Spastic diplegia is a form of cerebral palsy in which stiff, tight muscles primarily affect both legs, while the arms are mildly involved or not affected at all. It accounts for roughly 15 to 16% of all cerebral palsy cases and is strongly linked to premature birth. Children with spastic diplegia often walk on their toes with their knees pressed together, a pattern sometimes called a “scissoring” gait.
What Happens in the Brain
Spastic diplegia results from damage to the brain’s white matter, the pathways that carry signals from the brain to the muscles. The most common form of this damage is called periventricular leukomalacia, or PVL, which occurs in the tissue surrounding the fluid-filled spaces deep in the brain. PVL injures the nerve fibers that control movement, particularly the pathways responsible for leg function. Because the nerve fibers that serve the legs run closest to these vulnerable areas, the legs are hit hardest. The damage can also extend into deeper brain structures involved in coordination and movement planning, which is why some children have trouble with balance and fine motor control even in their hands.
This brain injury typically happens before, during, or shortly after birth. It is not progressive, meaning the damage itself doesn’t worsen over time, though its effects on the body can change as a child grows.
Why Premature Birth Is the Biggest Risk Factor
The developing brain is most vulnerable to white matter injury between roughly 24 and 34 weeks of gestation, which is exactly the window when many premature babies are born. Research tracking premature infants over time found a significant relationship between lower birth weight, earlier gestational age, and the later development of spastic diplegia. Adverse events during pregnancy or delivery, such as restricted blood flow or infection, further raise the risk.
Improvements in neonatal intensive care have created a paradox: better medical care prevents spastic diplegia in many low-birth-weight babies, but it also saves the lives of extremely premature infants who carry the highest risk. As a result, the overall number of cases has not dropped as much as might be expected.
How It Looks and Feels
The hallmark of spastic diplegia is increased muscle tone, or stiffness, in the legs. This stiffness is not constant. It intensifies during movement, making the muscles resist stretching in a way that feels like a ratchet or a catch. The muscles most affected are typically the calf muscles (causing toe-walking), the inner thigh muscles (pulling the legs together), and the hamstrings (bending the knees).
When a child with spastic diplegia walks, the pattern is distinctive. The extreme tightness in the inner thigh muscles can pull the legs across the midline, creating the scissoring appearance. Many children walk on their toes because the calf muscles are chronically shortened. The knees may stay slightly bent, and the hips may turn inward. The arms are usually less involved, though some children hold them in a slightly flexed position or have mild coordination difficulties in their hands.
Beyond movement, spastic diplegia can affect daily stamina. Walking takes significantly more energy when muscles are fighting against each other, so fatigue is a real and underrecognized challenge, especially over longer distances or uneven ground.
How Severity Is Classified
Doctors use the Gross Motor Function Classification System (GMFCS) to describe how much a child’s movement is affected. It has five levels based on what a child can do in real-world settings like home, school, and outdoors:
- Level I: Walks without limitations. Can run and jump, though speed and coordination may be slightly off.
- Level II: Walks but has trouble with long distances, uneven surfaces, running, or jumping. May use a wheeled device for longer outings.
- Level III: Walks indoors with a handheld device like a walker or crutches. Uses a wheelchair for longer distances at school or in the community.
- Level IV: Mostly uses a wheelchair. Can sit with support and has limited ability to move independently.
- Level V: Has significant difficulty controlling the head and trunk. Relies on a wheelchair and physical assistance for nearly all mobility.
Most children with spastic diplegia fall somewhere between Levels I and III. The GMFCS level tends to remain relatively stable after age six, making it a useful tool for planning long-term support.
Walking Outcomes
One of the first questions parents ask is whether their child will walk. In one study following 31 children with spastic diplegia, 58% achieved walking by their last assessment. About 23% walked independently, and another 35% walked with some form of assistance, such as a walker or crutches. The remaining 42% did not achieve walking and relied on wheelchairs. Children who could sit independently by age two and had milder spasticity were more likely to eventually walk.
These numbers span the full range of severity. A child classified at GMFCS Level I or II has a very different outlook than one at Level IV. The key predictors are early head and trunk control, the ability to sit without support, and the degree of underlying muscle strength beneath the spasticity.
Orthopedic Complications to Watch For
Because spastic muscles constantly pull on growing bones, children with spastic diplegia are at risk for skeletal problems that develop gradually. The most important one is hip displacement, where the thigh bone slowly migrates out of the hip socket. This is estimated to occur in 25 to 60% of all children with cerebral palsy, with complete dislocation affecting 10 to 15%.
Hip displacement often causes no pain in its early stages, which is why regular screening with X-rays is critical. Doctors measure how far the thigh bone has shifted using a metric called the migration percentage. A value below 33% is considered typical. Between 33% and 40%, closer monitoring is needed, and above 40% generally calls for surgical intervention. Screening programs ideally begin by age two. Scoliosis, severe muscle contractures, and pelvic tilting are other complications that can develop as the child grows, all driven by the ongoing imbalance between tight and weak muscles.
Braces and Orthotics
Ankle-foot orthoses (AFOs) are one of the most common tools for managing the gait problems of spastic diplegia. Several types serve different purposes:
- Solid AFOs lock the ankle in place entirely, which helps children who have very weak muscles or significant instability.
- Posterior leaf spring orthoses are flexible at the ankle, acting like a spring that helps lift the foot during walking and provides a small push-off at the end of each step. These work well for children with weakness in the muscles that pull the foot upward.
- Hinged AFOs are widely used in spastic diplegia specifically. They allow controlled movement at the ankle while stretching the tight calf muscles, helping to reduce stiffness and improve walking patterns over time.
- Carbon fiber AFOs are lighter and store energy during each step, improving walking efficiency and reducing fatigue.
The right type depends on the child’s specific pattern of tightness, their strength, and their GMFCS level. Orthotics are typically adjusted or replaced as the child grows.
Botulinum Toxin Injections
Injections of botulinum toxin (commonly known by the brand name Botox) are used to temporarily relax the tightest muscles, particularly in the calves and inner thighs. The toxin works by blocking nerve signals that tell muscles to contract, reducing stiffness for roughly three to six months per round of treatment. For lower limb spasticity, approved doses typically range from 300 to 400 units, distributed across the targeted muscles.
These injections do not cure spasticity. They create a window of reduced tightness during which physical therapy and bracing can be more effective. For young children, this window can be especially valuable because it allows the bones to grow in better alignment while the muscles are relaxed.
Selective Dorsal Rhizotomy
For children whose spasticity is the main barrier to better movement, a surgical procedure called selective dorsal rhizotomy (SDR) can permanently reduce leg stiffness. During SDR, a neurosurgeon identifies and cuts specific nerve rootlets in the lower spine that are sending abnormal signals to the leg muscles.
Not every child with spastic diplegia is a candidate. The best results occur in children who meet a specific profile: typically between ages 5 and 10, with moderate to severe spasticity, at least moderate muscle strength underneath the stiffness, reasonable balance (able to maintain a kneeling position for at least five seconds), no severe fixed joint deformities, and no involuntary movements or dystonia. Children who have good underlying strength but are held back by their spasticity tend to see the most dramatic improvements.
SDR permanently reduces spasticity, but it does not restore normal movement on its own. Intensive physical therapy for months after surgery is essential to build the strength and coordination needed to take advantage of the reduced stiffness. In countries with comprehensive rehabilitation services, adductor release surgery to reduce scissoring is also performed when needed.
Physical Therapy and Daily Life
Physical therapy is the foundation of managing spastic diplegia at every age and severity level. For young children, therapy focuses on building core strength, improving balance, and learning to move through developmental milestones. For school-age children and teens, the emphasis shifts toward functional goals: walking farther, navigating stairs, participating in physical activities, and maintaining range of motion as the body grows.
Stretching is a daily reality for most people with spastic diplegia. Because spastic muscles naturally shorten over time, regular stretching of the calves, hamstrings, and inner thighs helps preserve joint range and delay or prevent contractures. Strengthening the muscles that oppose the tight ones, particularly the muscles that straighten the knees and lift the feet, is equally important.
As children with spastic diplegia enter adulthood, some experience changes in their mobility. The physical demands of walking with an inefficient gait pattern can lead to joint pain, fatigue, and gradual loss of function over decades. Many adults find that using a wheelchair part-time for longer distances preserves their energy and reduces wear on their joints, while continuing to walk for shorter distances at home.