Hearing loss occurs when there is loss of sound sensitivity produced by an abnormality anywhere in the auditory system. A wide variety of conditions can cause hearing loss, including ear drum perforations, cholesteatoma etc.  

Hearing loss can be broadly classified as Conductive, Sensorineural of Mixed (both). 

Conductive Hearing Loss: Conductive hearing loss occurs when sound waves are prevented from passing from the air to the fluid-filled inner ear. This may be caused by a variety of problems including buildup of earwax (cerumen), infection, fluid in the middle ear, a perforated eardrum, or fixation of the ossicles, as in otosclerosis . Other causes include scarring, narrowing of the ear canal, tumors in the middle ear etc. Once the cause is found and removed or treated, hearing usually is restored.  

Sensorineural Hearing Loss: Sensorineural hearing loss develops when the auditory nerve or hair cells in the inner ear are damaged. The source may be located in the inner ear, the nerve from the inner ear to the brain, or in the brain. Sensorineural hearing loss, commonly referred to as "nerve deafness," frequently occurs as a result of the aging process in the form of presbycusis, which is a gradual loss occurring in both ears. Tumors such as acoustic neuromas can lead to sensorineural hearing losses, as can viral infections, Meniere's disease and meningitis. Sensorineural hearing loss can also be the result of repeated, continuous loud noise exposure, certain toxic medications, or as an inherited condition. Generally, it is non-reversible. Scientists have, however, made great progress in uncovering the genes responsible for a number of forms of congenital hearing impairments/ deafness, and this genetic research may in time lead to therapies for some congenital causes of hearing loss.  

Sensorineural hearing loss may be further differentiated as sensory or neural. Sensory hearing loss refers to loss caused by abnormalities in the cochlea, such as by damage from noise trauma, viral infection, drug toxicity, or Meniere's disease. Neural loss stems from problems in the auditory (eighth cranial) nerve, such as tumors or neurologic disorders. While tumors in this nerve may be life threatening, they are also often curable.  

Mixed Hearing Loss is a combination of both conductive and sensorineural hearing loss.  

Hearing loss may be partial or total. It may develop gradually or suddenly. People with hearing loss may experience difficulties hearing conversations, especially if there is background noise. Hissing, roaring, or ringing in the ears (tinnitus) occurs in some conditions, as may dizziness or problems with balance (vertigo).  

Evaluating Hearing Loss
Your Ear Nose Throat Surgeon will perform a number of examinations to determine the presence, extent, location, magnitude, and qualities of any hearing loss.  

A physical exam, using an otoscope or ear microscope, will evaluate the ear canal and tympanic membrane. The nose, nasopharynx, and upper respiratory tract will also be examined as the ear and nose has close anatomical relationship and disease conditions in one often affects the other ( eg one becomes temporarily deaf during a "cold".  

Audiometry involves the presentation of pure tones each ear via headphones or through a bone conductor transducer. A range of frequencies is used, and the patient's pattern of response is analyzed.  

Acoustic Reflex testing measures the contraction of a tiny ear muscle that responds to sounds at different volumes. The loudness at which the reflex occurs, or the absence of the acoustic reflex, provides important information.  

Tympanometry measures the impedance of the middle ear to sound. It uses an airtight seal and a microphone to deliver sound into the ear canal. The amount of sound that is absorbed or reflected from the middle ear is measured at the microphone at normal, positive, and negative air pressures. Tympanometry is useful in identifying middle-ear effusions in children. It is often used as a confirmatory test when microscopic examination is inconclusive. 

Otoacoustic Emissions (OAE) testing uses sensitive microphones inserted into the external auditory canal. The presence of otoacoustic emissions indicates that the outer hair cells are intact, and this can help distinguish sensory from neural hearing losses. OAE are especially robust in infants, and this painless test is often performed on newborns at the hospital nursery  when required. 

Auditory Brainstem Response (ABR) ( sometimes known as Evokes Response Audiometry (ERA))  testing is a specialized test performed by an audiologist. ABR picks up waveforms emitted by various parts of the nerve pathway when the inner ear responds to high pitched wide band noise. External electrodes are placed on the scalp and different parts of the head. Absence of these waveforms or even delays between different parts of these waveforms may separately indicate sensorineural deafness or nerve tumours ( Acoustic Neuroma). The main advantage is that this is an objective test not requiring the patient's response and hence useful in medicolegal  assessment of deafness but the main drawback is that it requires the patient to be still. Children very often require sedation for this to happen. 

Other tests: patients with conductive hearing losses may require computed tomography (CT) scans of the temporal bones. Those with unilateral or asymmetric sensorineural hearing loss should have magnetic resonance imaging (MRI) of the head. Many specialized centres do Electro-Nystagmography (ENG) for assessment of vertigo.