Complex sentence: The hearing loss of Pendred's syndrome is present from birth, and language acquisition is a significant problem if this is severe in childhood; the hearing loss is progressive and many proceed to deafness at a later age.
Split into simpler sentences: The hearing loss of Pendred's syndrome is present from birth, and language acquisition may be a significant problem if this is severe in childhood. Hearing typically worsens over the years, and many proceed to deafness at a later age.