Positive predictive value of myositis antibody line blot testing in patients with suspected idiopathic inflammatory myopathy.

INTRODUCTION/AIMS: Line blot (LB) is in widespread use for myositis antibody detection. Yet, studies of its positive predictive value (PPV) in patients with suspected idiopathic inflammatory myopathy (IIM), which would be of particular relevance to neuromuscular clinicians, are lacking. We aimed to determine the PPV of myositis antibody LB testing in patients with suspected IIM, and examine whether PPV was significantly impacted by intensity of antibody positivity. METHODS: This was a retrospective study of patients who underwent myositis antibody LB testing for suspected IIM between March 2019 and August 2022. RESULTS: Of 70 patients who underwent testing for suspected IIM and had positive myositis antibody LB results, 43 (61%) were female and the median age was 61 years (range: 10-83 years). Forty-four were classified as true-positives, yielding a PPV of 63%. The PPV of patients with weak-positive myositis antibody results (14/30, 47%) was significantly lower than the PPV of patients with moderate-positive or strong-positive myositis antibody results (30/40, 75%) (p = .02). DISCUSSION: Our study found that myositis antibody LB testing in patients with suspected IIM had a modest PPV, underscoring the need for antibody interpretation in the context of all available clinical and ancillary test data to avoid misdiagnosis. The significantly lower PPV in patients with weak-positive results emphasizes the particular importance of clinical correlation in such patients. Further study into the diagnostic performance of various LBs for myositis antibody detection is needed to inform their interpretation in clinical practice.

Evaluating yield and utilization of ganglioside antibody testing in clinical practice.

BACKGROUND AND OBJECTIVES: Ganglioside antibodies can help diagnose distinct acute and chronic inflammatory neuropathies including axonal variants of Guillain-Barre syndrome, Miller-Fisher syndrome (MFS), multifocal motor neuropathy, and chronic sensory ataxic neuropathies. Because ganglioside antibody testing may be routinely ordered in patients with suspected inflammatory neuropathy, we sought to evaluate its yield and utilization in clinical practice. METHODS: We performed a retrospective chart review of all patients at London Health Sciences Centre who underwent ganglioside antibody testing between April 2019 and August 2023. The disease phenotype was determined for each patient, and the proportion of all tests that yielded a true-positive result was calculated. Ganglioside antibody positivity was classified as a true-positive result if the disease phenotype was robustly associated with the detected ganglioside antibody and there was no other more likely diagnosis. RESULTS: We identified 92 patients who underwent ganglioside antibody testing. One patient (1%) was classified as having a true-positive result; this patient had GQ1b-IgG positivity with MFS. Among 92 patients tested, 20 patients (22%) had a disease phenotype that was considered to be robustly associated with ganglioside antibody positivity. CONCLUSIONS: The yield of ganglioside antibody testing in clinical practice is low. We found that this testing is frequently ordered in patients with disease phenotypes that are not robustly associated with ganglioside antibody positivity, indicating that suboptimal test utilization is a primary contributor to its low yield. Restricting ganglioside antibody testing to patients with characteristic disease phenotypes would be valuable to improving yield and utilization of this testing.

Pearls & Oy-sters: Gait Instability, Jaw Dystonia, and Horizontal Diplopia in a Woman With Anti-Ri Antibodies and Breast Cancer.

A 40-year-old woman was admitted for six months of progressive gait disturbance, lower limb-predominant weakness, stiffness, falls, jaw dystonia, horizontal diplopia, and weight loss. Neurological examination revealed horizontal gaze paresis, limited jaw opening with palpable masseter hypertrophy, and spastic paraparesis with sustained clonus and upgoing plantar responses. MRI revealed T2-hyperintense signal abnormalities in dorsal pons, medulla and upper cervical cord central grey matter extending to C3, without gadolinium enhancement. Cerebrospinal fluid (CSF) showed mildly elevated protein and immunoglobulin (IgG) index with CSF-specific oligoclonal bands. Neural autoantibody testing was positive for anti-Ri in CSF and serum by mouse brain indirect immunofluorescence and immunoblot. Testing for aquaporin 4 (AQP4)-IgG and myelin oligodendrocyte glycoprotein (MOG)-IgG by cell-based assay were negative. The patient received methylprednisolone 1 gram for 5 days and intravenous immunoglobulin 2 grams/kilogram over 2 days with prednisone taper, and botulinum toxin injections for jaw dystonia. PET-CT revealed an enlarged left axillary lymph node with high FDG uptake. Left axillary lymph node biopsy confirmed high-grade, locally invasive breast adenocarcinoma. Neurologic stabilization was documented at two-week follow-up after hospital discharge before modified radical mastectomy. Our case demonstrates a clinical triad highly suggestive of anti-Ri-associated paraneoplastic neurologic syndrome (Ri-PNS): gait instability, jaw dystonia, and horizontal gaze paresis. The more slowly progressive course and poor response to immunotherapy help distinguish it from AQP4-IgG seropositive neuromyelitis optica spectrum disorder (NMOSD) and MOG-IgG associated disease that share similar radiographic features. Early diagnosis, prompt immunotherapy and cancer treatment are paramount for disease stabilization.

"Obvious" indications for Neural antibody testing in Epilepsy or Seizures: The ONES checklist.

OBJECTIVE: Numerous predictive scores have been developed to help determine which patients with epilepsy or seizures of unknown etiology should undergo neural antibody testing. However, their diagnostic advantage compared to only performing testing in patients with "obvious" indications (e.g., broader features of autoimmune encephalitis, characteristic seizure semiologies) requires further study. We aimed to develop a checklist that identifies patients who have "obvious" indications for neural antibody testing and to compare its diagnostic performance to predictive scores. METHODS: We developed the "Obvious" indications for Neural antibody testing in Epilepsy or Seizures (ONES) checklist through literature review. We then retrospectively reviewed patients who underwent neural antibody testing for epilepsy or seizures at our center between March 2019 and January 2021, to determine and compare the sensitivity and specificity of the ONES checklist to the recently proposed Antibody Prevalence in Epilepsy and Encephalopathy (APE2)/Antibodies Contributing to Focal Epilepsy Signs and Symptoms (ACES) reflex score. RESULTS: One-hundred seventy patients who underwent neural antibody testing for epilepsy or seizures were identified. Seventy-four of 170 (43.5%) with a known etiology were excluded from sensitivity/specificity analyses; none had a true-positive neural antibody. Of the 96 patients with an unknown etiology, 14 (15%) had a true-positive neural antibody. The proportion of false-positives was significantly higher among patients with a known etiology (3/3, 100%) compared to an unknown etiology (2/16, 13%; p = .01). There was no significant difference of the APE2/ACES reflex score compared to the ONES checklist with regard to sensitivity (93% for both, p > .99) or specificity (71% vs. 78%, p = .18) for true-positive neural antibodies. SIGNIFICANCE: Compared to only performing neural antibody testing in patients with epilepsy or seizures of unknown etiology who have "obvious" indications, predictive scores confer no clear diagnostic advantage. Prespecified definitions of what constitutes a true-positive neural antibody is required in future studies to avoid false-positives that can confound results.