Optic neuropathy in classical methylmalonic acidemia.

Background: Classical MMA, caused by methylmalonyl-CoA mutase deficiency, may result in late-onset dysfunction in several organ systems. To date, 10 cases of optic neuropathy have been reported. The prevalence of optic neuropathy in visually asymptomatic patients has not been determined. This study sought to identify overt and subclinical optic neuropathy in a cohort with classical MMA. Methods and Materials: Neuroophthalmic examinations were performed on 21 patients identified with classical MMA, older than 10years. Diagnosis of optic neuropathy was determined by a combination of visual acuity, optic nerve appearance and electrodiagnostic tests. Tabulated data were analyzed for association of variables using SAS software. Significance was set at p < .05. Results: Two-thirds were Saudi nationals and one third, Syrian. Age range was 11-29years. Eleven (52.4%) patients had optic neuropathy. Nine (81.8%) of these were bilateral, seven (57.9% to 63.6%) reported decreased vision and four (33.1% to 36.4%) were asymptomatic. Two patients had catastrophic vision loss, following acute metabolic crises. Sixteen patients had chronic renal impairment while three had renal hypertension. Seventeen patients had short stature and eight, chronic pancreatitis. Methylmalonic acid levels ranged from 82 to 3,324µmol/L (Normal<1µmol/L). There was a significant association between optic neuropathy and female gender (p = .011) and none with age, nationality, renal impairment, pancreatitis or specific genotype. Conclusion: Optic neuropathy was a frequent finding in classical MMA. It was often bilateral and some cases were sub-clinical, lacking visual symptoms. These findings have important management implications. Full ophthalmic evaluations should be performed early and regularly in patients with MMA, even when patients are asymptomatic.

Utility of motor evoked potentials for intraoperative nerve root monitoring.

There is no entirely satisfactory way to monitor nerve root integrity during spinal surgery. In particular, standard free-running electromyography carries a high false-positive rate and some false-negative rate of injury. Stimulated electromyography to direct root stimulation can only be done intermittently, and roots are often inaccessible. This article reviews to what extent muscle motor evoked potential (MEP) monitoring might help. It presents background considerations, describes MEP methodology, and summarizes relevant experimental animal and clinical studies. Based on current evidence, root compromise can cause myotomal MEP deterioration that in some cases may be reversible. However, because of radicular overlap, limited sampling, confounding factors, and response variability, the effects range from no appreciable change to variable degrees of amplitude reduction to disappearance and some false-positive and false-negative results should be expected. For root monitoring, multichannel MEP recordings should span adjacent myotomes and avoid mixed myotome derivations. Only amplitude reduction warning criteria have been studied, but no percentage cutoff consensus has emerged, and this approach is troubled by response variability. There is some evidence that MEPs might reduce false electromyographic results. In conclusion, muscle MEPs could compliment electromyography but seem unlikely to completely solve the problem of nerve root monitoring.