Cystic Fibrosis beyond Childhood: A Case Report of Recurrent Heat-Induced Electrolyte Disturbances Unmasking a Late Diagnosis

INTRODUCTION: Cystic fibrosis (CF) is an autosomal recessive genetic disorder caused by pathogenic variants in the CFTR gene, leading to impaired chloride and bicarbonate transport across epithelial tissues. While typically diagnosed in infancy due to classic respiratory and gastrointestinal symptoms, atypical forms can present later in life with subtle or isolated manifestations. This case represents a rare monosymptomatic and atypical adult-onset presentation of CF, with electrolyte disturbances as the sole clinical manifestation. CASE PRESENTATION: We present the case of a 31-year-old man who developed recurrent episodes of profound electrolyte imbalance and acute kidney injury following intense physical activity in high environmental temperatures. He exhibited syncope, muscle cramps, and signs of severe dehydration. Laboratory tests revealed hyponatremia, hypokalemia, hypochloremia, hemoconcentration, and elevated creatinine levels, with urine findings consistent with extrarenal salt loss. A similar episode had occurred 6 years earlier. In the absence of an identifiable cause, CF was suspected. Sweat chloride testing confirmed the diagnosis in two separate samples. Genetic analysis revealed compound heterozygosity for a pathogenic CFTR variant and a variant of uncertain significance. Screening for other common manifestations of CF was negative. CONCLUSION: This case underscores the diagnostic challenge of adult-onset CF, particularly in regions where neonatal screening was not historically implemented. Electrolyte disturbances, especially in the context of heat stress, may represent the only clinical clue in patients with residual CFTR function. Prompt recognition and diagnosis are essential for initiating appropriate monitoring and treatment, including the potential use of CFTR modulator therapies that can significantly improve quality of life and long-term outcomes.