Hearing loss in homozygous merles follows a parallel developmental breakdown. The stria vascularis, a vascular tissue layer along the outer wall of the cochlear duct, supplies the endolymph that powers auditory hair cells. That tissue depends entirely on melanocytes to function.
When the lethal white pattern robs the inner ear of pigment, the stria vascularis atrophies during the first two to three weeks of life. The hair cells in the organ of Corti die from lack of nutrients, cutting communication to the auditory nerve. This congenital sensorineural deafness cannot be repaired surgically or medically. Brainstem Auditory Evoked Response (BAER) testing can confirm whether a double dapple puppy has unilateral or bilateral hearing loss by five weeks of age. Breeders rarely perform these tests before selling the litter.
