Does a very loud noise like a fire alarm affect a baby's hearing? The fire alarm went out of control and made a terrible noise for about 30 minutes. I tried to cover my baby's ears, but I worry. She seems normal and responds to my voice, but I am not 100 percent certain. Should I have her tested?

S.

"Noise-induced sensorineural hearing loss" is a potentially permanent hearing loss due to damage to the delicate sensory cells contained within the "cochlea" (the inner ear organ responsible for hearing). The degree of hearing loss depends on three factors: the intensity of the sound, the duration of the sound and certain genetic factors. (In other words, some folks are more prone to this type of hearing loss than others.)

The typical home smoke detector generates an armor-piercing noise, truly one of the most obnoxious noises known to man. Such an alarm would certainly have sufficient intensity to cause a noise-induced hearing loss. Is 30 minutes of this cacophony enough to permanently damage your baby's hearing? Maybe.

Fortunately, there are two excellent techniques for testing hearing in infants. If she is old enough to turn her head and look at a favored object (a toy, mom's face and so forth), then she can be "conditioned" to turn her head in response to sound. Here's how this works: The audiologist places headphones on your daughter's head. Your daughter may sit quietly in your lap, in a darkened room. The audiologist then simultaneously plays a tone through the headphones AND shines a light on a favorite toy. Your daughter glances over at the toy. Do this several times, and she will turn her head to look at the toy even when the light does not turn on -- she has learned to associate the tone with the light. Once she has been conditioned in this manner, the tone can be decreased in volume, step by step, in order for the audiologist to determine her "hearing threshold" (the lowest volume tone to which your daughter will respond). The audiologist would compare your daughter's responses against known "normal" values in order to determine whether she has a hearing loss.

An even more accurate way of determining hearing thresholds involves measurement of your daughter's brain waves. Her brain's electrical activity varies from one second to the next, based on how she is being stimulated by her environment. Part of this electrical activity originates in a portion of the brain known as the brainstem. By measuring this electrical activity in response to particular tones, the audiologist can precisely measure her hearing threshold. This test is known either as an ABR (auditory brainstem response) or BAER (brainstem auditory evoked response). It is painless and nearly risk-free. (I say "nearly" because ABR must be performed on a sleeping infant. If sedation is required to put her to sleep, then there is some risk associated with this sedation.)

Is all of this really necessary? For your peace of mind, I would say, "Yes." There is, of course, a small chance that her hearing was seriously damaged by this noise exposure. If the audiologist determines that she has such a hearing loss, then (depending on the degree of damage, of course) hearing aids may be recommended. Hearing aids can be worn even by very young infants. If you bear in mind the importance of hearing for speech development and socialization, you will understand why hearing aids should be provided as early as possible for infants who need them.