Clinical value of the tonal masking level difference

Abstract

The authors have studied the tonal masking level difference (MLD; re N₀S₀-N₀Sπ) in different groups of patients suffering either from hearing losses or from central nervous system disorders but with normal hearing in order to obtain a test for topographic diagnosis of disorders of the auditory pathway.

In patients with hearing losses, the MLD values were approximately normal in conductive lesions but significantly impaired in patients suffering either from Menière's disorder, from sensorineural lesions, or from acoustic neuromata.

In normally hearing patients, the tonal MLDs were abnormally small in all the cases with vestibular neuronitis and in 55% of the subjects with CNS lesions. The last group very often showed abnormal results on the ‘sensitized speech tests’ of Bocca and Calearo.

The behaviour of the MLDs in the different groups of patients shows that the binaural release from masking is affected by peripheral non-conductive hearing losses. In normally hearing patients, the MLD was reduced in bulbopontine lesions as well as in the CNS disease in which central auditory lesions were suggested by the abnormal sensitized speech tests.

The authors concluded that the tonal MLD may be used as a test for the diagnosis of central auditory lesions, only in normally hearing patients. Indeed in subjects with sensorineural hearing losses, the tonal MLD loses its diagnostic meaning, because the pathological condition of the peripheral auditory system affects the binaural release from masking.

Key Words:

• Auditory masking
• Masking level difference
• Conductive hearing loss
• Stapedial otosclerosis
• Menière's disorder
• Sensorineural hearing loss
• Acoustic neuroma
• Vestibular neuronitis
• Presbyacusis
• CNS diseases