Spinal muscular atrophy (SMA) causes progressive muscle weakness that primarily affects the muscles closest to the center of the body, like the thighs, hips, shoulders, and back. The specific symptoms vary widely depending on the type of SMA, which ranges from a severe form appearing before birth to a mild form that begins in adulthood. Across all types, SMA affects only motor nerves. Sensory function remains completely normal, meaning touch, temperature, and pain sensation are unaffected.
How SMA Affects the Body
SMA destroys motor neurons in the spinal cord, the nerve cells responsible for sending movement signals from the brain to the muscles. Without those signals, muscles weaken and gradually shrink. The pattern of weakness follows a consistent rule: muscles closer to the trunk weaken before muscles farther out. Thighs weaken before lower legs, upper arms before hands. Legs tend to weaken before arms.
Because motor neurons are the sole target, a physical exam will show flaccid (limp) weakness, low muscle tone, and absent reflexes. Tapping the knee or elbow with a reflex hammer produces no response. These findings, combined with normal sensation, are the hallmark that points clinicians toward SMA rather than other neuromuscular conditions.
Type 0: Symptoms Before Birth
Type 0 is the rarest and most severe form. It begins in the womb, where the earliest sign is noticeably decreased fetal movement. Babies are born with severe muscle weakness, very low muscle tone, and joint contractures, a condition where joints are locked in a fixed position because the muscles never developed enough strength to move them. Respiratory failure occurs at or shortly after birth, and most infants with type 0 do not survive beyond the first month of life.
Type 1: The Most Common Severe Form
Type 1, sometimes called Werdnig-Hoffmann disease, appears between birth and six months of age. It is the most common form of SMA. Affected infants are severely hypotonic, often described as “floppy,” with extremely weak muscles throughout the body. They cannot hold up their heads, cannot sit without support, and move very little on their own.
Breathing and feeding problems develop early. The muscles that control swallowing, chewing, and tongue movement are affected, creating difficulty feeding and increasing the risk of food or liquid entering the airway. Involuntary tongue twitching (fasciculations) is a characteristic sign that helps distinguish SMA from other causes of infant weakness. The muscles between the ribs weaken while the diaphragm initially remains stronger, giving the chest a bell-shaped appearance and creating a distinctive breathing pattern where the belly moves more than the chest.
Type 2: Able to Sit, Unable to Walk
Type 2 typically appears between 6 and 18 months of age. Children with this form reach the milestone of sitting independently but never gain the ability to stand or walk on their own. Muscle weakness is most pronounced in the legs and trunk, and it progresses over time, though more slowly than in type 1.
Breathing problems develop gradually. The cough becomes weaker as respiratory muscles lose strength, making it harder to clear mucus during colds or chest infections. Scoliosis is extremely common. Children with types 1c and 2 have roughly an 80% lifetime probability of needing scoliosis surgery, as the weakened trunk muscles cannot keep the spine aligned during growth. Joint contractures also become more frequent over time, particularly at the knees, hips, and elbows. Upper limb contractures affecting the fingers, wrists, and elbows are more common in type 2 than in milder forms.
Type 3: Walking With Increasing Difficulty
Type 3, also called Kugelberg-Welander disease, begins after 18 months of age, sometimes not until adolescence. Children with this form learn to walk independently, but muscle weakness gradually makes walking harder. Many eventually need a wheelchair, though the timeline varies enormously. Some lose the ability to walk in childhood, while others maintain it well into adulthood.
The weakness follows the same proximal pattern: climbing stairs becomes difficult before walking on flat ground does, and standing up from a seated position on the floor gets harder early on. Scoliosis and joint contractures can still develop, though less frequently and less severely than in type 2. Breathing problems are less prominent but can emerge later in the disease course, particularly during respiratory infections when a weak cough cannot clear secretions effectively.
Type 4: Adult-Onset SMA
Type 4 appears in the late teens or adulthood and is the mildest form. Symptoms begin with mild weakness in the proximal muscles, especially the thighs and hips. You might notice difficulty rising from a low chair, climbing stairs, or carrying heavy objects. The progression is slow, and most people with type 4 maintain the ability to walk throughout their lives. Breathing and swallowing problems are uncommon.
Why Severity Varies Between Types
SMA is caused by a missing or nonfunctional SMN1 gene, which produces a protein essential for motor neuron survival. A nearby backup gene called SMN2 produces a small amount of the same protein, roughly 10% of what each gene copy can generate. The number of SMN2 copies a person carries is the single biggest factor determining disease severity. More copies mean more backup protein and generally milder symptoms.
This relationship is strong but not absolute. Some patients with two SMN2 copies develop milder disease than expected, and some with three copies still develop type 1. Other genetic modifiers play a role, which is why two people with the same SMN2 copy number can have noticeably different disease courses.
Respiratory and Swallowing Symptoms Across Types
Breathing difficulty is the most dangerous aspect of SMA across nearly all types. Weak respiratory muscles reduce the ability to take deep breaths, and a weak cough means secretions build up in the airways. A healthy adult can generate a peak cough flow of 360 to 1,000 liters per minute. When cough strength drops below 270 liters per minute, clearing mucus during a chest infection becomes unreliable, and below 160 liters per minute, it becomes critically difficult.
Swallowing problems develop when the muscles of the mouth, tongue, and throat weaken. This can cause difficulty chewing, trouble moving food to the back of the throat, drooling from poor lip closure, and choking when food or liquid enters the airway instead of the esophagus. These symptoms are most prominent in types 0 through 2 but can appear in later-onset forms as the disease progresses.
How Early Treatment Changes the Picture
Newborn screening programs now identify SMA before symptoms appear by detecting the missing SMN1 gene. Starting treatment before motor neurons are lost has dramatically changed outcomes. In presymptomatic infants treated early, studies have shown improved survival, reduced need for permanent ventilation, and achievement of motor milestones like sitting and walking that would have been impossible under the natural disease course. The earlier treatment begins, the more motor neurons are preserved, which is why newborn screening has become a cornerstone of SMA care.
For children and adults already showing symptoms, treatment can slow progression and in some cases improve motor function, though it cannot reverse damage to motor neurons that have already been lost. The degree of benefit depends on how much motor neuron loss has occurred before treatment starts.