Palsy is a medical term for weakness or partial loss of movement in a part of the body. Unlike complete paralysis, where all voluntary movement is gone, palsy typically refers to conditions where some muscle control remains but is impaired. The word comes from the Old French “paralisie” and has been used for centuries to describe a wide range of conditions affecting the brain, nerves, or muscles. Today, it appears in the names of several distinct medical conditions, each with different causes, affected body parts, and outcomes.
How Palsy Differs From Paralysis
Paralysis is a broader term covering any partial or complete inability to move a body part. Palsy sits within that spectrum but leans toward the partial end. Someone with palsy can often still move the affected area, just with less strength, coordination, or control than normal. The muscles may be stiff, floppy, or prone to involuntary movements rather than completely unresponsive.
The underlying problem in all forms of palsy is a disruption in communication between the brain and the body. This can happen at several points along the chain: in the brain itself, along the spinal cord, or in the individual nerves that carry signals to muscles. Where that disruption occurs determines what type of palsy a person has and how severe it is.
Cerebral Palsy
Cerebral palsy is the most well-known form of palsy and the most common motor disability in childhood, affecting roughly 1.6 out of every 1,000 births in high-income countries. It results from damage to the developing brain, either before, during, or shortly after birth. The damage is permanent, but the condition itself doesn’t get progressively worse over time.
There are four main types. Spastic cerebral palsy accounts for about 80% of cases and causes muscles to be abnormally stiff, making movements rigid or awkward. It can affect the legs primarily (diplegia), one side of the body (hemiplegia), or all four limbs along with the trunk and face (quadriplegia), which is the most severe form. Dyskinetic cerebral palsy causes uncontrollable movements that can be slow and writhing or rapid and jerky. Muscle tone in this type can fluctuate dramatically, even within a single day. Ataxic cerebral palsy affects balance and coordination, making precise movements like writing or reaching for objects difficult. Some people have mixed cerebral palsy, with features of more than one type.
Management focuses on building functional ability. The strongest evidence supports targeted muscle strengthening, goal-directed functional training, and gait training to improve walking speed. For children with limited use of one hand, constraint-induced movement therapy, where the stronger hand is temporarily restricted to encourage use of the weaker one, has solid evidence behind it. Electrical stimulation combined with dynamic splinting can also improve muscle strength and range of motion. Approaches built around child-initiated movement and task-specific practice tend to produce the best motor outcomes.
Bell’s Palsy
Bell’s palsy causes sudden weakness or drooping on one side of the face. It happens when the seventh cranial nerve, which controls the facial muscles, becomes inflamed and swollen. The onset is rapid, often developing over hours, and can be alarming. One side of the face may droop, the eye on that side may resist closing, and smiling or other expressions become lopsided.
The exact cause isn’t fully settled, but the leading theory points to viral reactivation. Herpes simplex virus type 1 (the same virus behind cold sores) is considered the most likely trigger, reactivating near a nerve junction called the geniculate ganglion. Other viruses, including varicella zoster (the chickenpox virus), have also been implicated. The viral activity appears to spark an autoimmune-like inflammatory response that strips the protective coating from the facial nerve, disrupting its ability to transmit signals.
The lifetime risk of developing Bell’s palsy is about 1 in 60, with roughly 11 to 40 new cases per 100,000 people each year. The good news is that about 71% of untreated cases resolve completely on their own. Treatment with corticosteroids early on can improve those odds further. Most people see significant improvement within weeks to a few months.
Erb’s Palsy
Erb’s palsy is a birth injury affecting the upper arm. It occurs when the brachial plexus, the network of nerves running from the neck into the arm, is stretched or torn during delivery. Specifically, the fifth and sixth cervical nerve roots are damaged, usually by traction on the neck as the baby passes through the birth canal.
The two biggest risk factors are shoulder dystocia (when the baby’s shoulder gets caught behind the mother’s pelvic bone) and large birth weight. Other contributing factors include breech delivery, maternal obesity, maternal diabetes, and the use of vacuum or forceps during delivery. An infant with Erb’s palsy typically holds the affected arm limp at the side, with the elbow straight and the wrist turned inward. Many cases improve with physical therapy over the first year, though severe nerve tears can require surgical repair.
Bulbar and Pseudobulbar Palsy
These two conditions sound similar but involve different parts of the nervous system and produce distinct symptoms. Both affect the muscles of the face, throat, and tongue, causing difficulty with speaking, swallowing, and facial movement.
Bulbar palsy results from damage to the lower motor neurons in the brainstem. The affected muscles gradually waste away and may show visible twitching (fasciculations). Speech becomes nasal and slurred. It often occurs in conditions like ALS or brainstem tumors.
Pseudobulbar palsy, by contrast, results from damage to the upper motor pathways on both sides of the brain. The muscles don’t atrophy, but they become stiff and hard to control. Speech takes on a characteristic strained quality sometimes described as having a “Donald Duck” tone. One of the most distinctive features is emotional lability: episodes of uncontrollable laughing or crying that don’t match the person’s actual mood. In bulbar palsy, emotions remain unaffected.
Compression-Related Palsy
Some forms of palsy are caused by nothing more than sustained pressure on a nerve. The most familiar example is “Saturday night palsy,” which happens when the radial nerve in the upper arm is compressed for an extended period, typically from falling asleep with the arm draped over a chair or another hard surface. The name comes from its association with falling asleep in awkward positions after heavy drinking.
The radial nerve wraps around the humerus (the upper arm bone), making it vulnerable to compression at that point. The hallmark symptom is wrist drop: an inability to extend the wrist or fingers. The good news is that compressive radial neuropathy carries an excellent prognosis. Sensory recovery typically begins within about two and a half weeks, and full function returns within several weeks for most people.
How Palsy Is Diagnosed
Doctors rely heavily on physical examination and medical history, but when the cause or severity of palsy is unclear, nerve and muscle testing can help pin things down. The two most common tests are electromyography and nerve conduction studies, often performed together.
Electromyography measures the electrical activity inside muscles. A healthy muscle at rest produces no electrical signal. If a muscle is damaged or not receiving proper nerve input, it fires abnormally, either at rest or during movement. Nerve conduction studies send small electrical pulses along a nerve and measure how fast and strongly the signal travels. A damaged nerve produces a slower, weaker signal. Together, these tests help determine whether the problem originates in the muscles themselves or in the nerves supplying them, which directly shapes treatment decisions.
Brain imaging with MRI or CT scans is used when palsy is suspected to originate in the brain or brainstem, as in cerebral palsy or pseudobulbar palsy. For conditions like Bell’s palsy, diagnosis is primarily clinical, meaning the doctor can identify it from the pattern of symptoms without extensive testing.