Furunculosis of the External Auditory Canal

A localised staphylococcal abscess of a hair follicle in the cartilaginous outer third of the canal, the only part of the canal that bears hair and pilosebaceous units. It is a circumscribed collection, unlike the diffuse inflammation of acute otitis externa, and this distinction dictates that drainage rather than drops is often the definitive treatment.

• Staphylococcus aureus is the causative organism in almost all cases, with methicillin resistant strains in patients with healthcare exposure, prior colonisation or recurrent disease.
• Trauma from cotton buds, fingernails, scratching and hearing aid moulds initiates follicular inoculation.
• Recurrent or multiple furuncles suggest diabetes mellitus, nasal staphylococcal carriage, immunodeficiency, iron deficiency, or chronic corticosteroid use.

Because the cartilaginous canal skin has a subcutaneous layer, swelling is localised and pointing occurs. Infection may track through the fissures of Santorini into the parotid region or spread posteriorly, producing postauricular swelling that mimics mastoiditis.

• Severe localised pain out of proportion to the size of the lesion, aggravated by jaw movement, chewing, and any pressure on the tragus or traction on the pinna.
• Pain increases over 2 to 4 days, then abruptly settles if the furuncle discharges.
• Hearing is usually normal. Loss occurs only when the swelling completely occludes the canal.
• Scant bloodstained purulent discharge after rupture. Copious discharge points to a different diagnosis.
• Fever and systemic upset are uncommon and suggest spreading cellulitis.

Signs

• A tender, erythematous, dome shaped swelling confined to the outer canal, sometimes with a visible punctum or pointing yellow head.
• The medial canal and tympanic membrane are normal when they can be seen.
• Tender preauricular, postauricular or upper cervical lymphadenopathy.
• Swelling over the mastoid region when the furuncle arises from the posterior canal wall.

Distinguishing furunculosis from acute mastoiditis

• In furunculosis, hearing is normal, there is no history of preceding middle ear disease or discharge, the tympanic membrane is normal, and the postauricular sulcus is preserved.
• In mastoiditis, conductive hearing loss is present, there is a preceding otitis media, the sulcus is obliterated, the pinna is pushed forward and downward, and the tympanic membrane is abnormal.

Clinical, based on the localised swelling in the hair bearing canal with exquisite tenderness.

• Pus swab for culture and sensitivity when discharging or after incision, particularly to identify methicillin resistant Staphylococcus aureus.
• Capillary glucose and HbA1c in all patients, and especially with recurrent lesions.
• For recurrent furunculosis: full blood count, ferritin, HIV testing, and nasal swab for staphylococcal carriage. Consider immunoglobulin studies where furunculosis occurs alongside other recurrent pyogenic infections.
• Imaging is not required unless mastoiditis, parotid abscess or spreading deep neck infection is suspected, in which case contrast enhanced computed tomography is appropriate.

Differential diagnosis

• Diffuse acute otitis externa: circumferential oedema, no discrete swelling, greater discharge.
• Acute mastoiditis: distinguished from furunculosis by its clinical features.
• Perichondritis: the whole pinna is involved with lobule sparing.
• Herpes zoster oticus: vesicular eruption, severe neuralgic pain, possible facial palsy.
• Parotid abscess: swelling anterior and inferior to the tragus with pus at the parotid duct orifice and trismus.
• Malignancy in the older patient with a non resolving canal lesion.

Analgesia, which is the most immediate need

• Paracetamol 1 g orally every 6 hours, maximum 4 g in 24 hours, reduced to 3 g in adults under 50 kg or with hepatic risk.
• Ibuprofen 400 mg orally every 8 hours with food, maximum 2.4 g in 24 hours, avoided in renal impairment and peptic ulcer disease.
• Codeine phosphate 30 to 60 mg every 6 hours or oxycodone 5 mg every 4 to 6 hours for 48 to 72 hours in severe cases.
• Local heat applied externally is helpful and encourages pointing.

Topical measures

• Ichthammol 10 percent in glycerine on a ribbon gauze wick, changed daily for 3 to 5 days. It is hygroscopic, reduces oedema and relieves pain, and remains a valuable low cost measure.
• Aluminium acetate 8 to 13 percent on a wick is an alternative astringent.
• Topical antibiotic drops are of limited value because the lesion is a closed collection, but ciprofloxacin 0.3 percent with dexamethasone 0.1 percent, 4 drops twice daily, is reasonable once drainage has occurred.
• Mupirocin 2 percent ointment applied to the canal meatus twice daily for 5 days after drainage.

Systemic antibiotics

Indicated for surrounding cellulitis, fever, multiple furuncles, immunocompromise, diabetes, or failure of local measures.

• Flucloxacillin 500 mg orally four times daily for 5 to 7 days, taken 30 to 60 minutes before food. Paediatric dose 12.5 to 25 mg/kg four times daily.
• Clindamycin 300 to 450 mg orally four times daily in penicillin allergy, with counselling about Clostridioides difficile risk.
• Clarithromycin 500 mg orally twice daily as an alternative, noting QT prolongation and CYP3A4 interactions with statins and warfarin.
• Methicillin resistant Staphylococcus aureus: doxycycline 100 mg orally twice daily, or trimethoprim with sulfamethoxazole 960 mg orally twice daily, for 7 days. Avoid doxycycline in children under 8 years and in pregnancy.
• Severe cellulitis or systemic sepsis: flucloxacillin 2 g intravenously every 6 hours, or vancomycin with target trough 15 to 20 mg/L when resistant organisms are suspected.

Incision and drainage

• Indicated once the furuncle is fluctuant or pointing, and is the definitive treatment for pain.
• Under microscopic vision with topical or infiltrative local anaesthesia, or general anaesthesia in children.
• A single stab incision with a number 11 blade over the point of maximal fluctuance, kept superficial and parallel to the canal axis to avoid injuring cartilage and perichondrium.
• Express pus, send for culture, then insert a small wick to keep the cavity open for 24 to 48 hours.
• Do not incise a non fluctuant lesion, which causes chondritis without relieving pain.

Recurrent disease

• Decolonisation: mupirocin 2 percent nasal ointment applied to both anterior nares three times daily for 5 days, plus chlorhexidine 4 percent body wash daily for 5 days, plus hot washing of bed linen and towels. Repeat after 4 weeks if recurrence continues, and consider treating household contacts.
• Optimise glycaemic control and correct iron deficiency.
• Stop all cotton bud use permanently and review hearing aid mould hygiene.

Complications

• Spreading canal cellulitis and perichondritis of the pinna, requiring systemic antipseudomonal and antistaphylococcal cover and drainage of any subperichondrial collection.
• Parotid space infection through the fissures of Santorini.
• Canal stenosis after repeated episodes with scarring.
• Rarely, septicaemia in the immunocompromised.

Review at 48 hours if not drained, and at 5 to 7 days after drainage to confirm resolution and to exclude an underlying diffuse otitis externa.