[Postoperative dementia: toxicity of nitrous oxide].

INTRODUCTION: Post-operative neuropsychiatric manifestations represent a frequent situation and may be due to several aetiologies. The responsibility of vitamin B12 deficiency must be evoked, especially in case of anaesthesia with a currently used substance: nitrous oxide.

CASE REPORT: A 65 year-old man with no medical history, presented problems walking and memory loss 16 days after surgery for femoral prosthesis. Neurological examination revealed paraplegia with syndrome of combined degeneration of the spinal cord. The exploration of cognitive functions showed disorientation in time with memory disorders and disturbance of executive functioning. There was no apraxia, aphasia or agnosia. There were neither psychotic symptoms nor mood changes. MMS was at 18/30. Red blood count revealed an anaemia with macrocytosis (MGV=120 3). Vitamin B12 rate was very low (less than 30 g/l). Folate blood level was normal. Brain MRI showed moderate cerebral atrophy. Other investigations led to the diagnosis of Biermer's disease (fundic atrophy at biopsy with presence in the serum of antibodies to intrinsic factor). The diagnosis of neurological attack related to a vitamin B12 deficiency secondary to Biermer's disease was established, but the appearance of disorders in the post-operative period suggested the existence of an added factor. The recovery of informations revealed that anaesthesia was maintained by nitrous oxide during two hours and the patient exhibited pre-operative anaemia with macrocytosis. The hypothesis of decompensation of latent vitamin B12 deficiency by nitrous oxide was evoked. Replacement therapy by vitamin B12 induced real improvement of the cognitive impairment. MMS increased to 25/30.

DISCUSSION: Cognitive impairment due to vitamin B12 deficiency is rarely dominated by isolated memory disorders. An authentic dementia is exceptional. Our patient had a dementia diagnosed on the basis of DSM IV criteria including memory disorders, disturbance of executive functioning and significant impairment in social and occupational functioning, associated with a combined degeneration of the spinal cord, common in vitamin B12 deficiency. Furthermore, he had an unknown Biermer's disease responsible for pre-operative deficiency which was clinically latent (there was only macrocytosis anaemia). The appearance of problems in the post-operative period was due to an acute decompensation of the latent deficiency induced undoubtedly by nitrous oxide used in anaesthesia. According to Christensen, nitrous oxide causes irreversible oxidation of vitamin B12 cobalt's atom responsible for its inactivation and the appearance of clinical manifestations. Evolution under vitamin B12 replacement therapy depends on the rapidity of its founding. In our case, it led to an improvement, notably in cognitive functions.

CONCLUSION: Through this observation, the authors underline the necessity to search for vitamin B12 deficiency in the case of cognitive features following general anaesthesia.