Spinocerebellar ataxia, or SCA, is not a single condition but a group of inherited neurological disorders. These diseases are progressive, meaning their effects on the body worsen over time. The defining characteristic of SCA is ataxia, which is a loss of voluntary muscle coordination. This loss of control stems from the deterioration of cells within the cerebellum, the area of the brain responsible for coordinating movement and maintaining balance.
Genetic Origins of Spinocerebellar Ataxia
Most forms of spinocerebellar ataxia originate from a genetic mutation inherited from a parent. These conditions are passed down in an autosomal dominant pattern, which means a child has a 50% chance of inheriting the disease-causing gene if one parent is affected. This pattern affects males and females equally. The genetic flaw is often a trinucleotide repeat expansion.
This expansion is like a recurring typographical error in the body’s genetic code. In many types of SCA, the specific “typo” is a sequence of three DNA building blocks—cytosine, adenine, and guanine—referred to as a CAG repeat. While a certain number of these CAG repeats is normal, in SCA, the sequence is repeated too many times, which disrupts the function of the protein created from that gene’s instructions.
The number of these extra repeats impacts the disease. A higher number of CAG repeats is associated with an earlier age of onset and greater symptom severity. This number can also expand as the gene is passed from one generation to the next, a phenomenon that helps explain why symptoms might appear earlier or be more severe in a child than in their affected parent.
Recognizing the Symptoms
The primary sign of spinocerebellar ataxia is a progressive decline in coordination. This initially manifests as an unsteady and staggering walk, known as gait ataxia, causing wobbliness and frequent falls. Over time, this loss of muscle control extends to other parts of the body. The symptoms and their rate of progression can vary significantly from person to person, even within the same family.
Difficulties with speech (dysarthria) are common, causing it to become slurred, slow, and unclear. Many also develop dysphagia (difficulty swallowing), which can lead to serious complications. Fine motor skills deteriorate, making everyday tasks like writing, buttoning a shirt, or using utensils increasingly challenging.
Other neurological symptoms frequently appear. Abnormal eye movements, called nystagmus, where the eyes make repetitive, uncontrolled movements, are a common sign. Some individuals may also experience tremors, muscle stiffness, and a loss of sensation in their hands and feet. As a progressive disorder, these symptoms worsen over months or years, often requiring a wheelchair within 10 to 20 years of diagnosis.
Classifying the Different Types
Spinocerebellar ataxia is an umbrella term for a large and growing group of distinct genetic disorders. Scientists have identified more than 40 types, each caused by a unique gene mutation. These types are distinguished numerically in the order their associated gene was discovered, such as SCA1, SCA2, and so on. This classification is important because while all types share ataxia as a central feature, they can present with unique symptoms and progress at different rates.
For example, SCA3, also known as Machado-Joseph disease, is one of the most common types found worldwide. It is often characterized by a wide range of symptoms that can include not just ataxia but also dystonia (involuntary muscle contractions), bulging eyes, and issues with muscle twitching. Another common type, SCA6, generally has a later onset and progresses very slowly, with symptoms that can include episodic vertigo in addition to the characteristic ataxia and slurred speech.
While SCA1 and SCA2 can involve vision problems, other types might be associated with cognitive impairment or peripheral neuropathy.
Diagnosis and Symptom Management
The diagnostic process begins when an individual shows signs of balance and coordination problems. A physician will conduct a neurological exam to assess gait, hand-eye coordination, speech, and reflexes. A review of family medical history is also done, as similar symptoms in relatives can point toward an inherited condition.
To rule out other potential causes of ataxia, such as stroke or tumors, a doctor may order brain imaging scans like an MRI. These scans can often reveal atrophy, or shrinkage, of the cerebellum, a hallmark of SCA. The definitive diagnosis for many types of SCA comes from genetic testing. A blood test can identify the specific gene mutation, confirming the diagnosis and classifying the exact type, which allows for a more accurate prognosis.
There is no cure for spinocerebellar ataxia. Treatment focuses on managing symptoms to maintain function and quality of life. This involves a multidisciplinary team of specialists. Physical therapy improves balance and strengthens muscles, while occupational therapy helps individuals adapt daily routines and use assistive devices like walkers or wheelchairs. Speech therapy can aid with both communication and swallowing difficulties. No medication can stop the ataxia’s progression, but some drugs can help manage related symptoms like muscle stiffness, tremors, or depression.