Overview

Phenylketonuria (PKU) is an autosomal recessive disorder where the processing of the amino acid phenylalanine is impaired. This results in accumulation of phenylalanine, which if untreated results in severe intellectual disability.

Dietary restriction of phenylalanine is highly effective in avoiding the neurocognitive sequalae of PKU.

[Spronsen et al, 2021]

Pathophysiology

Most cases of PKU are due to a mutation in the gene coding for phenylalanine hydroxylase; the remaining cases are due to a number of genes involved with tetrahydrobiopentin (BH4) handling.

Clinically, patients are grouped into classic, moderate or mild PKU according to the amount of phenylalanine they can tolerate per day.

Impacts on anaesthesia

Nitrous Oxide

Single case report One patient who received general anaesthesia with nitrous oxide exposure of 65 minutes has been reported to have gradual onset of spastic paresis in the weeks following surgery. This was attributed to dietary vitamin B12 deficiency exacerbated by nitrous oxide exposure.

Given the restrictive PKU diet, it would be reasonable to keep nitrous oxide exposure as low as reasonably possible.

[Lee et al, 1999]

Propofol

Single case report A 1 year old patient with PKU who received a 12mg/kg/h infusion of propofol for an MRI developed fever and metabolic acidosis, similar to the propofol infusion syndrome.

Given the theoretical inhibition of mitochondrial complex I by both propofol and hyperphenylalaninaemia, it is plausible that these patients may be susceptible to propofol infusion syndrome.

[Rayadurg et al, 2018]

Oral medications

Expert opinion Aspartame and gelatin contain significant levels of phenylalanine. Opinion from a pharmacist may be helpful when choosing a suitable formulation.

Ingested blood

Expert opinion Blood swallowed during surgery involving the oropharyngeal cavity represents a significant load of phenylalanine. It may be prudent to suction gastric contents or use a pharyngeal pack in these cases.