The first is the collection and amplification of the sound, using the external ear, ear canal, eardrum and the ear bones in the middle ear.  Disease in this area of the ear will cause a "Conductive Loss" or loss of sound loudness.  Surgery will often restore the hearing to near normal levels and if the patient is unable to undergo surgery a hearing aid will greatly benefit the patient.  Even an simple auditory amplifier which costs under $50 can return the hearing to normal.  This type of hearing loss is most commonly found in children and presents as an ear infection or ear fluid.  Examples of Conductive Hearing Losses:

	Eustachian tube dysfunction - Negative pressure in the ear can cause a mild hearing loss, but can also lead to more serious conditions such as ear fluid or a cholesteatoma.  Treatment of negative pressure is by popping the ears and observation by your doctor.    Search PubMed for Cholesteatoma
	Ear fluid - Often treated with myringotomy (ear) tubes.  Search PubMed for Myringotomy Tubes
	Hole in eardrum - Treated with a tympanoplasty.   Search PubMed for Tympanoplasty
	Fixed middle ear bones - If involving the malleus or incus an ossiculoplasty can be performed.  If the stapes is fixed, a stapedectomy can be performed.   Search PubMed for Ossiculoplasy   Search PubMed for Stapedectomy

The second is the processing of the sound by the cochlea and converting the sound into nerve impulses and transmitting them to the brain via the auditory nerve.  The processing of the sound is very complex, parts of the sound are actually recreated by the inner ear and can be heard by use of a microphone.  This process is the basis for otoacoustic emission testing in children.  Hearing losses involving this process are called sensorineural.  Sensory for losses involving the cochlea and neural for losses involving the auditory nerve.  Sensorineural hearing losses are most common in the adult.  Unfortunately, there is no cure for these losses but the most common type, noise-induced hearing loss, are preventable.  If the loss is determined to be neuronal in type, a central lesion, such as a brain tumor, needs to be sought for.   Hearing aids will help patients with this type of hearing loss, but usually do not restore the hearing to normal.  Examples of sensorineural hearing loss:

	Noise-induced hearing loss   Search PubMed for Noise Induced Hearing Loss
	Presbycusis - Hearing loss associated with aging.   Search PubMed for Presbycusis
	Meniere's Disease - Hearing loss associated with increased pressure in the endolymph (fluid contained in the cochlea).    Search PubMed for Meniere's Disease
	Sometimes a hearing loss comes on quickly, without warning and for no know reason.  These hearing losses are called a " Sudden Sensorineural Hearing Loss ".  All other causes, such as an acoustic neuroma, should be excluded before this diagnosis is made.  Steroids, when given early, have been show to help some patients.  View Article  View Article.  The use of antiviral agents are controversial with some authors having shown them to be ineffective.  View Article   Physicians have also advocated the use of agents to increase blood flow, such as plasma expanders and vasodilators but these medications have not been proven to be effective.  View Article  Treatment should also include strict limits on heavy lifting and straining.
	Tumors of the auditory nerve - Most commonly a slowly growing benign tumor called an acoustic neuroma.  Search PubMed for Acoustic Neuroma