Why Does Muscle Tone Change in Canavan Disease Over Time?

The transition from being “floppy” (hypotonia) to becoming stiff (often spasticity or dystonia) is a common pattern in severe, early-onset Canavan disease ^[1][2]. This shift happens because of how the disease progressively affects the brain’s communication pathways over time, rather than due to a problem originating in the muscles themselves ^[3]. However, Canavan disease varies widely—some children may have persistent low tone, mixed tone, or milder symptoms, and the timing of these changes is not the same for every child ^[4][2].

Understanding the Terminology

When discussing muscle tone, your care team may use specific terms to describe what your child is experiencing ^[2][5]:

• Hypotonia: Low muscle tone, often described as feeling “floppy” ^[1].
• Hypertonia: The broad medical term for increased muscle tone or stiffness ^[2].
• Spasticity: A specific type of stiffness where the brain has difficulty regulating automatic reflexes, so a quick stretch or movement causes the limb to catch or tighten ^[5].
• Dystonia: Involuntary, patterned muscle contractions that can cause twisting movements or abnormal postures ^[5].
• Contracture: A fixed loss of joint movement that can develop over time when a limb remains in a shortened position ^[2].

The Role of NAA and White Matter

Canavan disease is caused by the deficiency of an enzyme called aspartoacylase, which normally breaks down a brain chemical called N-acetylaspartic acid (NAA) ^[3]. Because the enzyme isn’t working properly, NAA accumulates in the brain ^[3][6].

This accumulation of NAA is strongly associated with ongoing injury to the brain’s white matter—the tissue that contains nerve fibers and connects different parts of the brain ^[3][7]. Canavan disease is a leukodystrophy involving abnormal white-matter development (dysmyelination) and spongy degeneration (vacuolar changes) ^[8][9]. While the exact mechanisms are complex and still being studied, this progressive damage disrupts how the brain sends signals to the body ^[10].

From Floppy to Stiff: How It Happens

In typical severe Canavan disease, early brain injury commonly causes central hypotonia ^[1]. This early low muscle tone contributes to head lag and delayed motor milestones, making infants feel “floppy” ^[11][5].

As the disease progresses and white matter injury worsens, the brain pathways that normally regulate voluntary movement and suppress reflexes become impaired ^[8][5]. The brain loses its ability to smoothly coordinate these signals. This upper-motor-neuron injury can lead to spasticity, where automatic stretch reflexes become exaggerated, or dystonia ^[2][5].

It is also important to know that sudden increases in stiffness are not always due to disease progression. Treatable issues like pain, constipation, a full bladder, infections, reflux, or poorly fitting equipment can temporarily worsen muscle tone and should be evaluated ^[5].

Managing Tone and Safety

Because stiffness can impact your child’s comfort, positioning, and daily care, management is highly individualized. A neurologist or rehabilitation team will assess what type of tone changes your child has ^[5]. Physical and occupational therapies, seating assessments, stretching, and sometimes orthotics (braces) can improve comfort and help reduce the risk of contractures ^[2].

Medications may be considered to help relax muscles, but they must be carefully monitored by your clinician because they can cause side effects such as sleepiness, weakness, or worsened swallowing and breathing ^[5].

Breathing and Swallowing Safety
Changes in muscle tone and underlying neurologic progression can affect the muscles used for swallowing and breathing ^[8]. Contact your care team if you notice ^[5]:

• Coughing or choking during or after feeds
• Wet or gurgly breathing or voice
• Prolonged or exhausting feeds
• Repeated chest infections

Seek urgent or emergency medical care if your child experiences severe breathing difficulty, pauses in breathing, a blue or gray skin color, or choking that does not resolve ^[5].