Isolated bulbar phenotype of amyotrophic lateral sclerosis

Abstract

Typical bulbar-onset ALS generally portends a poor prognosis. To determine whether a relatively isolated bulbar phenotype (IBP) may have a better prognosis, patients with bulbar onset presentations were prospectively assessed, with IBP defined by an absence of limb progression over an initial six-month period. Clinical features and neurophysiological characteristics were compared. From a cohort of 300 consecutive referrals, 32 patients with bulbar onset disease (21 females, 11 males) were identified and compared to 23 age-matched control subjects. In total, patients were followed for 54 months. Twelve patients were identified with IBP (nine female, three male) and 20 had more typical bulbar ALS (12 female, eight male). Clinically, IBP was characterized by greater female predominance and upper motor neuron bulbar involvement. Compound motor action potential amplitudes were preserved in IBP compared to bulbar ALS (IBP, 7.1 mV; bulbar ALS, 4.2 mV, p <0.05), as was the neurophysiological index (IBP, 1.2; bulbar ALS 0.5, p <0.05). Furthermore, short interval intracortical inhibition was normal in IBP and reduced in typical bulbar ALS. In conclusion, patients with IBP were typically female with prominent upper motor neuron bulbar features and had normal cortical excitability. Biomarkers of cortical excitability may prove useful for further classifying ALS.