Traber et al (1) recently reported a case of “Subacute bilateral visual loss in methylmalonic acidemia” in a 23-year-old woman. Late-onset optic nerve damage has been described only rarely in organic acidemias. We present a similar case in propionic acidemia.
A 24-year-old man diagnosed with propionic acidemia at birth had been controlled with strict observance of a low–propiogenic amino acid diet, medical treatment (enalapril, lansoprazole, tiamina, carnitine, and carvedilol), and prompt management of any acute metabolic decompensation. At the time of admission for elective vasectomy, the patient's chronic manifestations were mild developmental delay and cardiomyopathy. Surgery was performed under local anesthesia (mepivacaine and ropivacaine) with intravenous sedation. None of the drugs that have been described to cause problems in patients with propionic acidemia were used, and surgery was monitored by the patient's internist. Despite these precautions, the patient experienced transient hyperammonemia (177 mmol/L), which was promptly controlled with carglumic acid and hydrocarbonated diet with no proteic components.
After surgery, the patient complained of diminished vision, and on examination 2 months later, visual acuity was 20/160, right eye and counting fingers, left eye. He was unable to identify any of the pseudoisochromatic color plates. Ophthalmoscopic examination revealed bilateral optic disc pallor and, with optical coherence tomography, there was marked thinning bilaterally of the temporal retinal nerve fiber layer (Fig. 1).
Magnetic resonance imaging (MRI) of the brain and orbits showed moderate parietal cortical atrophy bilaterally, hyperintense lesions involving the parietal periatrial white matter and reduced calibre of the optic chiasm (Fig. 2).
Progressive and symmetric optic atrophy is a manifestation of several organic acidemias, including biotinidase deficiency (2–4), 3-methylglutaconic aciduria (Costeff syndrome) (5), methylmalonic aciduria and homocystinuria (CBL C disease) (6), and propionic acidemia. They all share a common pathophysiologic mechanism of deficiency of mitochondrial enzymes involved in the catabolism of long-chain fatty acids and amino acids, leading to chronic metabolic decompensation (7). The deficient enzyme in propionic acidemia is propionyl-CoA carboxylase, which is essential for the conversion of propionyl-CoA to methylmalonyl-CoA (8).
To our knowledge, only 4 cases of propionic acidemia and optic nerve atrophy have been previously reported. Ianchulev et al (9) reported 3 boys (ages 2, 9, and 10 years) who developed optic atrophy, and Williams et al (10) described optic atrophy with visual dysfunction in a 20-year-old woman. In the last case, concomitant nutritional deficiency (vitamins B1 and B6) may have been responsible for the optic neuropathy because her vision stabilized with appropriate supplementation.
In the case reported by Traber et al (1) MRI showed enhancement of both optic nerves. Their patient developed rapid onset of decreased vision, and the MRI was performed 2 weeks after the onset. In contrast, our patient's visual loss was more chronic, and MRI was obtained approximately 2 months after the onset of visual failure.
We believe that the late occurrence of optic neuropathy in our patient may, in part, be the result of strict control of his disease since birth. Fasting, the stress of the surgical procedure, and hyperammonemia that developed during surgery may have triggered optic nerve damage.
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