No One Single Test for SPS
A diagnosis of Stiff Person Syndrome (SPS) depends on multiple factors. There is no one single lab test or study that determines the diagnosis. A detailed medical history, physical exam, blood tests, and other lab tests can support the diagnosis. Importantly, these tests and their interpretation should be done by a physician who has expertise in the evaluation of SPS. The history should be consistent with one of the forms of SPS described in Types of SPS, most commonly including muscle stiffness of the legs or trunk, usually worsening over months or years.
Since SPS symptoms may overlap with other disorders, an evaluation for SPS would include multiple other blood tests and possibly a spinal tap to examine cerebral spinal fluid (CSF).
70-80% of People with SPS have GAD Antibodies
One useful blood test can detect the presence of antibodies to glutamic acid decarboxylase (GAD). The antibodies are abnormal while the GAD is a normal enzyme. About 70 – 80% of people with SPS have antibodies against GAD. These antibodies can be detected by a blood test.
The absence of GAD antibodies does not rule out SPS.
The presence of extremely high levels of GAD antibodies strongly supports the diagnosis of SPS in the context of the appropriate medical history and physical exam findings. GAD antibodies may also be measured in the cerebrospinal fluid obtained by a spinal tap. However, GAD antibodies can also be found in other autoimmune neurological disorders and in other autoimmune disorders that are not neurologic, including type 1 diabetes mellitus, thyroiditis, pernicious anemia, celiac disease, and very rarely in certain cancers.
It is worth emphasizing that having elevated GAD antibodies does not by itself determine a diagnosis of SPS. The medical history, exam and lab findings must also be consistent. Extremely high levels of GAD antibodies are most suggestive of an SPS diagnosis. This highlights the importance of being evaluated by an appropriate medical expert who has the experience to correctly assess these different factors.
Other Antibodies Could Indicate SPS
Glycine receptor antibodies are the second most common SPS antibodies, found in about 10% of SPS patients, followed by amphiphysin antibodies and DPPX (dipeptidyl peptidase-like protein 6) antibodies.
Some neurologists also recommend an electromyogram (EMG), which records electrical activity in affected skeletal muscles, including the legs. The EMG findings can support the diagnosis of SPS by demonstrating abnormal continuous muscle activity in the tested muscles. An EMG may also help distinguish SPS from other neuromuscular disorders. EMG testing can be uncomfortable, though not to the extent that it requires sedation or pain medication. In fact, it is important that patients avoid sedation and refrain from using muscle relaxants or pain medication in advance of an EMG, as use of these drugs can interfere with accurate EMG results.
Imaging Not As Useful In Diagnosis
Imaging studies such as x-rays, CT scans, MRIs, and ultrasounds do not have findings that are unique to SPS, though may be ordered by your doctor to evaluate for other possible disorders.