Transcranial crossover intensity as a function of earmold length and material in transcranial CROS hearing aid fittings

The effect of earmold length and material on crossover amplification in the transcranial CROS (contralateral routing of signals) fitting technique was measured systematically. Transcranial CROS is intended specifically for individuals with an unaidable hearing loss in one ear, who maintain normal, or near-normal, hearing sensitivity in the other ear. The Transcranial CROS method involves stimulating the contralateral cochlea via bone conduction with a hearing aid worn on the unaidable ear. Over the past fifteen years, reports of this approach to fitting profound unilateral hearing losses have appeared from time to time in the literature. The purposes of the present study were: (1) To determine whether the point of amplification crossover varies as a function of earmold length (earmold termination approximately 5, 10 & 15 mm from the tympanic membrane); (2) To determine whether the point of amplification crossover varies as a function of type of material (poly flex versus hard acrylic); (3) To determine whether an EARTONE 3A insert phone can predict the point of crossover as measured by custom earmolds. It was speculated that these results would assist clinicians in identifying the optimum choice of earmold for transcranial CROS fittings.; Crossover point was measured in three subjects, each with a profound or total hearing loss in one ear and hearing loss of no worse than mild-to-moderate degree in the other ear. Crossover point for the insert phone and the six earmold types were compared. No statistically significant difference at crossover point was observed between the different earmold types for length or material. A statistically significant interaction between frequency and material was observed, however the effect was too small to be of practical significance. Crossover point measured with EARTONE 3A insert phones was highly correlated with crossover point measured with custom earmolds. Results suggest no one type of earmold has a clear advantage over others in engendering cross-hearing, and that an insert phone can predict point of crossover for custom earmolds. The practical implications of these data on transcranial BTE fittings are discussed.