Non-diabetic Ketoacidosis in a Patient With Amyotrophic Lateral Sclerosis: The Role of Stewart's Approach in Analyzing Acid-Base Disorders.

Amyotrophic lateral sclerosis (ALS) is often complicated by severe malnutrition, increasing the risk of metabolic disturbances. Non-diabetic ketoacidosis (NDKA) is a rare but serious complication, typically related to prolonged fasting or catabolic states. A 62-year-old female patient with ALS and hypothyroidism presented with pneumonia and tetraplegia. Her body mass index (BMI) was 17 kg/m². Laboratory findings showed a high anion gap (AG) metabolic acidosis (pH 7.23, bicarbonate 13 mmol/L, partial pressure of carbon dioxide (pCO₂) 28 mmHg) without hyperlactatemia, but with significant ketonemia (5 mmol/L), severe hypophosphatemia, and signs of systemic inflammation. Upon admission, she received an intravenous infusion of 4.2% sodium bicarbonate. The simplified strong ion difference (SID) was preserved, excluding dilutional or hyperchloremic causes. Stewart's physicochemical approach, supported by a Gamblegram, revealed an acidosis due to unmeasured fixed acids, specifically ketone bodies. In light of this, bicarbonate therapy was discontinued, and nutritional correction with glucose hydration led to rapid clinical and biochemical improvement. This case illustrates the diagnostic and therapeutic value of Stewart's model in complex acid-base disturbances and underscores the need for early nutritional assessment in ALS patients. To our knowledge, this is the first reported case of NDKA in ALS, highlighting a rare but clinically relevant metabolic complication.