

Rarely, psychological causes, such as some states of catatonia, can cause hemiplegia. People with parasomnia—a sleep disorder leading to unusual nighttime behavior—may experience nighttime episodes of hemiplegia.

Cardiovascular Problems

Cardiovascular problems, particularly aneurysms and hemorrhages in the brain, can lead to hemiplegia or hemiparesis.

Adverse Reactions to Surgery or Medicine

Reactions to surgery, medication, or anesthesia can cause permanent damage to the nervous system. While rare, this can lead to paralysis or weakness in the body—including hemiplegia.

Loss of Oxygen to the Brain

A loss of oxygen to the brain due to choking, drowning, anaphylactic shock or cardiovascular diseases can result in permanent brain damage. This, in turn, may lead to hemiplegia.