Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant genetic condition characterized by acute, recurrent episodes of brachial plexus neuropathies caused by heterozygous pathogenic variants in the SEPT9 gene. We present a case that underscores the importance of comprehensive genetic evaluation and variant reclassification in the context of strong family history. A 9-year-old male presented to a large referral neuromuscular clinic with severe subacute left arm pain, weakness, and muscle atrophy, mirroring a similar episode in his right arm one year earlier. Magnetic resonance imaging (MRI) of the cervical spine and left brachial plexus was normal. Electromyography (EMG) and nerve conduction studies (NCS) indicated severe left brachial plexopathy affecting the upper more than middle trunks, with active denervation in the deltoid and biceps. Paternal family history revealed multiple relatives with similar episodes, including his father, aunt, grandfather, and first cousin once removed. Considering the personal and family history, we were suspicious of a hereditary brachial plexus neuritis. During the visit, we learned that the cousin had undergone genetic testing eight months earlier, resulting in a variant of uncertain significance (VUS)—a duplication of exon 2 in the SEPT9 gene – dismissed as clinically insignificant. Further evaluation of the family’s clinical history, electrophysiological findings, familial variant testing, and literature review prompted reassessment by the reporting laboratory. This resulted in reclassification of the SEPT9 variant as pathogenic, enabling definitive diagnosis and directly informing genetic counseling for the patient and his family. This case underscores the critical role of thorough variant research, especially for VUS results, in the context of strong family histories. Genetic healthcare providers must remain diligent in reevaluating VUS findings to ensure accurate diagnoses, guide management, and inform at-risk relatives. Variant reclassification can significantly impact clinical decision-making and genetic counseling practices, particularly in conditions with clear clinical patterns and known genetic etiologies.