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Case Report/Case Series |

Hypokalemic Periodic Paralysis Induced by Thymic Hyperplasia and Relieved by Thymectomy

Renrong Yang, MD1; Karin Jurkat-Rott, MD2; Jinlin Cao, MD1; Guofeng Wang, MD3; Hans-Peter Seelig, MD4; Changping Yang, MD1; Guibao Liu, MD1; Lin Pan, MD5; Haiyan Zheng, MD1; Frank Lehmann-Horn, MD2
[+] Author Affiliations
1Department of Thoracic Surgery, 117 PLA Hospital, Hangzhou, China
2Division of Neurophysiology, Neuromuscular Disease Center, and Rare Disease Center, Ulm University, Ulm, Germany
3Department of Pathology, Second Affiliated Hospital of Zhejiang University School of Medicine, Hangzhou, China
4Medizinisches Versorgungszentrum, Karlsruhe, Germany
5Department of Neurology, 117 PLA Hospital, Hangzhou, China
JAMA Neurol. 2013;70(11):1436-1439. doi:10.1001/jamaneurol.2013.3918.
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Importance  Hypokalemic periodic paralysis is a muscle channelopathy based on mutations or predisposing variants or secondary to potassium wasting. In contrast to myasthenia gravis, an association with thymic hyperplasia has not yet been reported, to our knowledge.

Observations  We report a male patient in his mid-20s with progressive episodes of flaccid muscle weakness, associated low serum potassium levels, and a pathologic decrement in the long exercise test. Because the familial inheritance in the family was initially unknown, thorough diagnostic tests were performed including contrast-enhanced computed tomography scan, which displayed a mass in the anterior mediastinum. The test results for autoantibodies against myasthenia gravis (acetylcholine receptor, muscle-specific tyrosine kinase, and low-density lipoprotein receptor–related protein 4) and other end plate channelopathies were negative, and test results for hypokalemia-inducing hormones (thyroid, corticotropin, and cortisol) were negative. Surgery identified a thymus of 13 × 8 × 3 cm3. Histologic analysis was consistent with thymic hyperplasia of the follicular subtype and immunohistologic analysis showed cytokeratin 5/6 in hyperplastic epithelial cells. A 2-year follow-up revealed the postoperative absence of weakness episodes. As in 30% of familial cases, molecular genetics testing failed to identify a mutation in periodic paralysis genes.

Conclusions and Relevance  Thymic hyperplasia can clinically manifest susceptibility to hypokalemic periodic paralysis. For patients with late onset or increasing weakness episodes, we recommend imaging to assess for thymic enlargement and thymectomy at thymic hyperplasia.

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Figure 1.
Long Exercise Tests

The compound muscle action potentials of the patient and his affected uncle show a typical late decrement in contrast to the healthy control. For details, see the Methods subsection.

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Figure 2.
Images of the Enlarged Thymus

A, Mass without involvement of the surrounding tissue on the contrast-enhanced computed tomography scan of the anterior mediastinum. B, Hyperplastic thymus with lobular appearance and fibrous capsule. C, Lobular architecture, cortex, medulla, and Hassall corpuscles and adipose tissue in the stroma (hematoxylin-eosin, original magnification ×100). D, Immunostaining of cytokeratin 5/6 in hyperplastic epithelial cells (original magnification ×200).

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