Imaging and neurological manifestations of an unusual thymic carcinoma

Our case is noteworthy for a number of reasons. While thymic carcinoma has been found on rare occasion to metastasize to the CNS, this is the first case where metastasis to the brain was observed prior to any of the more common sites. It is also the first case where a thymic metastasis to the brain has demonstrated drop-metastasis to the spine, leading to painful and painless radiculopathies, as well as a profound encephalopathy. We speculate that microscopic dissemination of tumor cells may have occurred secondary to the cerebellar operative trauma, resulting in numerous spinal metastases. Alternatively, potential tumor spread of malignant thymoma to the spinal canal via hematogenous or lymphatic routes is a reasonable explanation [7]. This case is also significant for its presentation with LEMS and Addison's disease. Though LEMS may be seen in association with other autoimmune states, this is only the second case report of an association between LEMS and Addison's disease [14]. Furthermore, this is one of few reports of a thymic malignancy causing LEMS. This is clinically important because most patients with generalized weakness and a thymic mass who are found to have a decremental motor response to low rates of repetitive stimulation are presumed to have myasthenia gravis, a disorder in which there is a postsynaptic defect in neuromuscular transmission. In this case, the clue to the diagnosis of a presynaptic defect in neuromuscular transmission was the presence of low compound motor action potential amplitudes. The diagnosis of LEMS was then confirmed by demonstrating postexercise facilitation and incremental electromotor responses to high rates (20 Hz) of repetitive stimulation. Our patient had a modest response to treatment with pyridostigmine, and a nonsustained response to prednisone and intravenous immunoglobulin, as do many patients with myasthenia gravis. This underscores the importance of not mistaking LEMS for myasthenia gravis, since the former is often associated with an underlying malignancy that should be searched for aggressively [13]. This case illustrates the importance of considering the possibility of CNS metastases in cases of thymic malignancy. It also places LEMS more firmly among the paraneoplastic syndromes that may arise with thymic carcinoma. Unfortunately, despite treatment for his malignancy and for the associated autoimmune disorders, our case eventually succumbed to his disease, as is typical for metastatic thymic carcinoma.