CHAPTER 37 OTITIS EXTERNA, MEDIA, AND INTERNA 547
Myringotomy: spinal needle or sterile catheter is inserted through the ventral aspect
of the pars tensa caudal to the manubrium of the malleolus to sample fluid within the
bulla for cytologic examination, culture and sensitivity testing; frequent contamina-
tion of samples from the external ear canal occurs during sampling (Figure 37.24).Histopathologic examination of tissue obtained by biopsy forceps from the external
ear canal or middle ear.
Brainstem auditory evoked responses: only way to detect bilateral or (especially) uni-
lateral hearing loss; can differentiate deficits originating in the middle ear, cochlea, or
central auditory pathways.CSF analysis: detect CNS involvement.
Culture of exudates for identification and sensitivity: may be useful with per-
sistent/nonresponsive infection; most indicated when rod bacteria found in exu-
date samples or systemic antibiotics are prescribed; concentration of antibiotics
instilled into the ear are often significantly higher than used to determine sensitivity
in vitro.Cytology
Microscopic evaluation of exudates: single most important diagnostic tool after com-
plete examination of the ear canal.
Appearance of the exudates (guidelines – not definitive):
Yeast infections: yellow-tan and adherent (especially in proximal horizontal
canal) to brown or gray and waxy in deeper canal
Bacterial infections: yellow-green, purulent (especiallyPseudomonas) to brown-black thin exudate (especially staphylococcal)
Mucoid or viscous: associated with biofilm production.Gross appearance of exudates does not allow an accurate diagnosis of the type of
infection; microscopic examination is necessary.
Samples obtained from the proximal and distal external ear canal, as well as from
the middle ear, are frequently different; cytological examination and submission of
samples from each location may be necessary for accurate assessment of otitis externa
and otitis media.Exudates and infection within each canal can differ; samples should be examined from
each canal; separate samples may need to be submitted for culture and sensitivity if
cytology results demonstrate disparate populations of organisms.Infections within the canal can change with prolonged or recurrent therapy; repeat
examination of exudates is required in chronic cases.
Response to therapy also monitored by repeat cytological examination.
Preparations: made from both canals: contents of each canal may differ; spread sam-
ples thinly on a glass microscope slide; examine both unstained and Romanowsky
(Diff-Quik®) stained samples: stain chambers should be changed regularly.Heat fixing may help with waxy debris but is not required for most samples.
Mites: presumptive diagnosis.
Type(s) of bacteria or yeast: assist in the choice of therapy as well as determine
whether culture is needed.