Introduction
Paraneoplastic neurological syndromes (PNS) are rare immune-mediated nervous system disorders associated with tumours and occur in <1% of cancer patients. SOX1 (Sry-like high mobility group box) is part of a family of transcription factors that play a role in the developing central nervous system. Anti-SOX1 antibodies are onco-neurological autoantibodies associated with various neurological syndromes, including Lambert Eaton myasthenic syndrome, paraneoplastic cerebellar disorder, and paraneoplastic limbic encephalitis. Neurological symptoms can precede cancer diagnosis by up to 4 years. These anti-SOX1 antibody mediated PNS are most commonly reported with small cell lung cancer, where up to 36.5% of patients have positive SOX1 antibodies, but can also occur in non-small cell lung cancers. Very few other cancers have been reported in the literature. Papillary thyroid cancer is the most common thyroid malignancy with an excellent survival rate of >90% and is rarely associated with paraneoplastic syndromes.
Case
We present the case of a previously well 65-year-old gentleman with SOX1 paraneoplastic limbic encephalitis and multifocal papillary carcinoma presenting with rapidly progressive cognitive impairment and neuropsychiatric symptoms. His initial MOCA was 24/30 and ACE was 69/100. Investigations yielded serum positive but CSF negative SOX1 antibodies. PET scan showed an FDG-avid thyroid lesion and he underwent total thyroidectomy confirming a stage 2, BRAF positive, multifocal papillary thyroid cancer with extrathyroidal extension (pT3a, N1, Mx). He was subsequently treated with I-131 dose of 3700MBq and commenced on thyroxine suppressive therapy. His neurological symptoms were refractory to initial methylprednisolone and IVIG however significantly improved four weeks post thyroidectomy and additional immunosuppression. His MOCA and ACE assessment improved to 29/30 and 96/100 respectively after three months.
Conclusions
SOX1 limbic encephalitis is a rare PNS occurring in this case in association with papillary thyroid cancer which responded well to treatment. However, development of further malignancies is still a possibility.