Mercury exposure leading to functional vitamin B12 deficiency and subacute combined degeneration, a case report and literature review

Isidora Semnic¹Valentino Racki^1,2*Vladimira Vuletić^1,2

• ¹Department of Neurology, Faculty of Medicine, University of Rijeka, Rijeka, Rijeka, Croatia
• ²Department of Neurology, Clinical Hospital Center Rijeka, Rijeka, Croatia

Introduction: The association between neurological symptomatology and heavy metal exposure has been reported in the literature. A few cases of extrapyramidal symptomatology and subacute combined degeneration were described as a manifestation of mercury intoxication. We highlight a case of a patient with Parkinsonian features (tremor, rigidity, bradykinesia), pyramidal deficits, dysarthria, paresthesia, mild cognitive decline, and emotional lability with proven elevated mercury levels in the blood and hair and elevated arsenic in urine. Case: A 60-year-old man, with history of exposure to mercury while working in the Centre for Waste Management and Environmental Protection presented to the neurologist after 10- months of persistant tremors, muscle spasms, paresthesia, and irritability, followed by the appearance of bradykinesia, slurred speech, rigidity, insomnia, and subtle cognitive decline. Laboratory investigations disclosed functional vitamin B12 and vitamin D deficiencies, while toxicological quantitative analysis showed elevated blood mercury levels (15.2 μg/L) and hair roots (3μg/g). MRI of the brain was normal, and an MRI of the posterior cervical spine detected signs of myelopathy. FDG PET of the brain registered bilateral temporal and parietal glucose hypometabolism, most pronounced inleft inferior parietal and left superior temporal regions. SPECT imaging of dopaminergic neurons in the striatum was negative, and the patient was levodopa-unresponsive. Multivitamin therapy (Vitamin B, E, and D) with selenium added to symptomatic therapy (benzodiazepines, muscle relaxants, antidepressive) provided minimal relief, leading to the introduction of N-acetyl cysteine, resulting in a moderate improvement of symptoms. Physical and speech therapy were of great importance in this case. Discussion: This case is unique because it represents the development of therapy-resistant extrapyramidal symptoms developed during 3 years of mercury exposure, likely leading to a subacute combined degeneration due to functional vitamin B12 deficiency. Epidemiological data provide information about methylmercury poisoning known as Minamata disease- somatosensory deficits, ataxia, parkinsonism, dysarthria, visual and hearing problems Conclusion: Toxicological screening for heavy metals in blood and urine should be considered in patients presenting with unexplained, levodopa-resistant extrapyramidal symptoms, behavioral and sleep disturbances, cognitive decline, and other non-specific neurological signs.