To describe a rare case of heroin inhalation–induced unilateral hippocampal restricted diffusion and toxic myelitis.

Hippocampal injury and myelitis are serious complications of heroin inhalation. Hippocampal damage proposed mechanisms include hypoxic–ischemic neurotoxicity and mitochondrial dysfunction. Microscopy shows hemorrhagic and ischemic CA1 and subiculum involvement, reflecting neuronal-vascular disruption. Although frequent improvement with supportive care, irreversible atrophy has been reported. Spinal cord involvement is rarer. Pathophysiology, including neurotoxicity, ischemia, and immune-mediated reactions, with histology revealing gray and white matter necrosis, correlates with poor outcomes. Typical opioid-related hippocampal injury is symmetric. Isolated unilateral involvement is exceedingly rare, and to our knowledge, coexisting myelitis has not been previously reported.

A 62-year-old man with recent heroin inhalation presented with abdominal pain. Urinalysis was positive for nitrites, ketones, cocaine, cannabis, and opioids. A complicated urinary tract infection was suspected, and antibiotics were started. Three days later, he developed sudden unresponsiveness and generalized weakness. Examination revealed lethargy, flaccid quadriparesis, and severe dysautonomia. He suffered a cardiac arrest and was admitted to the intensive care unit following return of spontaneous circulation.

Brain and cervical-spine MRI showed right hippocampus and spinal cord (C1–C4) restricted diffusion, with severe spinal canal stenosis. Cerebrospinal fluid analysis revealed pink, cloudy fluid with pleocytosis (26 WBCs/mm^³), polymorphonuclear predominance, 513 RBCs/mm^³, hyperglycorrhachia (105mg/dL), and hyperproteinorrachia (721mg/dL), with negative infectious and autoimmune testing. Plasmapheresis followed by methylprednisolone did not result in clinical improvement. Based on clinical findings, heroin inhalation–induced unilateral hippocampal restricted diffusion and toxic myelitis was diagnosed.

Although bilateral hippocampal damage from cocaine and opioids is well-documented, unilateral injury has only been previously linked to heroin. This exceptionally rare presentation helped identify the myelitis’ etiology. As the opioid epidemic grows, awareness of rare but serious neurological complications is crucial. Early recognition and treatment might improve outcomes; however, complete recovery is rare.