Subacute Combined Degeneration From Isolated Folate Deficiency Secondary to Nutrition and Co-Trimoxazole Use in a Patient With IgG4-Related Disease.

Subacute combined degeneration (SCD) is characterized by demyelination primarily affecting the dorsal column and lateral corticospinal tracts. It typically presents with paresthesia in the feet and progresses to involve the upper extremities. SCD is commonly associated with nutritional deficiencies, particularly deficiencies of vitamin B12, folate, or copper. Neurological disorders solely caused by isolated folate deficiency are uncommon. Classic presentations of folate deficiency include fatigue, lethargy, glossitis, diarrhea, and the gradual development of megaloblastic anemia. Early SCD diagnosis and treatment, even in the absence of hematological signs, are imperative to prevent irreversible neurological deficits and achieve favorable outcomes. We present a patient with IgG4-related disease who developed subacute sensory ataxia and generalized hyperreflexia. She was eventually diagnosed with SCD, which was attributed to isolated folate deficiency caused by critical illness-related malnutrition and concomitant co-trimoxazole use. Her clinical and neurological examination findings improved at the subsequent 2-week follow-up after effective treatment. Finally, she was able to walk and run independently. This case emphasizes the importance of early recognition of this rare presentation and prompt treatment, which can significantly contribute to better neurological outcomes.