The patient’s history and physical examination usually provide sufficient information to allow the clinician to make the diagnosis of otitis externa (OE). Most persons with OE are treated empirically.
Thus, laboratory studies typically are not needed. However, Gram staining and culture of any discharge from the auditory canal may be helpful if the patient is immunocompromised, if the usual treatment measures are ineffective, or if a fungal cause is suspected. However, as many as 40% of all cases of OE do not produce a dominant pathogen. Adults with OE may benefit from a blood glucose check or a urine dipstick test to evaluate for occult diabetes.
Histologic examination of the skin of the external canal shows acute inflammation with exudate.
CT, MRI, Bone Scan, and Gallium Scan
Imaging studies are not required for most cases of OE. However, radiologic investigation may be helpful if an invasive infection such as necrotizing (malignant) OE is suspected or if the diagnosis of mastoiditis is being considered.
High-resolution computed tomography (CT) is preferred and better depicts bony erosion.  Radionucleotide bone scanning and gallium scanning have been used to make the diagnosis. Magnetic resonance imaging (MRI), though not used as often, may be considered secondarily or if soft tissue extension is the predominant concern. 
In cases of external ear infection, otoscopic examination must be performed in conjunction with evaluation of related structures (eg, the external ear and the head and neck). For example, the auricle should be examined for swelling, deformity, and erythema; the face, for evidence of facial nerve paresis or other cranial neuropathy; and the neck, for masses.
An otoscope consists of a head and a handle and is used to examine the external auditory canal (EAC), the tympanic membrane, and the middle ear. A magnifying lens enhances the clinician’s view. The following two types of head are available for the otoscope:
Diagnostic head – This head is fixed to the otoscope and does not allow the use of microinstruments through the scope
Working (operating) head – This head has a magnifying lens that can slide to the side, enabling passage of microinstruments through the speculum into the EAC and the middle ear
A pneumatic attachment on the diagnostic head allows assessment of tympanic membrane motion by generating positive pressure in the EAC, causing the tympanic membrane to deflect medially. When pressure is released, the tympanic membrane expands laterally. This technique is an important tool in the diagnosis of middle ear effusions, vascular lesions, and inner ear fistulas.
For optimal viewing of the tympanic membrane in an adult, retract the auricle posteriorly and superiorly to straighten the EAC; for optimal viewing in a child, pull the auricle posteriorly. Remove any debris or cerumen to allow an adequate examination. Proceed with the examination as follows:
First, examine the EAC for masses, skin changes, and otorrhea
Next, examine all parts of the tympanic membrane (eg, pars tensa and pars flaccida)
Next, assess the motion of the tympanic membrane by means of pneumatic otoscopy
Finally, attempt a thorough examination of the middle ear contents through the tympanic membrane, though this examination may be limited by the opacity of the membrane itself
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