Stiff person syndrome

Overview

Stiff person syndrome (SPS) is a rare, chronic neurological disorder characterized by fluctuating muscle rigidity and painful spasms affecting the trunk and limbs. The stiffness typically worsens with emotional stress, sudden movement, or noise, and can severely impair mobility and quality of life. SPS is believed to be an autoimmune condition affecting the central nervous system, leading to dysfunction in the inhibitory pathways that control muscle tone.

Although SPS is rare, early diagnosis and appropriate treatment are crucial to managing symptoms and preventing disability.

Causes

Stiff person syndrome is primarily caused by an autoimmune attack on components of the central nervous system that regulate muscle relaxation. The majority of patients have antibodies against glutamic acid decarboxylase (GAD), an enzyme involved in the synthesis of gamma-aminobutyric acid (GABA), the brain’s main inhibitory neurotransmitter. Reduced GABA activity results in increased muscle tone and spasms.

Other causes and associations include:

• Autoimmune diseases such as type 1 diabetes, thyroiditis, or pernicious anemia

• Paraneoplastic SPS linked to cancers like breast or lung carcinoma

• Genetic predisposition may play a role but is not well defined

Symptoms

The clinical presentation of stiff person syndrome can vary, but common symptoms include:

• Progressive muscle stiffness in the trunk and proximal limbs

• Painful muscle spasms triggered by stress, noise, or sudden movements

• Difficulty walking and impaired posture due to rigidity

• Anxiety and exaggerated startle response

• Muscle spasms causing falls or injury

• In some cases, involvement of other muscles such as the face or diaphragm

Diagnosis

Diagnosis of SPS is based on clinical features supported by laboratory and electrophysiological studies:

• Clinical evaluation: Identification of characteristic stiffness and spasms, particularly in the axial muscles.

• Serologic testing: Detection of anti-GAD antibodies in the blood or cerebrospinal fluid, though some patients may be antibody-negative.

• Electromyography (EMG): Continuous motor unit activity at rest, supporting abnormal muscle activation.

• MRI of the brain and spine: Usually normal but performed to exclude other neurological disorders.

• Exclusion of differential diagnoses: Such as multiple sclerosis, Parkinson’s disease, or other movement disorders.

Treatment

There is no cure for SPS, but various treatments can reduce symptoms and improve function:

• GABA-enhancing drugs: Benzodiazepines (e.g., diazepam) are the mainstay to reduce muscle stiffness and spasms.

• Muscle relaxants: Baclofen may be used alone or in combination with benzodiazepines.

• Immunotherapy: Intravenous immunoglobulin (IVIG), plasmapheresis, or corticosteroids may help modulate the autoimmune response.

• Physical therapy: To maintain mobility, flexibility, and prevent contractures.

• Symptomatic management: Pain control, treatment of anxiety, and fall prevention.

Prognosis

The prognosis of stiff person syndrome varies widely. Some patients experience slow progression and good response to treatment, while others may develop severe disability due to persistent stiffness and frequent spasms. Early diagnosis and initiation of therapy improve long-term outcomes.

Complications can include fractures from falls, respiratory difficulties if respiratory muscles are involved, and significant psychosocial impact due to chronic pain and disability. With comprehensive multidisciplinary care, many individuals can maintain a reasonable quality of life.