Nitrous Oxide

Brand Names:

Clinical Names: Nitrous Oxide

Summary

generic name: Nitrous Oxide

type of drug: Anesthetic gas.

used to treat: Commonly used as an anesthetic during dental procedures but also during different types of surgery when other types of anesthesia are contraindicated.

adverse effects: Neurologic degeneration, megaloblastic bone marrow changes and other adverse reactions have been found to be associated with nitrous oxide anesthesia, especially on a chronic or repeated basis.

overview of interactions:
• nutrient affected by drug: Vitamin B12 (Cobalamin)

• nutrient affected by drug: Folic Acid (Folate)

• substance affecting drug toxicity: Alcohol



Interactions

adverse effects: The various adverse effect due to chronic exposure to nitrous oxide may have significant implications for patients, especially those with compromised or depleted systems. However, chronic exposure to nitrous oxide among dentists, dental hygenists and other dental staff could easily pose a more obvious and potentially significant risks. After studying the bone marrow of 21 dentists Sweeney et al concluded that direct evidence existed that occupational exposure to nitrous oxide was associated with depression of vitamin B12 activity resulting in measurable changes in bone marrow secondary to impaired synthesis of deoxyribonucleic acid. Thus, the nutritional recommendations discussed below may be particularly relevant to this population of healthcare providers. Risks include neurologic degeneration and megaloblastic bone marrow changes as well as the potential for increased incidence of spontaneous abortion, infertility or birth defects among female workers.
(Ostreicher DS. NY State Dent J 1994 Mar;60(3):47-49; Sweeney B, et al. Br Med J (Clin Res Ed) 1985 Aug 31;291(6495):567-569.)

nutrients affected by drug: Vitamin B12 (Cobalamin) and Folic Acid (Folate)

• mechanism: Extended use of nitrous oxide interferes with the normal metabolic activity of vitamin B12 and folic acid. Nitrous oxide inactivates the enzyme methionine synthase by oxidation of enzyme-bound cobalamin, which is formed from the cofactor methylcobalamin during the catalytic cycle. Essentially, the B12 loses its function as a result of oxidation. The resulting inhibition of homocysteine remethylation increases the homocysteine efflux and thereby the level of extracellular homocysteine, in both patients and cultured cells. The adverse effect upon folate levels increases risk of birth defects among pregnant women repeatedly exposed to nitrous oxide.

• research: In both human case reports and animal studies researchers have found that nitrous oxide causes megaloblastosis by interfering with vitamin B12 metabolism. Adverse changes in bone marrow have been found in cases where nitrous oxide is administered for periods over four hours long. As mentioned above, Sweeney et al. found measurable changes in bone marrow in their study of 21 dentists. Further studies by Koblin et al, and others, indicate that individuals with an underlying B12 deficiency are particularly susceptible.
(Amess JA, et al. Lancet. 1978 Aug 12;2(8085):339-342; Deacon R, et al. Lancet. 1978 Nov 11;2(8098):1023-1024; Ermens AA, et al. Clin Pharmacol Ther 1991;49:385-393; Flippo TS, Holder WD Jr. Arch Surg 1993;128:1391-1395; Sweeney B, et al. Br Med J (Clin Res Ed) 1985 Aug 31;291(6495):567-569; Koblin DD, et al. Anesth Analg 1990;71:610-617.)

Nitrous oxide abuse has been found to cause myeloneuropathy, involving symmetric abnormal signal in the posterior columns of the cervical spinal cord. Such myeloneuropathy is caused by inactivation of vitamin B12 by nitrous oxide. This syndrome can also be seen in patients with borderline vitamin B12 deficiency who have recently been anesthetized with nitrous oxide.
(Pema PJ, et al. AJNR Am J Neuroradiol 1998 May;19(5):894-896.)

• nutritional support: Individuals preparing for surgical procedures in which nitrous oxide will be used could benefit from supplementation with vitamin B12 and folic acid. A dosage of vitamin B12 (100 mcg per day) and folic acid (1000 mcg per day) for one week before and one week after prolonged exposure to nitrous oxide will provide effective protection with no risk. Such supplementation is not usually indicated with short term use of nitrous oxide, under two hours duration, during routine dental procedures, unless there is a current vitamin B12 deficiency. Dentists and others with chronic exposure to nitrous oxide could potentially benefit from similar levels of nutritional support; this could be especially important for women of child-bearing age working in such settings.

substance affecting drug toxicity: Alcohol

• research: The chronic administration of ethanol or brief exposure to nitrous oxide (N2O) decreases the activity of hepatic methionine synthase and disrupts normal metabolic processes that require folate and vitamin B12. This combination of these drug actions has clinical relevance since alcoholic patients often require surgery and receive N2O as a component of their anesthetic.
(Koblin DD, Everman BW. Alcohol Clin Exp Res 1991 Jun;15(3):543-548.)

• nutritional support: In general individuals with patterns of high consumption of alcohol benefit from supplementation with vitamin B12 due to the substance's action of depleting the nutrient. This need is amplified when nitrous oxide administration compounds this adverse effect upon vitamin B12 status. Similar concern might be raised with regard to dentists or other dental office personnel with a history of high alcohol consumption who may be chronically exposed to low levels of exposure to nitrous oxide.

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References

Amess JA, Burman JF, Rees GM, Nancekievill DG, Mollin DL. Megaloblastic haemopoiesis in patients receiving nitrous oxide. Lancet. 1978 Aug 12;2(8085):339-342.
Abstract: In a prospective study the incidence of megaloblastic change after ventilation with nitrous oxide for periods of up to 24 h has been determined and the cause of the altered D.N.A. synthesis studied with the deoxyuridine (dU) suppression test in 22 patients undergoing cardiac bypass surgery. 8 patients who received nitrous oxide and oxygen for 24 h had megaloblastic bone-marrow aspirates and abnormal dU suppression tests at the end of ventilation. 5 patients who received no nitrous oxide had normoblastic aspirates and normal dU suppression test. Of the remaining 9 patients, who received nitrous oxide during the operation only, 3 had abnormal dU suppression tests at 24 h. The abnormality revealed by the dU suppression tests was identical with that found in vitamin-B12 deficiency, but the patients' serum-B12 concentrations were normal. These results suggest that nitrous oxide interferes with the function of vitamin B12. Nitrous oxide oxidises vitamin B12 in vitro, and probably also in vivo when premixed 50% nitrous oxide and 50% oxygen mixture ('Entonox') is given.

Amos RJ, Amess JAL, Hinds CJ, Mollin DL. Incidence and pathogenesis of acute megaloblastic bone-marrow change in patients receiving intensive care. Lancet 1982 Oct 16;2(8303):835-839.
Abstract: The incidence and pathogenesis of acute megaloblastic bone-marrow change and of abnormalities in DNA synthesis, as assessed with the deoxyuridine(dU) suppression test, have been investigated in a prospective study of 70 seriously ill patients admitted to an intensive-care unit. On admission megaloblastic bone-marrow change was present in 22 patients, 18 of whom had been anaesthetised with nitrous oxide for 2-6 h during surgical procedures before admission. 16 of these 18 patients died, compared with 7 of 22 patients in whom haemopoiesis remained normoblastic despite receiving equivalent amounts of nitrous oxide. An abnormal dU-suppression test developed only in patients who had received nitrous oxide; on admission an abnormal dU-suppression test was found in 39 of the 42 patients tested who had been exposed to the anaesthetic. The abnormality produced in the dU-suppression test by nitrous oxide in patients admitted to the intensive-care unit was more severe and recovery was slower than the abnormality seen in patients undergoing cardiac-bypass surgery. During the recovery period from the effects of nitrous oxide the pattern of correction of the dU-suppression test changed from that of vitamin-B12 deficiency to folate deficiency.

Amos RJ, Amess JA, Hinds CJ, Mollin DL. Investigations into the effect of nitrous oxide anesthesia on folate metabolism in patient receiving intensive care. Chemioterapia. 1985 Oct;4(5):393-399.
Abstract: Anaesthesia with nitrous oxide inactivates vitamin B12 and impairs DNA synthesis in bone marrow cells. The secondary development of folate deficiency may prolong the recovery from these abnormalities in some seriously ill patients. This was investigated, using the deoxyuridine (dU) suppression test, in 48 patients admitted to an Intensive Care Unit (ICU). On admission to the ICU, following nitrous oxide anaesthesia, the pattern of correction of the abnormal dU-suppression tests was typical of that seen in vitamin B12 deficiency; 3 days later the pattern had changed to that usually seen in folate deficiency. During the same period the serum folate levels fell to subnormal values. Further evidence for the development of folate deficiency was provided by treatment with physiological amounts of folic acid which accelerated the recovery of the bone marrow abnormalities present after nitrous oxide anaesthesia. Urinary folate excretion was measured by radioassay, and increased following anaesthesia with nitrous oxide, however, this was insufficient by itself to explain the development of folate deficiency.

Amos RJ, Amess JA, Nancekievill DG, Rees GM. Prevention of nitrous oxide-induced megaloblastic changes in bone marrow using folinic acid. Br J Anaesth. 1984 Feb;56(2):103-107.
Abstract: Prolonged anaesthesia with nitrous oxide inactivates vitamin B12 and impairs DNA synthesis in bone marrow cells. The use of parenteral folinic acid in the prevention of these toxic effects has been studied in 11 patients, ventilated artificially with nitrous oxide in oxygen for 24 h. Bone marrow aspirates were performed before and after exposure to nitrous oxide. They were assessed morphologically and with the deoxyuridine suppression test. Folinic acid 30 mg immediately before anaesthesia and 30 mg 12 h later, prevented the toxic effects of nitrous oxide in four out of five patients, whereas smaller amounts of folinic acid (between 3 and 36 mg in 24 h), were ineffective.

Armstrong P, Rae PW, Gray WM, Spence AA. Nitrous oxide and formiminoglutamic acid: excretion in surgical patients and anaesthetists. Br J Anaesth 1991 Feb;66(2):163-169 .
Abstract: We have investigated the possible toxicity of nitrous oxide on vitamin B12 and its sequelae upon folic acid metabolism using the urine formiminoglutamic acid excretion test, an index of the functional state of folate metabolism. Ten control subjects not exposed to nitrous oxide and five patients receiving limb surgery under local anaesthesia excreted normal amounts of formiminoglutamic acid in urine for 6 days. Fifty patients received nitrous oxide anaesthesia for similar surgery and, of these, 22 had a dose-dependent increase in excretion on the first 2 days after operation. There were large individual variations. Exposure to 70% nitrous oxide appeared to cause abnormal metabolism of folate when exposure was greater than 90 min. Ten anaesthetists demonstrated normal excretion of formiminoglutamic acid; their exposure to nitrous oxide was typical of that in other studies of theatre environmental pollution.

Burman JF, Amess JA, Mollin DL. Nitrous oxide and vitamin B12. Lancet. 1978 Nov 25;2(8100):1153-1154. (Letter)

Christensen B, Ueland PM. Methionine synthase inactivation by nitrous oxide during methionine loading of normal human fibroblasts. Homocysteine remethylation as determinant of enzyme inactivation and homocysteine export. J Pharmacol Exp Ther 1993 Dec;267(3):1298-1303.
Abstract: Nitrous oxide inactivates the enzyme methionine synthase by oxidation of enzyme bound cobalamin, which is formed from the cofactor methylcobalamin during the catalytic cycle. The resulting inhibition of homocysteine remethylation increases the homocysteine efflux and thereby the level of extracellular homocysteine, both in patients and cultured cells. In the present work we measured the kinetics of enzyme inactivation and homocysteine export rate in two human fibroblast cell lines exposed to nitrous oxide and cultured in the presence of low to supraphysiological concentrations (15-100 microM) of methionine. Both the rate and extent of methionine synthase inactivation were reduced by increasing methionine concentration in the culture medium. In cells not exposed to nitrous oxide, methionine increased the homocysteine export rate in a dose-dependent manner. Nitrous oxide increased the export at low methionine concentrations, so that for treated cells the export was high and essentially independent of the extracellular methionine level. Neither methionine nor nitrous oxide significantly affected the amount of S-adenosylmethionine or folate in these cells. These data agree with methionine synthase as a low Km and methionine conserving enzyme, highlight the importance of methionine synthase activity as a determinant of homocysteine export and point to the possibility of protecting the enzyme by reducing catalytic turnover through product inhibition.

Deacon R, Lumb M, Perry J, Chanarin I, Minty B, Halsey MJ, Nunn JF. Selective inactivation of Vitamin B12 in rats by nitrous oxide. Lancet. 1978 Nov 11;2(8098):1023-1024.

Deacon R, Lumb MJ, Perry J. Vitamin B12, folate and nitrous oxide. Med Lab Sci. 1982 Apr;39(2):171-178. (Review)

Ermens AA, Schoester M, Lindemans J, Abels J. Effect of nitrous oxide and methotrexate on folate coenzyme pools of blast cells from leukemia patients. Leuk Res 1991;15(2-3):165-171.
Abstract: The effects of methotrexate (inhibiting dihydrofolate reductase) and nitrous oxide (inactivating methionine synthase) on intracellular folate coenzyme levels of leukemic cells were studied. Blast cells from 10 cases of acute myeloid leukemia (AML) and 5 cases of acute lymphoid leukemia (ALL) were incubated with 5 x 10(-8) M [3H] 5-formyltetrahydrofolate (5-formylTHF) for 18 h to label intracellular folate pools, which were subsequently quantitated by high performance liquid chromatography (HPLC). In AML, 5-methylTHF made up 53% of the total folate pool followed by 10-formylTHF (26%), 5-formylTHF (10%), THF (9%) and DHF (1%). Cells from ALL differed from AML (p less than 0.05) with respect to 10-formylTHF (17%) and DHF (10%). Exposure to nitrous oxide (8 h) caused an equal decrease of 10-formylTHF and 5-formylTHF in both AML (30%) and ALL (45%), whereas 5-methylTHF increased (130%). Methotrexate (4 h, 10(-6) M) caused an accumulation of DHF and a decrease of 5-methylTHF in both AML (32%) and ALL (12%). A specific reduction of the 10-formylTHF (50%) and 5-formylTHF (25%) pools was noticed in ALL. Exposure to nitrous oxide prior to methotrexate treatment aggravated the reduction of 10-formylTHF and 5-formylTHF presumably by impaired replenishment from the 5-methylTHF pool. In conclusion, this study demonstrates a significant difference in folate coenzyme distribution between cells from AML and ALL. Moreover it is shown that nitrous oxide and methotrexate treatment of leukemic cells cause an accumulation of 5-methylTHF and DHF respectively at the expense of other folate forms. The presence of substantial amounts of DHF in cells from ALL together with the specific reduction of 10-formylTHF (necessary for purine synthesis) during MTX treatment may in part explain the efficacy of methotrexate in the treatment of ALL.

Ermens AA, Refsum H, Rupreht J, Spijkers LJ, Guttormsen AB, Lindemans J, Ueland PM, Abels J. Monitoring cobalamin inactivation during nitrous oxide anesthesia by determination of homocysteine and folate plasma and urine. Clin Pharmacol Ther. 1991 Apr;49(4):385-393.
Abstract: The effects of nitrous oxide-induced cobalamin inactivation on homocysteine and folate metabolism have been investigated. Plasma levels of cobalamin, folate, homocysteine, and methionine were determined in 40 patients before and after operation under nitrous oxide anesthesia (range of exposure time, 70 to 720 minutes). Twelve patients anesthetized with total intravenous anesthesia served as control subjects (range of exposure time, 115 to 600 minutes). Postoperative plasma levels of folate and homocysteine increased (p less than 0.001) up to 220% and 310%, respectively, in nitrous oxide-exposed patients, whereas plasma levels of methionine decreased (p less than 0.025). Response occurred after 75 minutes of nitrous oxide exposure. The percentage increase of plasma folate and homocysteine correlated significantly with exposure time (p less than 0.025 and p less than 0.0001, respectively). In eight patients receiving nitrous oxide anesthesia plasma homocysteine levels had not returned to preoperative levels within 1 week (p less than 0.01). Urinary excretion of folate and homocysteine increased during and after nitrous oxide exposure (p less than 0.01 and p less than 0.002, respectively) and correlated with exposure time (p less than 0.01 and p less than 0.005, respectively). It can be concluded that disturbance of homocysteine and folate metabolism by nitrous oxide develops with little delay and return to normal levels requires several days. Elevation of plasma homocysteine levels may therefore be used for monitoring nitrous oxide-induced cobalamin inactivation.

Everman BW, Koblin DD. Aging, chronic administration of ethanol, and acute exposure to nitrous oxide: effects on vitamin B12 and folate status in rats. Mech Ageing Dev. 1992 Mar 1;62(3):229-243.
Abstract: Elderly patients with alcoholism often require surgery and receive nitrous oxide (N2O) as a component of their anesthetic. Since aging, ethanol, and N2O may all perturb folate and/or vitamin B12 metabolism, we examined the combined influence of these parameters on vitamin B12/folate status in a rodent model. Aged male Fischer 344 rats (24 months old) were given a liquid ethanol diet (35% of calories as ethanol) and control rats were pair-fed a liquid diet with carbohydrate substituting for the caloric content of ethanol. After receiving liquid diets for 7 weeks, rats were exposed to 60% N2O/40% 0(2) for 6 h. Urinary excretion of formic acid, formiminoglutamic acid (FIGLU), and methylmalonic acid (MMA) were used as indirect markers of folate/vitamin B12 status. In both the aged ethanol-fed and control groups, excretion of formic acid and FIGLU markedly increased the first day after N2O exposure and returned towards background values by the second day. No changes occurred in MMA excretion. Exposure to N2O decreased methionine synthase activities in liver, kidney and brain, and recovery of methionine synthase activities occurred over a period of 4 days in both the aged ethanol-fed and control groups. Ethanol treatment for 7 weeks combined with acute exposure to N2O did not deplete the aged rats of folate or vitamin B12 in blood, liver, kidney or brain. Thus, in this animal model, aging, chronic ethanol administration, and acute N2O exposure did not act synergistically to produce prolonged and severe disturbances in folate and vitamin B12 metabolism.

Fiskerstrand T, Ueland PM, Refsum H. Folate depletion induced by methotrexate affects methionine synthase activity and its susceptibility to inactivation by nitrous oxide. J Pharmacol Exp Ther 1997 Sep;282(3):1305-1311.
Abstract: We compared the effects of methotrexate (MTX) and nitrous oxide on the methionine (Met) synthase system in two variants of a human glioma cell line. The cells were protected from cytotoxic effect of MTX by adding thymidine and hypoxanthine to the cell culture medium. MTX (0-1 microM) was associated with a dose- and time-dependent reduction in 5-methyltetrahydrofolate (5-methyl-THF) in both cell lines. Already after 3 hr of exposure, 5-methyl-THF was reduced by 50% and after additional 48 hr, the level was undetectable. In addition to reduction in folate level, homocysteine (Hcy) remethylation in intact cells was markedly inhibited as judged by an increased export of Hcy from the cells, and Met synthase activity in cell extracts and level of cellular methylcobalamin (CH3Cbl) declined. MTX reduced Hcy remethylation and CH3Cbl level more efficiently than nitrous oxide. In both cell variants, the inactivation of Met synthase by nitrous oxide was almost completely prevented in cells pre-exposed to MTX. This indicates that there is no catalytic turnover in cells exposed to MTX, and emphasizes the importance of the sequence of administration for synergistic effect of this drug combination. In conclusion, our data show that MTX through depletion of 5-methyl-THF reduces both the Met synthase activity and the cellular CH3Cbl level. Moreover, the effect of MTX on the Hcy remethylation is more pronounced than the inhibition caused by nitrous oxide. These observations should be taken into account in studies on MTX pharmacodynamics.

Flippo TS, Holder WD Jr. Neurologic degeneration associated with nitrous oxide anesthesia in patients with vitamin B12 deficiency. Arch Surg. 1993 Dec;128(12):1391-1395. (Review)
Abstract: Vitamin B12 (cyanocobalamin) is an integral component of two biochemical reactions in man: the conversion of L-methylmalonyl coenzyme A into succinyl coenzyme A and the formation of methionine by methylation of homocysteine. The transmethylation reaction is essential to DNA synthesis and to the maintenance of the myelin sheath by the methylation of myelin basic protein. Active vitamin B12 contains cobalt in its reduced form (Co+). Nitrous oxide produces irreversible oxidation to the Co++ and Co forms that renders vitamin B12 inactive. Five cases (four from the literature and one new case) are presented in which patients unsuspected of having vitamin B12 deficiency developed subacute combined degeneration of the spinal cord following nitrous oxide anesthesia. Patients with vitamin B12 deficiency are exceedingly sensitive to neurologic deterioration following nitrous oxide anesthesia. If unrecognized, the neurologic deterioration becomes irreversible and may result in death.

Gillman MA. Folinic acid prevents megaloblastic changes associated with nitrous
oxide. Anesth Analg 1988 Oct;67(10):1018-1019 . (Letter)

Koblin DD, Everman BW. Vitamin B12 and folate status in rats after chronic administration of ethanol and acute exposure to nitrous oxide. Alcohol Clin Exp Res. 1991 Jun;15(3):543-548.
Abstract: The chronic administration of ethanol or brief exposure to nitrous oxide (N2O) decreases the activity of hepatic methionine synthase and disrupts normal metabolic processes that require folate and vitamin B12. This combination of drugs has clinical relevance since alcoholic patients often require surgery and receive N2O as a component of their anesthetic. To assess this clinical problem using a rodent model, rats were given a liquid ethanol diet (35% of calories as ethanol) and control rats were pair-fed a liquid diet with carbohydrate substituting for the caloric content of ethanol. After receiving liquid diets for 6 weeks, rats were exposed to 60% N2O/40% O2 for 6 hr. Urinary excretions of formic acid and formiminoglutamic acid (FIGLU) were used as indirect markers of folate status. In both the ethanol-fed and control groups, excretion of formic acid and FIGLU markedly increased the first day after N2O and returned towards background values by the second day after N2O exposure. Ethanol treatment alone decreased methionine synthase activities in liver, but not kidney or brain. Exposure to N2O further decreased methionine synthase activities, and recovery of methionine synthase activity after N2O occurred over a period of 4 days at the same rate in both the ethanol-fed and control groups. Ethanol treatment for 6 weeks combined with acute exposure to N2O did not deplete the rats of vitamin B12 in blood, liver, kidney, or brain. We conclude that in this animal model, chronic treatment with ethanol does not markedly exacerbate the disturbances in folate/vitamin B12 metabolism caused by brief exposure to N2O.

Koblin DD, Tomerson BW, Waldman FM, Lampe GH, Wauk LZ, Eger EI 2d. Effect of nitrous oxide on folate and vitamin B12 metabolism in patients. Anesth Analg. 1990 Dec;71(6):610-617.
Abstract: Exposure to nitrous oxide (N2O) markedly enhances excretion of formic acid and formiminoglutamic acid (FIGLU) in the urine of rats, suggesting a disruption in the normal pathways of folic acid metabolism secondary to an N2O-induced inactivation of methionine synthase. We tested whether surgical patients (23 having total hip replacements and 26 having resection of acoustic neuromas) exposed to isoflurane alone or combined with N2O responded similarly. We found no increase in urinary formic acid and FIGLU in patients exposed to N2O for hip replacement, but a small, transient increase in the FIGLU-to-creatinine ratio in those undergoing resection of acoustic neuromas (mean duration of anesthesia = 9.3 h). This increase peaked at the end of anesthetic exposure and returned toward control levels by the first day after anesthesia and surgery. Low preoperative levels of red blood cell folate and low-normal levels of serum vitamin B12 did not predict an increase in formic acid or FIGLU in response to N2O. Although an occasional patient may prove highly susceptible to and develop signs of severe vitamin B12 and folic acid deficiency after exposure to N2O, our findings suggest that this is a rare event.

Lumb M, Perry J, Deacon R, Chanarin I. Changes in tissue folates accompanying nitrous oxide-induced inactivation of vitamin B12 in the rat. Am J Clin Nutr. 1981 Nov;34(11):2412-2417.
Abstract: The anesthetic gas, nitrous oxide, oxidizes cob(I)alamin and thus inactivates methionine synthetase which requires cobalamin as a coenzyme. The effect on folates in liver, kidney, marrow, plasma, and brain in rats breathing a 1/1 nitrous oxide/oxygen mixture is described. There is loss of folate from tissues, most marked in liver, that affects folate polyglutamates to a greater extent than folate monoglutamates. Both methyl- and nonmethyl-analogues are affected. There is a transient rise in the levels of 5-methyltetrahydropteroylpolyglutamate in all tissues 8 h after starting nitrous oxide, which falls thereafter. In marrow and brain there is also a transient rise in methyltetrahydropteroylmonoglutamate. Plasma folate increased markedly throughout the period of exposure to nitrous oxide. It is suggested that these changes are due to the action of nitrous oxide in depressing tissue uptake of folate from plasma, in promoting loss of folate into urine and in inhibiting folate polyglutamate synthesis.

Lumb M, Perry J, Deacon R, Chanarin I. Recovery of tissue folates after inactivation of cobalamin by nitrous oxide. The significance of dietary folate. Am J Clin Nutr. 1981 Nov;34(11):2418-2422.
Abstract: The anesthetic gas, nitrous oxide (N2O), oxidizes the cobalt moiety in the vitamin B12 molecule and in this way inactivates methionine synthetase which requires reduced cobalamin. In rats this is followed by a disappearance of folates from the tissues, this loss being most marked in the liver. Returning the animals to a normal atmosphere leads to restoration of most of the pre-N2O folate levels within 5 days. The plasma folate, which rises on exposure to N2O, falls within several hours. The restoration of tissue folates does not take place if the rats are placed on a low folate diet after withdrawal from an N2O environment. Thus the fall in tissue folate levels is due to loss from the body either by excretion or increased catabolism and not to redistribution of folate. Return of normal folate levels requires a dietary source of folate.

Lumb M, Perry J, Deacon R, Chanarin I. Urinary folate loss following inactivation of vitamin B12 by nitrous oxide in rats. Br J Haematol. 1982 Jun;51(2):235-242.
Abstract: Rats were injected with [2-14C]H4PteGlu daily for 3 d and thereafter one group left in air and a second group in an atmosphere of nitrous oxide/oxygen (1/1). Nitrous oxide inactivates cobalamin. The N2O-treated rats excreted large amounts of L. casei-active folate into the urine. The urinary folate co-chromatographed with authentic 3H-labelled 5-methyltetrahydrofolate. Both groups of animals excreted 14C-labelled breakdown products in the urine but there was no evidence of increased folate catabolism in the N2O-treated rats. It was concluded that the folate deficiency that develops in the N2O-treated rat is due to massive urinary loss of folate. This appears to be secondary to impaired cellular uptake of folate which leads to a raised plasma folate level.

Nestor PJ, Stark RJ. Vitamin B12 myeloneuropathy precipitated by nitrous oxide anaesthesia. Med J Aust 1996 Aug 5;165(3):174. (Letter)

Nilsson-Ehle H. Age-related changes in cobalamin (vitamin B12) handling. Implications for therapy. Drugs Aging 1998 Apr;12(4):277-292 . (Review)
Abstract: Cobalamin (vitamin B12) deficiency is more common in the elderly than in younger patients. This is because of the increased prevalence of cobalamin malabsorption in this age group, which is mainly caused by (autoimmune) atrophic body gastritis. Cobalamin supplementation is affordable and nontoxic, and it may prevent irreversible neurological damage if started early. Elderly individuals with cobalamin deficiency may present with neuropsychiatric or metabolic deficiencies, without frank macrocytic anaemia. An investigation of symptoms and/or signs includes the diagnosis of deficiency as well as any underlying cause. Deficiency states can still exist even when serum cobalamin levels are higher than the traditional lower reference limit. Cobalamin-responsive elevations of serum methylmalonic acid (MMA) and homocysteine are helpful laboratory tools for the diagnosis. The health-related reference ranges for homocysteine and MMA appear to vary with age and gender. Atrophic body gastritis is indirectly diagnosed by measuring serum levels of gastrin and pepsinogens, and it may cause dietary cobalamin malabsorption despite a normal traditional Schilling's test. The use of gastroscopy may also be considered to diagnose dysplasia, bacterial overgrowth and intestinal villous atrophy in healthy patients with atrophic body gastritis or concomitant iron or folic acid deficiency. Elderly patients respond to cobalamin treatment as fully as younger patients, with complete haematological recovery and complete or good partial resolution of neurological deficits. Chronic dementia responds poorly but should, nevertheless, be treated if there is a metabolic deficiency (as indicated by elevated homocysteine and/or MMA levels). Patients who are at risk from cobalamin deficiency include those with a gastrointestinal predisposition (e.g. atrophic body gastritis or previous partial gastrectomy), autoimmune disorders [type 1 (insulin-dependent) diabetes mellitus and thyroid disorders], those receiving long term therapy with gastric acid inhibitors or biguanides, and those undergoing nitrous oxide anaesthesia. To date, inadequate cobalamin intake has not proven to be a major risk factor. Intervention trials of cobalamin, folic acid and pyridoxine (vitamin B6) in unselected elderly populations are currently under way.

Nunn JF. Clinical aspects of the interaction between nitrous oxide and vitamin B12. Br J Anaesth. 1987 Jan;59(1):3-13. (Review)

Nunn JF, Chanarin I, Tanner AG, Owen ER. Megaloblastic bone marrow changes after repeated nitrous oxide anesthesia. Reversal with folic acid. Br J Anaesth 1986 Dec;58(12): 1469-1470.
Abstract: A seriously ill patient was found to have megaloblastic changes in his bone marrow after a nitrous oxide anaesthesia which lasted for 105 min. After an interval of 7 h a second nitrous oxide anaesthetic was administered, during which the patient received 30 mg of folinic acid. His marrow was normal when assessed 4 h later.

Nunn JF. Clinical aspects of the interaction between nitrous oxide and vitamin B12. Br J Anaesth. 1987 Jan;59(1):3-13. (Review)

Ostreicher DS. Vitamin B12 supplements as protection against nitrous oxide inhalation. NY State Dent J 1994 Mar;60(3):47-49.
Abstract: Chronic exposure to ambient nitrous oxide levels found in dental offices may pose unrecognized health risks, among them, increased incidence of spontaneous abortion or infertility among female workers. Vitamin B12 supplements can reverse some of the effects of chronic N2O exposure.

Pema PJ, Horak HA, Wyatt RH. Myelopathy caused by nitrous oxide toxicity. AJNR Am J Neuroradiol 1998 May;19(5):894-896.
Abstract: We describe a case of myeloneuropathy resulting from nitrous oxide abuse. MR imaging of the spine revealed symmetric abnormal signal in the posterior columns of the cervical cord. Myeloneuropathy is caused by inactivation of vitamin B12 by nitrous oxide. This syndrome can also be seen in patients with borderline vitamin B12 deficiency who have recently been anesthetized with nitrous oxide.

Skacel PO, Hewlett AM, Lewis JD, Lumb M, Nunn JF, Chanarin I. Studies on the haemopoietic toxicity of nitrous oxide in man. Br J Haematol. 1983 Feb;53(2):189-200.
Abstract: Nitrous oxide inactivates vitamin B12 and in man can produce a megaloblastic anaemia. Haematological and biochemical changes were studied in nine surgical patients ventilated with 70% N2O for up to 24 h and in three control patients. There was a rise in the numbers of hypersegmented neutrophils in peripheral blood following N2O. Serial bone marrow aspirates showed gross megaloblastic change after 24 h of N2O which had reverted to normoblastic but dyserythropoietic haemopoiesis by 1 week. Giant forms of early myeloid precursors were also seen after 24 h ventilation with N2O but by 1 week abnormalities were evident in more mature cells, metamyelocytes and segmented neutrophils. Megaloblastosis was associated with abnormal dU suppression which showed a correction pattern similar to that seen in vitamin B12 deficiency. Administration of N2O was also associated with a progressive rise in serum folate and fall in serum methionine levels. No similar patterns were seen in the three control patients.

Sweeney B, Bingham RM, Amos RJ, Petty AC, Cole PV. Toxicity of bone marrow in dentists exposed to nitrous oxide. Br Med J (Clin Res Ed) 1985 Aug 31;291(6495):567-569.
Abstract: The morphology of the bone marrow of 21 dentists who habitually used nitrous oxide in their surgeries was investigated. Exposure to nitrous oxide was measured with an atmospheric sampling device, and each dentist was invited to fill in a questionnaire giving details of medical history, diet, and intake of alcohol. During the trial a full neurological and haematological investigation was carried out and a bone marrow aspirate was examined both morphologically and by the deoxyuridine suppression test. Mean exposures to nitrous oxide ranged from 159 to 4600 parts per million. In all subjects serum vitamin B12 and folate concentrations were within normal limits. Abnormal results of deoxyuridine suppression tests were obtained in three of the 20 dentists tested; two of these three had abnormal white cells in their peripheral blood films. This study provides direct evidence that occupational exposure to nitrous oxide may cause depression of vitamin B12 activity resulting in measurable changes in bone marrow secondary to impaired synthesis of deoxyribonucleic acid.

Takacs J. [N2O-induced acute funicular myelosis in latent vitamin B 12 deficiency]. Anasthesiol Intensivmed Notfallmed Schmerzther 1996 Oct;31(8):525-528. [Article in German]
Abstract: The neurotoxicity of nitrous oxide ("laughing gas") had already been observed in 1956 when using N2O in the long-term sedation of tetanus patients. In 1967 Parbrook described leukopenic effects during long-term exposure to N2O. It was only in 1978 that further studies were conducted on myeloneuropathies and myelodepression under the influence of N2O. The basic cause is vitamin B12 deficiency and the irreversible oxidation of coenzyme B12 by N2O. Between 1986 and 1995 eight cases of acute funicular myelosis associated with latent vitamin B12 deficiency subsequent to nitrous oxide anaesthesia were reported. In our hospital, two further patients now have this disease. Two observations must be emphasised when assessing the 10 patients mentioned above: 1. There was no long-term exposure to N2O (> 6 hrs); the periods of anaesthesia were between 1.5 and 3.5 hrs. 2. Vitamin B12 deficiency was not known preoperatively, and there was no marked pernicious anaemia, so that the only pointers to the risk patients were supplied by the mean corpuscular haemoglobin (MCH) and the mean corpuscular volume (MCV) of the blood picture.

van Achterbergh SM, Vorster BJ, Heyns AD. The effect of sepsis and short-term exposure to nitrous oxide on the bone marrow and the metabolism of vitamin B12 and folate. S Afr Med J 1990 Sep 1;78(5):260-263.
Abstract: It is recognised that prolonged anaesthesia with nitrous oxide (N2O) induces megaloblastic anaemia by oxidising vitamin B12. To determine whether sepsis aggravates the effect of N2O on haemopoiesis 5 patients with severe sepsis, who required surgery and were exposed to short-term (45-105 minutes) N2O anaesthesia, were studied. None had evidence of pre-operative vitamin B12 or folate deficiency. The effect of the combination of N2O anaesthesia and sepsis on DNA synthesis in bone marrow cells was assessed morphologically, and by the deoxyuridine suppression test. In 3 patients exposed to the longest duration (75-105 minutes) of N2O, addition of folinic acid and vitamin B12 partially improved the utilisation of deoxyuridine in vitro. No patient had evidence of megaloblastic haemopoiesis as judged by bone marrow morphology. It is concluded that prolonged N2O anaesthesia in patients with severe sepsis may adversely affect DNA synthesis. Although this effect did not manifest as overt megaloblastic erythropoiesis, it may be prudent to avoid N2O in such patients.