Abstract
BACKGROUND: Pernicious anemia, an autoimmune disease, presents with gradual, nonspecific symptoms, often leading to delayed diagnosis owing to its overlap with other conditions and variability in laboratory findings, such as neurological symptoms without anemia or macrocytosis. CASE PRESENTATION: This case describes a 40-year-old Iranian woman with a decade-long history of nonspecific symptoms, including fatigue, widespread musculoskeletal pain, paresthesia, cognitive disturbances, and optic neuritis; misattributed to conditions such as fibromyalgia, hypothyroidism, and autoimmune diseases. Despite annual monitoring for normocytic anemia, her critically low vitamin B12 levels (< 150 pg/mL) and a diagnosis of pernicious anemia were identified only after persistent symptoms prompted further evaluation, revealing atrophic gastritis as the underlying cause. Neurologic improvement with parenteral B12 therapy, alongside management of fibromyalgia, emphasizes the importance of considering vitamin B12 deficiency even in the absence of classic hematologic findings. CONCLUSION: This case highlights the diagnostic challenges of pernicious anemia, where nonspecific symptoms and overlapping comorbidities obscure diagnosis, underscoring the need for a systematic, multidisciplinary approach and timely recognition of vitamin B12 deficiency to prevent irreversible complications.