<b><i>Introduction:</i></b> Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are required redox equivalents for essential biochemical reactions. Their hydrated forms, NADHX and NAD(P)HX, are inhibitors for several dehydrogenases and cause harmful byproducts. NAD(P)HX dehydratase (NAXD) and NAD(P)HX epimerase (NAXE) together form the nicotinamide repair system. <b><i>Case Presentation:</i></b> A 7-month-old boy was admitted due to myoclonic seizures, impaired consciousness, and rapid loss of head control. One of his siblings regressed after a febrile seizure and died at 7 months. He had lethargy and axial hypotonia but skin lesions and organomegaly were not noted. Basal metabolic tests were within normal limits except serum and cerebrospinal fluid lactate levels, which were mildly elevated. Mitochondrial cocktail was added to the antiepileptic treatment with suspicion of mitochondrial disease. Whole-exome sequencing showed a novel homozygous mutation (c.247G&#x3e;A) in the <i>NAXD</i> gene. His seizures stopped within a few weeks. However, he died at the age of 18 months. <b><i>Discussion:</i></b> Prominent features of NAXD deficiency are progressive neurological deterioration after fever, cardiomyopathy, skin lesions, and premature death. Unlike the cases reported in the literature, our patient had neither preceding fever nor skin lesion during follow-up. It appears that cases show phenotypic diversity.

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Background: Myotonia congenita is the most common form of nondystrophic myotonia and is caused by Mendelian inherited mutations in the CLCN1 gene encoding the voltage-gated chloride channel of skeletal muscle. Objective: The study aimed to describe the clinical and genetic spectrum of Myotonia congenita in a large pediatric cohort. Methods: Demographic, genetic, and clinical data of the patients aged under 18 years at time of first clinical attendance from 11 centers in different geographical regions of Türkiye were retrospectively investigated. Results: Fifty-four patients (mean age:15.2 years (±5.5), 76% males, with 85% Becker, 15% Thomsen form) from 40 families were included. Consanguineous marriage rate was 67%. 70,5% of patients had a family member with Myotonia congenita. The mean age of disease onset was 5.7 (±4.9) years. Overall 23 different mutations (2/23 were novel) were detected in 52 patients, and large exon deletions were identified in two siblings. Thomsen and Becker forms were observed concomitantly in one family. Carbamazepine (46.3%), mexiletine (27.8%), phenytoin (9.3%) were preferred for treatment. Conclusions: The clinical and genetic heterogeneity, as well as the limited response to current treatment options, constitutes an ongoing challenge. In our cohort, recessive Myotonia congenita was more frequent and novel mutations will contribute to the literature.