Propriospinal myoclonus (PSM) is a rare movement disorder characterized by sudden, involuntary muscle jerks, which falls under the broader category of myoclonus. Myoclonus describes a quick, shock-like contraction of muscles, but PSM is distinct because it originates specifically within the spinal cord rather than the brain. This unique origin means the jerks follow a particular pattern, often leading to it being misdiagnosed as other movement disorders. Distinguishing PSM from other types of myoclonus is important for appropriate management.
Understanding Propriospinal Myoclonus
Propriospinal myoclonus is primarily characterized by repetitive, arrhythmic jerks that largely affect the axial muscles of the body, such as the trunk, neck, hips, and knees. These movements are typically flexor, meaning they cause a bending of the trunk, but they can also be extensor in nature. A defining feature is the sequential spread of the muscle activation, which often starts in the midthoracic region of the spinal cord.
The involuntary jerks then propagate slowly either upward (rostrocaudal) or downward (caudorostral) to adjacent muscle groups. This characteristic sequential pattern distinguishes PSM from other forms of myoclonus. Patients often notice the movements increase when they are lying down, particularly during the relaxed wakefulness that precedes sleep. The jerking movements are usually painless and can sometimes be triggered by external stimuli or sudden movement.
The Spinal Cord Circuitry
The unique sequential movement of PSM is directly linked to the specialized network of nerve cells within the spinal cord known as the propriospinal tract. This tract is composed of long interneurons that run both up and down the spinal cord, acting as a specialized internal wiring system that connects different spinal segments. These interneurons are responsible for coordinating actions across multiple levels of the body, such as coordinating movement between the trunk and the limbs.
In PSM, an abnormal signal, or “generator,” is thought to become hyperexcitable, often starting in the thoracic spinal segments. This abnormal electrical impulse then travels along the propriospinal interneurons to adjacent segments above and below the initial point. This process is like a chain reaction, where the signal is relayed sequentially from one spinal segment to the next. The propagation of this signal is relatively slow, typically between 5 and 15 meters per second.
Identifying Associated Causes and Triggers
The cause of propriospinal myoclonus is often divided into primary (idiopathic) and secondary forms, but a large number of cases are now also considered functional movement disorders (FMD). In cases of secondary PSM, the myoclonus is linked to an underlying condition that affects the spinal cord circuitry. These associated conditions can include structural lesions, such as tumors or syringomyelia, or inflammatory processes like myelitis.
Other causes that can trigger secondary PSM include spinal cord injury (SCI), various infections, and exposure to certain medications. However, a significant portion of reported cases are classified as idiopathic, meaning no clear underlying cause is identified. Certain situational factors can act as triggers, such as sudden movement, stress, or various sensory stimuli.
Clinical Diagnosis and Treatment Approaches
The diagnosis of propriospinal myoclonus relies heavily on clinical observation combined with specific neurophysiological testing, particularly Electromyography (EMG). A multichannel EMG study is used to record muscle activity simultaneously across different parts of the trunk and limbs. The characteristic finding is the demonstration of the fixed, sequential muscle activation pattern, which confirms the signal’s origin in the spinal cord and its propagation via the propriospinal pathways.
The EMG may show a slow conduction velocity and a specific sequence of muscle firing, often starting in the thoracic muscles and spreading outward. Other tests, such as Magnetic Resonance Imaging (MRI), are often performed to rule out structural causes, like a spinal cord lesion or tumor. Pharmacological management typically aims to suppress the hyperexcitability of the spinal neurons.
The most commonly used medication class is the benzodiazepines, with clonazepam often considered a first-line agent for spinal myoclonus. Certain anti-epileptic drugs, such as levetiracetam and valproate, may also be attempted, although their success can vary. For highly focal or segmental jerks, botulinum toxin injections may be used to temporarily paralyze the specific muscles involved. Treatment is focused on managing symptoms and improving quality of life, especially since there is no definitive cure for the idiopathic form.