Incidentally Detected Subclinical Limbic Encephalitis on ¹⁸F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in a Patient with Myasthenia Gravis and Sarcoidosis: A Rare Immunological Triad?

A 62-year-old-male on treatment for myasthenia gravis with azathioprine and pyridostigmine for 3 years presented with low-grade fever, evening rise of temperature, mild headache, irritability, loss of appetite, and loss of weight for 3 months' duration. The laboratory investigation revealed hypercalcemia (13.5 mg/dl) and an elevated C-reactive protein level of 31.6 mg/dl. His X-ray chest was within normal limits. However, ultrasonography and computed tomography of the abdomen were suggestive of hepatosplenomegaly. His blood and urine cultures were sterile, and his bone marrow was normocellular. Tumor markers were within normal limits Figure 1. The ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) scan maximum intensity projection (MIP) image (a) showed diffuse hypermetabolism in the enlarged liver (17.3 cm) and spleen (15.4 cm) (solid arrows). The fused PET/CT, corresponding PET, and CT images of the brain (b-d, f-h) showed hypermetabolism in the bilateral mesial temporal lobes (arrowheads), basal ganglia, and thalami suggestive of autoimmune limbic encephalitis. The coronal fused PET/CT and CT images of bilateral lungs (e, i) showed FDG avid ground glass haziness. The fused PET/CT, corresponding PET, and CT images of the pelvic region (j-l) showed FDG avid (SUV max 13.2) lytic lesion with soft tissue component in the left ischial tuberosity (dashed arrows). Biopsy from the left ischial lesion revealed epithelioid granulomatous inflammation with multinucleated Langerhans giant cells and foreign body type giant cells suggestive of sarcoidosis. AFB stain was negative. Furthermore, there is no scan evidence of thymoma which is showed in the axial fused PET/CT image (m).

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Figure 1 The ¹⁸F-FDG PET/CT scan maximum intensity projection (MIP) image (a) showed diffuse hypermetabolism in the enlarged liver (17.3 cm) and spleen (15.4 cm) (solid arrows). The fused PET/CT, corresponding PET, and CT images of the brain (b-d, f-h) showed hypermetabolism in the bilateral mesial temporal lobes (arrowheads), basal ganglia, and thalami suggestive of autoimmune limbic encephalitis. The coronal fused PET/CT and CT images of bilateral lungs (e,i) showed FDG avid ground glass haziness. The fused PET/CT, corresponding PET and CT images of the pelvic region (j-l) showed FDG avid (SUV max 13.2) lytic lesion with soft tissue component in the left ischial tuberosity (dashed arrows). Biopsy from the left ischial lesion revealed epithelioid granulomatous inflammation with multinucleated Langerhans giant cells and foreign body type giant cells suggestive of sarcoidosis. AFB stain was negative. Furthermore, there is no scan evidence of thymoma which is showed in the axial fused PET/CT image (m).

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Based on PET/CT findings and histopathological examination demonstrating encephalitis and sarcoidosis, he was started on steroid therapy, following which fever subsided, appetite improved, and he started gaining weight. The steroid dose was eventually tapered from 60 mg to 5 mg once a day Figure 2. Follow-up ¹⁸F-FDG PET/CT MIP image (a) showed normalization of hepatosplenomegaly (solid arrows), axial fused PET/CT, corresponding PET and CT images of the brain (b-d, f-h) showed complete resolution of hypermetabolism involving mesial temporal lobes (arrowheads) and reduction in the ¹⁸F-FDG avidity in the basal ganglia and thalami. The coronal fused PET/CT and CT images of bilateral lungs (e, i) showed resolution of previously noted FDG avid ground glass haziness. The axial fused PET/CT, corresponding PET and CT images of the pelvic region (h-j), showed resolution of FDG avid soft tissue lesion with sclerosis in the ischial tuberosity (dashed arrows).

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Figure 2 Follow-up ¹⁸F-FDG PET/CT MIP image (a) showed normalization of hepatosplenomegaly (solid arrows), axial fused PET/CT, corresponding PET and CT images of the brain (b-d, f-h) showed complete resolution of hypermetabolism involving mesial temporal lobes (arrowheads) and reduction in the 18F-FDG avidity in the basal ganglia and thalami. The coronal fused PET/CT and CT images of bilateral lungs (e,i) showed resolution of previously noted FDG avid ground glass haziness. The axial fused PET/CT, corresponding PET and CT images of the pelvic region (h-j), showed resolution of FDG avid soft tissue lesion with sclerosis in the ischial tuberosity (dashed arrows).

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Limbic encephalitis is the inflammation of the medial temporal lobe structures due to direct infectious or indirect autoimmune etiology.[12] Sarcoidosis is a rare cause of limbic encephalitis, has been reported by few authors.[345] The association has also been reported between limbic encephalitis and myasthenia gravis and sarcoidosis, the three different diseases with immune dysregulation as the common pathology.[65678910] This patient had a combination of myasthenia gravis, sarcoidosis, and limbic encephalitis, and ¹⁸F-FDG PET/CT was helpful in the early identification of limbic encephalitis even before the onset of symptoms and the skeletal lesion. The knowledge about such rare combinations of autoimmune manifestations will be helpful for the better interpretation of the PET/CT and management of patients.