The data presented represent the largest series of children with the severe neonatal form of propionic acidaemia to be investigated with MRS, and is to our knowledge the first to study children during severe acute encephalopathic episodes. The selective vulnerability of the basal ganglia in PA is well established, and our data provide further insight in to processes occurring within the basal ganglia. MRI results found that basal ganglia appeared normal in children who had not had a severe acute encephalopathic episode beyond the neonatal period, as similarly reported in glutaric aciduria [9
], but were acutely damaged during episodes of acute illness. Similarly brain metabolite alterations were more marked during severe acute episodes.
Decreased tNAA is seen in many pathologies, and is thought to reflect loss of viable neurons [10
] including following acute ischemic stroke [11
]. Elevated lactate suggests a switch to anaerobic respiration, reflecting local ischaemia or mitochondrial dysfunction. As previously reported, elevated lactate was seen in two of the cases during metabolic stability, suggesting that brain tissue metabolism is not normal even when metabolism is well controlled.
The alterations seen in glutamine and glutamate in basal ganglia are of particular note. Glx was significantly decreased during severe acute episodes, with a smaller (non-significant) decrease noted in basal ganglia in studies acquired during metabolic stability. The ratio of Glx/tNAA was lower in the propionic acidaemia cohort than in cohorts of children with other defined inherited metabolic disorders including a cohort with confirmed respiratory chain disorders, while the Glx/NAA ratio was elevated in a cohort with the urea cycle disorder argininosuccinic aciduria (JED, unpublished data). This suggests that the alterations seen in Glx here were not simply a reflection of neuronal loss represented by lower tNAA.
Although the spectral signals from glutamine and glutamate overlap, there is increasing evidence that they can be differentiated in the analysis of cohorts even at 1.5Tesla using software such as LCModel. Such evidence includes Montecarlo simulations of normal brain spectra fitted with LCModel [12
], comparisons of experimental and simulated basis sets [13
], and the finding of a significant correlation between in vivo and in vitro results for both glutamate and glutamine detected in brain tumours [14
A previous MRS study reported elevated basal ganglia Glx in three metabolically stable propionic acidaemia patients who had normal ammonia and near-normal glutamine and glutamate in plasma and cerebrospinal fluid leading the authors to suggest differential mechanisms regulating brain parenchymal metabolism [6
]. However this study relied on peak area estimation and ratio to creatine, whereas the current study provides more robust quantification. Moreover the reduced glutamine detected by brain MRS is consistent with the plasma glutamine reduction previously reported in PA [16
], also seen in plasma glutamine levels in our cohort (data not shown).
The administration of the alternative pathway drugs used to control hyperammonaemia depletes plasma glutamine and glycine and may contribute to the decreased brain Glx seen. Furthermore, all children were haemofiltered during severe acute episodes which may further deplete glutamine due to its high sieving coefficient [17
]. However CVVH was instigated after the MRI/MRS in 80% of cases suggesting that glutamine levels were already low before CVVH was commenced.
The acute decrease seen in Glx supports the hypothesis that they are consumed to replenish a depleted Krebs cycle [4
], and that this decrease is a marker of compromised aerobic cellular respiration within brain tissue. It remains to be determined whether the decreased Glx is directly detrimental or simply a secondary marker of compromised metabolic status. Interestingly the decrease in Glx was greater in the basal ganglia than in white matter, correlating with the more vulnerable brain region.
There is a need for brain protective strategies during acute metabolic decompensations, with potential methods including brain cooling, adequate energy provision, and anaplerotic therapies. MRS could provide a tool to monitor the effect of dietary supplements aimed at increasing glutamine levels, specifically monitoring metabolite levels within the target-organ of interest, namely the brain.
The two children who had liver transplant have had no severe acute encephalopathic episodes beyond the neonatal period, and have normal appearing basal ganglia on MRI. MRS demonstrated normal/high tNAA reflecting the absence of major neuronal loss and correlating with their good neurocognitive status, and no reliably detected lactate. The small numbers preclude statistical significance, but Glx was slightly increased in white matter but decreased in basal ganglia compared to the comparator group. These results support the hypothesis that liver transplantation provides systemic metabolic stability by providing a hepatic pool of functional propionyl CoA carboxylase, thus preventing further acute metabolic decompensations which are associated with the risk of brain infarction.
The data presented here have yielded some interesting observations, however further prospective data collection and analysis could overcome some of the specific limitations of the present work. In particular, use of higher field strength MRI scanners (3Tesla and above) would provide more robust differentiation of the metabolites of interest, notably glutamine and glutamate. Serial data collection in specific patients could also confirm the alterations in brain metabolites seen before, during and after metabolic decompensations. Finally analysis of a larger cohort of post-liver transplant patients as well as longer follow up with serial studies will provide stronger evidence for the therapeutic value of liver transplantation in PA.