SMA most commonly stands for spinal muscular atrophy, a genetic disease that causes progressive muscle weakness by destroying the nerve cells responsible for movement. It affects roughly 1 in 6,000 to 10,000 live births. SMA can also refer to the superior mesenteric artery, a major blood vessel in the abdomen, or to “SMA syndrome,” a rare condition involving that artery. This article covers all three meanings so you can find the one relevant to you.
Spinal Muscular Atrophy: The Basics
Spinal muscular atrophy is caused by a shortage of a protein called survival motor neuron (SMN) protein, which keeps motor neurons healthy. Without enough of it, motor neurons in the spinal cord gradually die. Since motor neurons are the nerve cells that tell your muscles to move, losing them leads to muscle weakness and wasting throughout the body. Breathing, swallowing, and walking can all be affected depending on severity.
The root cause is a change in a gene called SMN1. Nearly everyone with SMA has lost both working copies of this gene. Humans carry a backup gene called SMN2, but it only produces about 10% functional protein. The number of SMN2 copies a person has is the single biggest factor in how severe the disease becomes: more copies mean more functional protein and, generally, milder symptoms.
About 1 in 25 to 50 people carry a single faulty copy of SMN1 without knowing it. Two carriers have a 25% chance of having a child with SMA.
The Five Types of SMA
SMA is traditionally divided into five types based on when symptoms appear and how severe they are. The number of SMN2 gene copies a person carries largely predicts which type they develop.
Type 0 is the rarest and most severe form. Symptoms begin before birth, and infants are born with profound weakness, breathing difficulty, and trouble feeding. These babies typically have just one copy of SMN2.
Type 1 (also called Werdnig-Hoffmann disease) is the most common form. Symptoms appear before 6 months of age and include severe muscle weakness, difficulty breathing, coughing, and swallowing. Most children with Type 1 have one or two SMN2 copies. Without treatment, historical studies showed a mean age at death of 8 to 10 months, though some children survived years longer. In one German study, only 32% of untreated children with Type 1 survived to age 2.
Type 2 appears between 6 and 18 months. Children can learn to sit independently but are unable to stand or walk without assistance. Most have three copies of SMN2.
Type 3 (Kugelberg-Welander disease) shows up after 18 months. Children can walk on their own but often struggle with running, climbing stairs, or getting up from a chair. Three or four SMN2 copies are typical.
Type 4 is the mildest form, developing in adulthood (after age 18). It causes mild to moderate leg weakness and progresses slowly. People with Type 4 usually have four SMN2 copies.
How SMA Is Diagnosed
Genetic testing is the standard diagnostic tool. The most common mutation, a deletion of a specific section of the SMN1 gene, is essentially 100% specific for SMA, meaning a positive result confirms the diagnosis with near-certainty. If the deletion isn’t found on both copies but SMA is still suspected, further gene sequencing can look for smaller mutations in the remaining SMN1 copy. Doctors also count SMN2 copies, since that number helps predict disease severity and guides treatment decisions.
Newborn screening has changed the landscape dramatically. SMA was added to the U.S. recommended screening panel, and by mid-2020, 24 states were universally screening all newborns, with at least 10 more preparing to launch. Catching SMA before symptoms appear makes a significant difference: in one clinical study of presymptomatic infants who received early treatment, 100% survived without permanent ventilator support, all could sit independently, and 88% could walk independently.
Treatment Options
Three disease-modifying therapies are now available, all aimed at increasing the amount of functional SMN protein in the body. They work through different mechanisms and are given in different ways.
The first is a gene therapy delivered as a single intravenous infusion. It works by delivering a functional copy of the SMN1 gene directly to motor neurons using a harmless virus as a carrier. Because it’s a one-time treatment, it’s an appealing option for young children, and studies show infants treated before 3 months of age reach motor milestones earlier than those treated later.
The second therapy is injected into the spinal fluid on a recurring schedule. It works by coaxing the SMN2 backup gene to produce more functional protein than it normally would.
The third is taken by mouth daily, also targeting SMN2 to boost its protein output. Its oral form makes it practical for patients of all ages and avoids the need for spinal injections.
None of these treatments cure SMA, but they can dramatically slow progression and, when started early enough, allow children to reach milestones like sitting and walking that would have been impossible a generation ago.
Living With SMA: Day-to-Day Management
Beyond disease-modifying drugs, SMA requires ongoing support across several areas. Respiratory care is central, especially for Types 1 and 2. Most children with severe SMA use a breathing support machine (BiPAP) daily and a cough-assist device at least once or twice a day to help clear secretions from the airways.
Nutrition is another constant concern. Muscle atrophy shifts body composition toward higher fat mass and lower lean mass, and swallowing difficulties can make eating by mouth unsafe. In one survey of children with Type 1, 43 out of 44 depended on a feeding tube for their essential calorie intake. Bone health also requires monitoring, since reduced mobility raises the risk of osteoporosis.
Children with Types 2 and 3 often benefit from physical therapy, bracing, and sometimes surgery to manage scoliosis or joint contractures that develop as muscles weaken unevenly. The goal across all types is to preserve function, prevent complications, and maintain quality of life as long as possible.
SMA as a Blood Vessel: The Superior Mesenteric Artery
In anatomy, SMA stands for the superior mesenteric artery, the second major branch off the body’s largest artery (the aorta). It originates in the upper abdomen and supplies blood to a large stretch of the digestive tract: the lower part of the duodenum, the entire small intestine, and the large intestine from the cecum through roughly the first two-thirds of the transverse colon. It also helps supply the head of the pancreas. If you’ve seen “SMA” on an imaging report or surgical note, this is likely what it refers to.
SMA Syndrome: A Compression Problem
Superior mesenteric artery syndrome is a separate condition where the SMA and the aorta pinch the duodenum (the first part of the small intestine) between them. Normally, the angle between these two vessels is 38 to 65 degrees, leaving 10 to 28 mm of space. When that angle narrows below about 25 degrees, the gap shrinks to less than 10 mm and compresses the duodenum.
This compression blocks the normal flow of food through the intestine. The most common symptoms are upper abdominal pain, nausea, and vomiting. Some people also experience bloating, early fullness, and weight loss. Symptoms often worsen when lying flat. SMA syndrome typically occurs in people who have lost significant body weight rapidly, since the fat pad that normally cushions the space between the aorta and the SMA shrinks along with it. Treatment focuses on restoring weight and nutrition, and in persistent cases, surgery to relieve the compression.