Describe the special audiological tests.

Speech Audiometry and Site-of-lesion Tests

• Speech audiometry presents standardised word lists at varying intensities and measures the percentage correctly repeated
• The speech reception (or recognition) threshold (SRT) is the level at which 50% of spondee words are repeated correctly, and should agree within 10 dB of the pure tone average — a large discrepancy suggests a non-organic loss.

Objective Tests

• Objective tests require NO response from the patient.
• Otoacoustic emissions (OAE) are sounds generated by the outer hair cells and recorded in the ear canal — direct evidence that the cochlear amplifier is working
• They are present when outer hair cell function and the middle ear are normal, and absent with a hearing loss beyond 30 dB OR with any middle ear problem — so an effusion abolishes them and gives a false "refer".
• Their great advantages are speed, low cost and the need for no response.
• Their critical limitation is that they are present in auditory neuropathy, in which the hair cells work but the signal is not transmitted — so emissions alone would miss these children, which is why high-risk babies also have a brainstem response test
• Brainstem evoked response audiometry (BERA / ABR) records the electrical activity of the auditory pathway in the first 10 milliseconds after a click, averaged over many presentations
• Five waves are identified, generated broadly by the cochlear nerve (I), cochlear nucleus (II), superior olive (III), lateral lemniscus (IV) and inferior colliculus (V). Wave V is the most robust and is used for threshold estimation
• Its uses — estimating threshold in infants and the untestable; newborn screening, as automated ABR; detecting retrocochlear lesions by a prolonged wave I to V interval or an interaural wave V difference of more than 0.2 ms; diagnosing auditory neuropathy (absent or grossly abnormal ABR with present emissions.
• Its limitations — it is a test of synchronous neural firing rather than of hearing; it is weighted to 2 to 4 kHz and gives little low-frequency information; and its sensitivity for small schwannomas is inferior to MRI, which has replaced it as the screening test
• Auditory steady-state response (ASSR) gives frequency-specific threshold estimates and complements BERA, particularly in severe and profound loss where BERA gives no response