# Stiff-person syndrome with concurrent Graves disease: a rare autoimmune overlap

## TL;DR

A rare case of stiff-person syndrome and Graves disease highlights a possible autoimmune overlap and successful treatment with plasma exchange and rituximab.

## Contribution

Reports a rare co-occurrence of stiff-person syndrome and Graves disease with a novel treatment response.

## Key findings

- Therapeutic plasma exchange and rituximab led to rapid improvement in both neuropsychiatric and thyroid symptoms.
- Thyroid-stimulating immunoglobulin and thyrotropin receptor antibody levels normalized after treatment.
- Elevated antiglutamic acid decarboxylase 65 antibodies confirmed autoimmune encephalitis within the SPS spectrum.

## Abstract

Stiff-person syndrome (SPS) is an autoimmune neurologic disorder characterized by progressive rigidity and spasms, often linked to antiglutamic acid decarboxylase 65 antibodies. While SPS has established associations with autoimmune diseases like type 1 diabetes and celiac disease, co-occurrence with hyperthyroidism, particularly Graves disease (GD), is rare. We present the case of a 34-year-old woman with paranoia, hallucinations, and altered mental status. Labs revealed thyrotoxicosis, with positive thyroid-stimulating immunoglobulin and thyrotropin receptor antibodies, confirming GD. Despite antithyroid treatment, her neuropsychiatric symptoms progressed. Though the neurologic workup was unremarkable, cerebrospinal fluid studies revealed oligoclonal bands with markedly elevated antiglutamic acid decarboxylase 65 antibodies, consistent with autoimmune encephalitis within the SPS spectrum. Therapeutic plasma exchange and rituximab resulted in rapid clinical improvement. At 6 weeks, thyroid-stimulating immunoglobulin and thyrotropin receptor antibody levels normalized, and she remained stable off antithyroid therapy, suggesting resolution of GD. This case illustrates the rare coexistence of SPS and GD.