Stiff Person Syndrome

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Definition

Stiff person syndrome (SPS) is a rare acquired neurological disorder characterized by fluctuating muscle rigidity and stiffness, painful spasms, and continuous motor unit activity (1, 2, 3). This condition was originally described as “stiff man syndrome” (SMS) by Moersch and Woltman in 1956. They reported observations from 14 cases of patients who experienced progressive stiffness, episodic painful muscle spasms, and difficulty walking without any other neurological signs (4). The rigidity and continuous motor unit activity is most pronounced in muscles of the trunk (thoracolumbar paraspinals and rectus abdominis), but may also occur in the legs and arms, and persists even when relaxation is attempted (3).

SPS includes the following variants: classical SPS, focal SMS, Stiff Leg Syndrome (SLS), jerking SMS, progressive encephalomyelitis with rigidity and myoclonus (PERM) paraneoplastic SMS, gait ataxia, dysarthria, and abnormal eye movements (1, 3).

Etiology / Epidemiology

The exact etiology of SPS remains unclear, however it is widely accepted as an autoimmune disease (1, 2, 3, 4). The majority of SPS cases contain the glutamic acid decarboxylase (GAD) autoantibody. GAD is a GABA synthesizing enzyme, with two primary isoforms: GAD-65 and GAD-67 (1, 2). GAD-65 is primarily associated with SPS; it is also associated with diabetes mellitus, cerebellar ataxia, and limbic encephalitis (1). When GAD is inhibited by the anti-GAD in SPS, GABA becomes less readily available, thus reducing GABA’s inhibitory effect. This process leads to continuous stimulation of muscles by motor neurons, resulting in muscle rigidity (1, 3).

SPS is a very rare disease, prevalent in 1-2 in a million (1, 2). It is mostly reported in women, with 2 women affected per every one man (1, 5). SPS usually manifests later in life; the patient typically presents with initial symptoms between 40-60 years of age, although onset has been reported in patients younger and older (1, 3, 5). Over half of SPS patients have, or will develop, diabetes at some point. SPS is also related to thyroid disease and vitiligo (5).

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