This past month I have suddenly had two teenagers present for surgery claiming to have 5,10 methylenetetrahydrolate reductase deficiency. Ironically, I had just reviewed two articles on this exact issue in my past journals of anesthesiology. This was nice, because it allowed me to offer a degree of comfort to the patients by appearing to be informed of the disease and potential pitfalls. What made me particularly interested in doing a write up about this particular genetic polymorphism was that the mother of the second patient gave me a paper claiming that these patients should avoid propofol, epinephrine, lactated ringers, and N20. (here is the link) The mother was hyper anxious and emotionally charged in discussing the anesthetic. She appeared ready to defend her position and stated that her daughter had an "allergy" to propofol and lactated ringers. I reassured her that it would be no problem to avoid propofol and nitrous oxide for her anesthetic and the mother appeared to back down in a somewhat disappointed manner as if she was hoping for more push back from the doctor which could provide her an opportunity to demonstrate to her other family members her superior intellect. However, I was intrigued enough to spend some time looking at the anesthetic literature related to this deficiency to better understand how propofol, bupivacaine, lactated ringers or epinephrine might have been included in the list of things to avoid by this very concerned mother.
In general, the main concern related to 5,10 methylenetetrahydrofolate reductase (MTHFR) deficiency is related to increased homocysteine in the blood. Homocysteine is an amino acid that has been associated with increased cardiovascular disease. In a large cohort  of (~18,000 men and woman) in western Norway, total plasma homocysteine was associated with increased risk of cardiovascular morbidity, general mortality, and depression with neurocognitive deficits in the elderly. This cohort study, demonstrated an association, but cannot be said to prove that elevated total plasma homocysteine caused these outcomes. However, other case control studies have also found an association with elevated plasma homocysteine levels and increased vascular disease. Graham IM et al. showed in a case control study that increased plasma total homocysteine levels is an independent risk factor for vascular diseases similar to that conferred from smoking or hyperlipidemia. It also was shown to powerfully increase the already elevated risk associated with smoking and hypertension . It is estimated that plasma levels above 10 micromol/L are associated with a doubling of vascular risk and levels greater than 20 micromol/L can confer a TEN fold increased risk of vascular disease. Furthermore, acute increases have also been shown to cause endothelial dysfunction and provide procoagulant effects . Chambers et al. hypothesize that hypomethylation is the major biochemical mechanism in homocystinemia vascular disease in addition to inhibition of HDL biosynthesis in humans.
Hyperhomocysteinemia can result from a number of causes. These include vitamin deficiencies such as Vitamin B6 (pyridoxine), Vitamin B12, and folic acid. Renal insuficiency can also be a culprit. Genetic defects that are relevant to homocysteine levels and anesthesia practice include methyltetrahydrofolate reductase deficiency and cystathionine β synthase deficiency.
Cystanthionine β Synthase deficiency is a congenital disorder that is also known as homocystinuria and is associated with defects in collagen such that patients suffer from a marfanoid like body with defects in bone and issues with the eyes. They are prone to hypoglycemia, therefore, patients having surgery will require short fasting periods and supplementation during fasting with IV dextrose. Furthermore, thromboembolic complications are high and measures must be taken to reduce these complications in the perioperative period.
Acute increases in homocysteine were found to occur in patients given nitrous oxide. These acute increases of homocysteine levels were associated with cardiovascular damage in a 2000 clinical study. Badner et al  looked at patients undergoing carotid endarterectomy who were randomized to a nitrous oxide(N2O) group (more than 50%) vs. no nitrous. They found that in those receiving N2O, homocysteine levels were significantly increased from an average baseline of 12.7 μmol/L to 15.5 μmol/L in the PACU. Although, this was a very slight increase, it was statistically greater than the non N2O group. Furthermore, this resulted in patients in the N2O group experiencing more frequent episodes of ischemia in the first 48 hours post op and a longer average duration of ischemia post op. There was no increased cardiovascular morbidity noted although this wasn't an end point of the study. Importantly, they found that the univariate predictors of myocardial ischemia in these patients were N2O use (RR 1.9), homocysteine greater than 17 μmol (RR 2.0), and pre and intraop ischemia (RR 3.7). These same authors noted that the potential causes of elevated ischemic risk in the patients with elevated homocysteine can be traced back to homocyteine's effects on the vascular endothelial lining and known procoagulant effects such as increased platelet adhesivness, factor V activation, protein C inhibition and antithrombin and plasminogen activator binding. It is likely that these effects are mediated by the consumption of nitric oxide (potent vasodilator).
N2O inhibits vitamin B12 (cobalamin) by irreversibly oxidizing the cobalt atom (from +1 to +3 valence state) of cobalamin. This leads to subsequent inhibition of enzymes requiring cobalamin in its coenzyme form. Because this is an irreversible inhibition, the reduction of cobalamin lasts several days. Among the many enzymes, methionine synthase is crucial because it's located at the juncture of two pathways: homocysteine remethylation and the folate cycle. (fee fig)
fig (see reference 8.)
Therefore, when cobalamin is oxidized via N2O, homocysteine can no longer be converted into methionine and builds up in the blood. As noted in the above chart, a multitude of problems can now arise, because purines, thymidine, and RNA/DNA methylation all depend on the proper function of this pathway (see fig). In particular, S-adenosylmethionine (from methionine) is critical in the methylation of myelin sheath phospholipids resulting in decreased myelin formation. Furthermore, elevated homocysteine levels are thought to lead to increased concentrations of S-adenosyl homocysteine (SAH), a feedback inhibitor of methylation reactions. In this case, patients with severe vitamin B12 deficiency exposed to nitrous oxide are at particular risk of subacute combined degeneration of the spinal cord. Degeneration in the spinal cord occurs primarily in the posterior and lateral columns, but can in rare occasions occur in peripheral nerves and white matter in the brain. There have been a number of case reports related to this in susceptible individuals in the literature. In general, patients in the case reports are found to 1) be deficient in vitamin B12, or 2) abuse N2O. Patients present from days to weeks after an exposure to N2O with ataxia, sensory deficits that are symmetrical, with deficitis of propioception and vibration sensory discrimination (posterior columns). These patients often have a megalobastic anemia (or no anemia, but elevated MCV) which goes along with vitaminB12 defiency. In severe cases, death or permanent disability are the result. In many cases, high doses of vitamin B12 can result in a resolution.
The methylenetetrahydrofolate reductase gene (see fig above) (MTHFR) has two distinct polymorphisms that result in deficits and have a combined prevalence of 20% in the Western European population. Two prominent case reports [9,10] related these polymorphisms to catastrophic neurologic outcomes in children which have lead to further studies being conducted. In a 2008 study, , 140 healthy patients were carefully evaluated to determine how the above two polymorphisms affected homocysteine levels after N2O (66%) anesthesia. They found significantly higher homocysteine levels in patients who were homozygous for MTHFR 677T or MTHFR 1298C (5.6 increase vs. 1.8 μM)
Fig. 2. Plasma homocysteine concentrations in the different groups based on methylenetetrahydrofolate reductase ( MTHFR ) 677/1298 genotype at three different time points: preoperative, after 2 h of anesthesia, and at the end of surgery. Both homozygous groups developed significantly higher homocysteine concentrations than the other groups (*** P < 0.001). Genotype combinations ( MTHFR 677/1298): wt/wt: CC/AA; het/wt: CT/AA; wt/het: CC/AC; het/het: CT/AC; wt/hom: CC/CC; hom/wt: TT/AA
This same study was also able to show that even in patients with normal genetic (wild type) function at the MTHFR locus, prolonged (greater than four hours) exposure to N2O could substantially increase homocysteine levels. In this group of patients with prolonged exposure, there was an approximate 80% increase in homocysteine levels which is similar to the increase experienced by those with the shorter exposures but homozygous for MTHFR polymorphisms.
More recently, a study of pediatric patients was conducted who had known MTHFR deficiency. In this cohort, they found 12 patients with known MTHFR defiency (ages 3.5 months to 9 years). All twelve patients had normal homocysteine levels preoperatively. The authors found no increase in homocysteine levels in these twelve at risk patients. Four of the twelve had a TIVA with propofol and the remainder underwent sevoflurane anesthesia. Nitrous oxide was avoided in all twelve. Although this study seems to suggest that in healthy pediatric patients with MTHFR deficiency, anesthesia is safe and homocysteine levels are not increased, there are reports of morbidity from MTHFR deficiency after "safe" no nitrous anesthesia. A case report from 2007 describes a patient who underwent urgent surgery with a preoperative diagnosis of homogyzous MTHFR deficiency. The patient was apparently well managed with coumadin and folic acid for prevention of ischemic insults. In the post operative period this patient developed a coronary ischemic insult and renal artery thrombosis .
Finally, there is evidence to suggest that despite acute elevations in homocysteine with adminstration of N2O, it may not be clinically relevant. In 2013, Nagele et al. published results in Anesthesiology , showing that even in patients with with at least two cardiovascular risk factors AND being homozygous for MTHFR deficiency, there was no difference in increase in Troponin I increases for 72 hours post operatively. They did find that patients homozygous for MTHFR deficiency had an increase in post operative homocysteine as has been previously shown. There were two arms of randomization (n=250) in patients determined to be homozygeous for MTHFR deficiency. One arm received 1mg vitamin B12 and 5 mg folic acid (before and after surgery) and the other arm received a saline placebo. All patients received a balanced anesthetic with 60% nitrous oxide for procedures lasting at least two hours. The results indicated that although vitamin supplementation did lower homocysteine levels,the incidence of elevation of troponin I was not different between groups.
This study provided further evidence that N2O results in an increase in homocysteine plasma levels and that these levels can be decreased by vitamin B12 supplementation. However, the study may have diminished concerns that an acute elevation of homocysteine levels after a short interval of anesthesia (~2 hrs) will lead to myocardial damage. The authors noted that there is a growing consensus that homocysteine may be a marker, rather than a cause of atherosclerotic disease and increased cardiovascular risk. The authors also noted that in this study, N2O did not result in an increase in homocysteine to a greater degree in MTHFR homozygous patients vs wild type genotype. They concluded that this difference was related to national mandatory folate fortification of all grain products in the US which can reduce the effects of MTHFR polymorphisms. This is in contrast to the study population in an earlier study conducted in Austria, a country without mandatory folate fortification.
In my case, I was presented with documentation by the mother that she apparently obtained from the web that indicated that I needed to avoid propofol, lactated ringers, bupivacaine and epinephrine in order to provide safe anesthesia. It is clear, after consulting the literature and gaining a greater understanding of the biochemicals pathways involved in MTHFR deficiency, that propofol, lactated ringers and epinephrine would not increase risk. It seems clear after reading the report that the authors seemed to have conflated MTHFR deficiency and a general mitochondrial disease. Indeed, there are a number of different congenital mitochondrial diseases and depending on the type encountered, propfol, lactated ringers, bupivacaine and epinephrine may be a relative contraindication. Mitochondrial diseases can be broken down into two major groups of related diseases. These are defects of the respiratory chain and defects in fatty acid transfer and metabolism. Propofol may have been on the list due to its relation to propofol infusion syndrome (PRIS) which leads to mitochondrial dysfunction and lactic acidosis. In fact, propofol is unque among parenteral anesthetics in that it is known to affect mitochondrial metabolism by at least four separate mechanisms. It can uncouple oxidative phosphorylation and inhibit complexes I, II, and IV. However the strongest effect of propofol is its inhibition of transport of long-chain acylcarnitine esters via inhibition of acylcarnitine transferase (carnitine palmitoyl transferease I). However, reveiws note that even in patients with mitochondrial defects, a limited one time bolus of propofol for induction of anesthesia seem generally well tolerated. The number and manifestations of mitochondrial disease are enormous and protean. Fortunately, MTHFR is not related to the function of the mitochondria and even patients homozygous for the defective gene of this enzyme seem to tolerate anesthesia without significant complications, even when given nitrous oxide. Now, I feel I would be better equipped to have a more involved and informative conversation with the mother. This will allow to me to push to maintain the freedom to use propofol, bupivacaine, LR, and epinephrine if I feel that they would be important to use.
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Blog with interesting cases and/or problems related to anesthesia with discussion based on best evidence in the literature.