Mechanisms Involved in the Neurotoxicity and Abuse Liability of Nitrous Oxide: A Narrative Review.

The recreational use of nitrous oxide (N2O) has increased over the years. At the same time, more N2O intoxications are presented to hospitals. The incidental use of N2O is relatively harmless, but heavy, frequent and chronic use comes with considerable health risks. Most importantly, N2O can inactivate the co-factor cobalamin, which, in turn, leads to paresthesia's, partial paralysis and generalized demyelinating polyneuropathy. In some patients, these disorders are irreversible. Several metabolic cascades have been identified by which N2O can cause harmful effects. Because these effects mostly occur after prolonged use, it raises the question of whether N2O has addictive properties, explaining its prolonged and frequent use at high dose. Several lines of evidence for N2O's dependence liability can be found in the literature, but the underlying mechanism of action remains controversial. N2O interacts with the opioid system, but N2O also acts as an N-methyl-D-aspartate (NMDA) receptor antagonist, by which it can cause dopamine disinhibition. In this narrative review, we provide a detailed description of animal and human evidence for N2O-induced abuse/dependence and for N2O-induced neurotoxicity.

[Acute and chronic toxicities associated with the use and misuse of nitrous oxide: An update]. / Toxicités aiguës et chroniques associées à l'usage et au mésusage du protoxyde d'azote : mise au point.

Nitrous oxide (N2O) is used since the eighteenth century as an anesthetic and analgesic but also for recreational use. If the labelled uses of N2O and their modalities are nowadays perfectly framed, the misuse of N2O takes very alarming proportions among teenagers and young adults. This misuse is the cause of acute (hypoxia, barotrauma, burns, neuropsychiatric disorders) and chronic complications if repeated (myeloneuropathy, anemia, thrombosis, inhalant use disorder). The main mechanism of the latter is mainly related to a functional deficit in vitamin B12 induced by N2O. The management of acute complications is symptomatic. The management of chronic complications is based on vitamin B12 supplementation. The best biomarker of chronic N2O exposure is the elevation of the plasmatic level of methylmalonic acid. In all cases of recreational misuses, addiction treatment is necessary to prevent complications or their worsening by providing information in order to stop consumption.

Recreational Nitrous Oxide Abuse: Prevalence, Neurotoxicity, and Treatment.

Nitrous oxide (N2O), also known as "laughing gas," is a colorless, nonirritating gas. Clinically, it is widely used as an inhaled anesthetic, analgesic, and anxiolytic. In recent years, recreational abuse of N2O has become increasingly common, especially among young adults and adolescents, but many of them lack awareness of the possible side effects associated with this drug. N2O abuse can damage multiple systems, especially the nervous system, but the exact mechanism of N2O toxicity remains controversial. At present, an increasing number of cases of nervous system damage caused by N2O abuse have been reported both at home and abroad. Discontinuation of N2O use and timely supplementation with vitamin B12 are essential for a good prognosis. Long-term abuse without timely treatment will eventually lead to irreversible neurological damage. In this article, we discuss the epidemiology of N2O abuse, neurotoxicity mechanisms, clinical manifestations, relevant auxiliary examinations, treatments, and prognosis to improve social awareness of N2O exposure risk, especially among users and clinicians.

Nitrous oxide-induced demyelination: Clinical presentation, diagnosis and treatment recommendations.

BACKGROUND: Recreational use of nitrous oxide (NO) in the general public has led to increasing reports of NO-induced demyelination (NOID). We describe the varying clinical presentations and pathophysiology, and offer a treatment paradigm. METHODS: A literature search of MEDLINE and EMBASE resulted in 42 publications with 37 studies meeting the inclusion criteria, for a total of 51 patients. Our case series included 5 patients seen from 2014 to 2018 followed over 3-60 months. RESULTS: Those with sensory symptoms and subjective weakness were categorized as having "mild" symptoms (25%). Symptoms indicating involvement outside the dorsal columns such as observer-graded weakness were categorized as "moderate" (61%). Patients with the aforementioned plus cognitive effects were categorized as "severe" (12%). There was no dose-dependent relationship between the amount of NO used and clinical impairment. There was a trend between the severity of neurologic impairment and serum levels of B12. Two patients were noncompliant. One initiated only oral therapy and did not improve. One received injections a month apart and worsened. CONCLUSIONS: Patients with NOID tend to have worse symptoms when presenting with lower serum vitamin B12 levels and have good recovery rates when treated with intramuscular B12 and oral supplementation.

No laughing matter: subacute degeneration of the spinal cord due to nitrous oxide inhalation.

BACKGROUND: Whilst the dangers of 'legal highs' have been widely publicised in the media, very few cases of the neurological syndrome associated with the inhalation of nitrous oxide (N2O) have been reported. Here we set out to raise awareness of subacute degeneration of the spinal cord arising from recreational N2O use so that formal surveillance programs and public health interventions can be designed. METHODS: Case series documenting the clinical and investigational features of ten consecutive cases of subacute degeneration of the spinal cord presenting to a hospital with a tertiary neurosciences service in East London. RESULTS: Sensory disturbance in the lower (± upper) limbs was the commonest presenting feature, along with gait abnormalities and sensory ataxia. MRI imaging of the spine showed the characteristic features of dorsal column hyperintensity on T2 weighted sequences. Serum B12 levels may be normal because subacute degeneration of the spinal cord in this situation is triggered by functional rather than absolute B12 deficiency. DISCUSSION: A high index of suspicion is required to prompt appropriate investigation, make the diagnosis and commence treatment early. This is the largest reported series of patients with subacute degeneration of the spinal cord induced by recreational use of N2O. However, the number of patients admitted to hospital likely represents the 'tip of the iceberg', with many less severe presentations remaining undetected. After raising awareness, attention should focus on measuring the extent of the problem, the groups affected, and devising ways to prevent potentially long-term neurological damage.

When the Laughing Stops: Subacute Combined Spinal Cord Degeneration Caused by Laughing Gas Use.

: Here we describe a case of subacute combined spinal cord degeneration caused by nitrous oxide (N2O, laughing gas) use. Because of its euphoric effects, the use of N2O has become increasingly popular in recent years. Unfortunately, the use of N2O leads to inactivation of vitamin B12. Vitamin B12 plays an essential role in the synthesis and maintenance of myelin, a fatty substance that surrounds nerve cells and is crucial for their functioning. Deficiency of vitamin B12 could typically result in degeneration of posterior and lateral columns of the spinal cord. Treatment with intramuscular vitamin B12 injections and abstinence of N2O generally leads to gradual improvement of symptoms. Our case demonstrates the importance of the methyl malonic acid test to detect early or mild vitamin B12 deficiency as a cause of myelopathy while serum vitamin B12 level may be normal. Written consent was obtained from our patient to publish the details of this individual case.

Nitrous Oxide-induced Subacute Combined Degeneration Presenting with Dystonia and Pseudoathetosis: A Case Report.

PURPOSE: Nitrous oxide (N2O) is neurotoxic by interfering with vitamin B12 bioavailability. The clinical picture is indistinguishable to that of subacute combined degeneration (SCD). A movement disorder might occur though it is not a characteristic feature. We report a patient with N2O-induced SCD, exhibiting a combination of different involuntary movements. CASE REPORT: A 20-year-old woman presented with one month of progressive unsteady gait, involuntary movements and tingling sensation in a stocking-glove distribution. She had used N2O and ketamine intermittently for recreational purposes for about two years. Neurological examination demonstrated normal cranial nerve functions except for dystonia in the facial muscle and tongue. Her muscle strength was full, but there were bilateral hyperreflexia and extensor plantar response. She exhibited dystonia in four limbs with athetoid movement in fingers and toes, worsened by eye closure. Vibration and proprioception were impaired. Laboratory tests revealed anemia (Hb: 9.9 g/dl) with normal mean corpuscular volume (85.7 fL) and decreased iron level (22 µg/dl) while other results were normal including serum vitamin B12 level (626 pg/ml). Magnetic resonance imaging showed a hyperintense lesion from C1 to C6 level in the posterior column. She was diagnosed as having SCD caused by N2O abuse, presenting with generalized dystonia and pseudoathetosis. The involuntary movements disappeared with vitamin B12 supplementation. CONCLUSION: Movement disorders may be the rare manifestations of SCD associated with N2O abuse. Early recognition of the etiology is vital because it is treatable with vitamin B12 and methionine.

Subacute combined degeneration of the spinal cord in a patient abusing nitrous oxide and self-medicating with cyanocobalamin.

PURPOSE: A case of subacute combined degeneration (SCD) of the spinal cord manifesting as severe ataxia and urinary retention in a patient with a history of heavy nitrous oxide abuse and self-supplementation with cyanocobalamin is reported. SUMMARY: A 27-year-old woman was treated in the emergency department for complaints of abdominal pain and inability to urinate for about 12 hours. The patient also complained of worsening lower-extremity weakness for 10 days and a "pins and needles" sensation in the lower extremities for approximately 1 year. She reported nitrous oxide abuse over 3 years (an average of 100-200 "whippit" cartridges daily on 3 or 4 days per week), as well as long-term self-medication with oral and i.m. cyanocobalamin for the purpose of preventing nitrous oxide-induced neurologic symptoms. Results of magnetic resonance imaging (MRI) were highly suggestive of SCD, which is typically seen in primary vitamin B12 deficiency but has been reported in the context of chronic nitrous oxide exposure. Treatment was initiated with cyanocobalamin 1000 µg i.m. daily, to be continued for 5 days and followed by a four-week regimen of 1000 µg i.m. weekly. The patient was discharged after 3 days, despite continued symptoms, with instructions to obtain ongoing care but was lost to follow-up. CONCLUSION: A patient who abused nitrous oxide chronically developed ataxia, paresthesia, and urinary retention while self-medicating with cyanocobalamin. A diagnosis of SCD was supported by MRI findings, symptoms, and the known relationship between nitrous oxide exposure and vitamin B12 deficiency.

Nitrous oxide induced myeloneuropathy: a case report.

We report the case of a 35-year-old male with a history of chronic, escalating nitrous oxide abuse who presented to the ER with a history of recent onset generalized weakness, altered sensorium, abnormal posturing of the hands, urinary complaints, and decreased balance. Physical examination was notable for pathologically brisk reflexes in all extremities, generalized flexion contracture of the fingers, decreased sensation in a stocking and glove distribution, and a weakly positive Babinski sign. The patient was noted to be a poor historian with decreased attention and concentration though otherwise generally alert and oriented. No discrete sensory level in the chest or trunk was detected, and the overall clinical appearance was felt to be most compatible with a mixed myeloneuropathic pattern of central and peripheral involvement. Laboratory findings were normal and noncontributory. Cervical spine MRI subsequently performed to rule out cord compression, intrinsic spinal cord mass, or demyelinating disease was notable for a long segment of increased T2 signal extending from C2-C3 to C6-C7 localizing to the dorsal columns of the cord in a typical "inverted V" fashion. No associated cord expansion was seen nor was there evidence of extrinsic compression; faint associated contrast enhancement was observed on post-gadolinium images. Further evaluation with nerve conduction velocity and electromyographic testing was deferred. Based on the exam findings, clinical history, and presentation, a diagnosis of nitrous oxide-related myeloneuropathy was made, and treatment with high-dose vitamin B12 supplementation was instituted. Recovery has been slow to date.

The effects of L-carnitine on the combination of, inhalation anesthetic-induced developmental, neuronal apoptosis in the rat frontal cortex.

The anesthetic gas nitrous oxide (N2O) and the volatile anesthetic isoflurane (ISO) are commonly used in surgical procedures for human infants and in veterinary and laboratory animal practice to produce loss of consciousness and analgesia. Recent reports indicate that exposure of the developing brain to general anesthetics that block N-methyl-D-aspartate (NMDA) glutamate receptors or potentiate GABA(A) receptors can trigger widespread apoptotic neurodegeneration. In the present study, the question arises whether a relatively low dose of ISO alone or its combination with N2O entails significant risk of inducing enhanced apoptosis. In addition, the role of L-carnitine to attenuate these effects was also examined. Postnatal day 7 (PND-7) rat pups were exposed to N2O (75%) or a low dose of ISO (0.55%) alone, or N2O plus ISO for 2, 4, 6 or 8 h with or without L-carnitine. The neurotoxic effects were evaluated 6 h after completion of anesthetic administration. No significant neurotoxic effects were observed for the animals exposed to N2O or ISO alone. However, enhanced apoptotic cell death was apparent when N2O was combined with ISO at exposure durations of 6 h or more. Co-administration of L-carnitine (300 or 500 mg/kg, i.p.) effectively protected neurons from the anesthetic-induced damage. These data indicate that 6 h or more of inhaled anesthetic exposure consisting of a combination of N2O and ISO results in enhanced neuronal apoptosis, and L-carnitine effectively blocks the neuronal apoptosis caused by inhalation anesthetics in the developing rat brain.

The role of antioxidant supplementation in occupational exposure to waste anaesthetic gases.

OBJECTIVES: Although the genotoxicity related to waste anaesthetic gases is controversial, a consistent number of observations have provided evidence for an increased level of DNA strand breaks. The goal of the research was to investigate this hypothesis and estimate the genoprotective role of antioxidant supplementation in technical anaesthesiology staff working in operating theatres. METHODS: Heparinized venous blood samples were collected from 17 exposed technical anaesthesiology staff (mean age 34.3 +/- 3.5 years) and non-exposed control group (mean age 32.2 +/- 3.4 years) and examined in the alkaline comet assay for DNA strand breakage. Vitamin E (300 mg/day) plus vitamin C (500 mg/day) were supplemented to the technical anaesthesiology staff for 12 weeks and blood samples were retaken and evaluated by comet assay. RESULTS: The DNA breakage observed in the lymphocytes of the technical anaesthesiology staff was 21.5 +/- 5.0, as calculated by total comet score (TCS). This score was significantly higher (P<0.001) than in the controls (8.6 +/- 4.7) before antioxidant treatment. Supplementation of vitamins E plus C significantly (P<0.01) reduced the mean TCS as 14.2 +/- 6.1. CONCLUSION: The results of our study indicate that occupational exposure to anaesthetic gases induces oxidative DNA damage. Supplementation of the diet for 12 weeks with vitamin C and vitamin E resulted in a significant decrease in the DNA damage.

Acute psychosis following ingestion of 'Rapture'.

OBJECTIVES: To describe a brief psychotic episode associated with ingestion of a dietary supplement containing piperazine. METHODS: A case report was undertaken. RESULTS: A 20-year-old man developed a brief psychotic episode associated with persecutory delusional beliefs and auditory and visual hallucinations, leading him to commit the offence of arson. This episode occurred 12 h following the ingestion of 'Rapture' in addition to small quantities of cannabis and nitrous oxide, in a young man with no prior psychiatric history. This episode resolved with the use of benzodiazepines and 6 months later he has had no recurrence of psychiatric symptoms. CONCLUSIONS: Given the reported mechanism of action of the 'herbal high' dietary supplements, it would seem possible that they may be able to precipitate a psychotic episode in vulnerable individuals. With the increasing use of these substances, it is important for clinicians to be alert to this possibility.

Effect of nitrous oxide on intracellular events of GT1-7 GnRH-secreting neurons.

Exposure to nitrous oxide (N2O) may result in decreased fertility and other reproductive problems in women working in dental offices. This study explored direct effects of N2O on gonadotropin-releasing hormone (GnRH) neurons involved in pituitary-gonadal function. An immortalized GnRH-secreting GT1-7 cell line was used to evaluate the effects of 60% N2O exposure for 24 hours on pro-GnRH mRNA levels, KCl-stimulated GnRH levels, and certain signaling pathways. N2O exposure for 24 hours decreased pro-GnRH mRNA levels by approximately 80% without affecting cell viability or intracellular GnRH levels. The ability of KCl to release GnRH stores was also significantly reduced. Western blot analyses of ERK1/2, and p38 phosphorylation and IkappaB did not indicate any change in phosphorylation, although p63 tyrosine phosphorylation was increased after short-term N2O exposure. Levels of c-fos, a nuclear transcription factor, were not changed by N2O exposure. These results indicate that N2O significantly decreases evoked release of GnRH as well as GnRH mRNA levels either by decreased synthesis or by mRNA stability. These results may be important in understanding adverse effects of N2O on female workers in dental practice where the gas is not scavenged.

Methionine prevents nitrous oxide-induced teratogenicity in rat embryos grown in culture.

BACKGROUND: Nitrous oxide (N2O)-induced teratogenicity in rats is commonly believed to be due to decreased tetrahydrofolate, which results in decreased DNA synthesis. The role of decreased methionine has been largely ignored as have the sympathomimetic effects of N2O. METHODS: A rat whole-embryo culture system was used to determine whether N2O-induced teratogenicity can be prevented with supplemental methionine or folinic acid and whether N2O-induced situs inversus is mediated by alpha 1-adrenergic stimulation. Embryos were explanted on day 9 of gestation, and those at stage 10b (late primitive streak stage) were cultured with or without N2O and the various chemicals, methionine (25 micrograms.ml-1), folinic acid (5 micrograms.ml-1), phenylephrine (range 0.5-50 microM) and prazosin (10 microM). Embryos in the N2O groups were exposed to a concentration of 75% for the first 24 h of culture. After 50 h of culture, embryos were examined for abnormalities including situs inversus. RESULTS: Treatment with N2O alone resulted in increased incidences of malformations and growth retardation. Methionine, but not folinic acid or prazosin, almost completely prevented N2O-induced malformations and growth retardation. N2O itself did not cause situs inversus but increased the incidence of phenylephrine-induced situs inversus. This additive effect was blocked by prazosin. CONCLUSIONS: Our results indicate that decreased methionine rather than decreased tetrahydrofolate plays the major role in N2O-induced teratogenicity in rats. They also indicate that N2O stimulates the alpha 1-adrenergic pathway in the embryo and thereby increases the incidence of phenylephrine-induced situs inversus.

Plasma amino acids and tissue methionine levels in fruit bats (Rousettus aegyptiacus) with nitrous oxide-induced vitamin B12 deficiency.

The effect of methylcobalamin inactivation by the gas nitrous oxide on plasma amino acid and tissue methionine levels in fruit bats (Rousettus aegyptiacus) was examined. Animals exposed to N2O-oxygen (1:1, v/v) for 90 min daily received a fruit diet with or without methionine or betaine supplements. Exposure and diets were continued for up to 17 weeks or until neurological impairment and muscular weakness was established. All the groups exposed to N2O had significantly lower liver, brain and plasma methionine concentrations except the methionine-supplemented animals which showed significantly raised levels. Plasma homocysteine, which was absent in controls, was present in all the N2O-exposed groups. Betaine supplementation resulted in reduced accumulation of homocysteine in plasma. However, plasma and liver methionine levels were only slightly increased compared with animals on the basal diet, and brain methionine levels were the lowest of all the groups studied. These results support the hypothesis that reduced methionine synthesis is an important contributor to the development of neurological impairment in this species and suggest that dietary supplementation with the methionine precursor betaine cannot replace the loss of vitamin B12-dependent methionine synthesis.