“Warm-Up”-Like Phenomen in Andersen–Tawil-Syndrome: A Case Report and Comparative Analysis to Myotonica Congenita

Background/Purpose: Andersen–Tawil-syndrome (ATS) is a distinct potassium channelopathy. Due to its rarity (1:500,000), establishment of evidence based therapeutic interventions is challenging. Treatment guidance in ATS, therefore, is mainly on individual basis. The aim of this presentation was to improve the knowledge on treatment of periodic paralysis in ATS.

Methods: Case description with literature review concerning periodic paralysis in a juvenile patient with ATS.

Results: The patient fulfilled the classic triad of ATS, that is, periodic paralysis, cardiac rhythm disorders, and facial features. Genetic analysis described a maternally inherited pathogenic variant (c.652C>T) in the KCNJ2 gene, with a dominant-negative effect. CK was moderately elevated at baseline (300 U/L), more during paralysis (507–917 U/L). Potassium level, also during paralysis, was normal or mildly decreased (3.3–3.9 mmol/L). Episodic muscle weakness was proximally predominant (MRC 3), and trigger factors included prolonged rest. Despite a mild fixed weakness, the muscle ultrasound depicted no hyperechogenicity. Oral supplementation of potassium showed some improvement. Daily intake of acetazolamide in a dose of 625 mg had a negative effect, in a moderate dose of 375 mg, a beneficial effect. During paralysis, there was a significant improvement in muscle weakness with exercise. Needle EMG depicted no myotonia. ([Fig. 1])

Conclusion: In the presented ATS patient, we describe a particular “warm-up” phenomenon of the periodic paralysis. Implementation of a targeted daily physiotherapy program resulted in improved weakness, in addition to a tailored dosing of acetazolamide and potassium. In contrast to a juvenile patient with myotonia congenita (CLCN1; c.1437_1450, c.2680C>T), the ATS patient did not depict clinical or electrophysiological myotonia, despite a comparable motor improvement (“warm-up”) under exercise after rest, and did not demonstrate any beneficial effect from add-on lamotrigine. In conclusion, individualized treatment strategies are important in ATS.

Fig. 1 Muscle ultrasound in the Andersen–Tawil syndrome (ATS) patient depicted no muscle hyperechogenicity despite a mild fixed muscle weakness. An MRI study (Vivekanandam et al and Muscle & Nerve, 2023) had demonstrated fatty replacement, primarily in the biceps femoris muscle, in ATS patients with advanced fixed muscle weakness.

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Artikel online veröffentlicht:
26. September 2025