2025 American Academy of Neurology Abstract Website

Objective:

Background:

Seronegativity is an important challenge in the diagnosis of myasthenia.

Design/Methods:

Results:

A 25 yo woman with Hashimoto’s thyroiditis presented to her PCP for facial weakness. She was treated for “Bell’s palsy” with Prednisone. Symptoms improved for a month but again worsened, with accompanying dysphagia and nasal speech. AChR antibodies were negative. MRI brain only showed an incidental arachnoid cyst adjacent to CN V-3. She was referred to ENT, rehab, and eventually cognitive behavioral therapy for “conversion disorder”. She returned in 3 months with facial weakness and pain, diagnosed as sinusitis, and again received steroids. Repeat AChR and MuSK Abs were negative. TPO Abs were elevated.

She was referred to neurology a few months later. Exam revealed fatigable weakness of the faciobulbar and proximal muscles. Repetitive nerve stimulation showed pathologic decremental responses. While repeat labs were pending, she had a positive response to Pyridostigmine and was started on IVIg and Prednisone. Repeat labs returned positive AChR Abs by live cell-based assay, a repeat routine AChR Ab RIA titer just above the cutoff (0.5 nmol/L, normal up to 0.4), as well as positive LRP4 Abs. CT chest showed normal thymus for age. She will likely be started on Efgartigimod as well as undergo thymectomy.

Conclusions:

Antibody testing may be negative early in the course of MG, and retesting can be helpful. Results may be confounded by treatment with immunomodulators including steroids. Cell-based assay has higher AChR Ab test sensitivity. NCS and therapeutic trials with medication can support the diagnosis when labs are ambiguous. Patients with LRP4 / MuSK Abs may rarely also harbor AChR Abs, broadening as well as challenging our understanding of disease pathophysiology. The association between thymoma and LRP4 antibodies is unclear. We will include her histopathological findings after her upcoming thymectomy.