Anderson Tawil syndrome (ATS) is a rare channelopathy traditionally characterised by periodic paralysis, cardiac arrhythmias and dysmorphic features. Mutations in the KCJN2 gene have been associated with ATS. Accurate and early diagnosis is important in facilitating treatment of episodic paralysis and preventing cardiac death. We aim to fully characterise the phenotype in a carefully stratified cohort including cognitive deficits and cardiac risk. 62 patients were identified with KCJN2 mutations. Comprehensive clinical information was collected for these patients. Cardiac symptoms were prominent - two thirds had daily or very frequent palpitations. 12.9% reported syncope. Serious cardiac complications requiring implantable cardiac defibrillator (ICD) occurred in 8.06% patients. 39% reported pain which has previously not been appreciated as part of ATS. Interestingly, 4 patients had fasciculations. 25% of patients had no decrement on Long exercise tests (LET) and 6.25% had less than 40% decrement despite having episodic weakness. Lower limb muscle MRI was abnormal in 6 patients. Excess sleepiness was reported, for which the aetiology requires further elucidation. While dysmorphic features exist, these can be subtle. Short stature is not ubiquitous. Heterogeneity within families was commonly seen. Evidently, the phenotypic spectrum of ATS is broader than currently appreciated. This is important to recognise in order to optimise accurate diagnosis as well as to develop screening and treatment regimens.