Microtia

Definition of Microtia

Microtia is a congenital hypoplastic deformity that can occur in isolation or in a syndrome. It presents a clinical spectrum of minor deficiencies to the complete absence of the auricle

Major research and surgical development was performed by Tanzer in the second half of the 20th century.

Multiple causes have been suggested. These include genetics, teratogens, and vascular abnormalities/ischemia¹. These embryological abnormalities are relatively rare.

• Prevalence is approximately 1-2 per 10,0000 births^2-4
• Male predominance
• Ratio of right/left/bilateral is 5:3:1⁴

Embryology of Microtia

Microtia occurs during the embryological development of the first (mandibular) and second (hyoid) pharyngeal (branchial) arches. More specifically, the six hillocks of His.

Important embryology structures to consider are:

• Ectoderm: helps differentiate mesoderm into cartilage & forms the inner ear.
• 1st pharyngeal arch: 3 anterior hillocks
• 2nd pharyngeal arch: 3 posterior hillocks.
• 1st pharyngeal cleft: forms the external auditory meatus.
• 1st pharyngeal pouch: develops into the middle ear and Eustachian tube
• Otic placode in the ectoderm invaginates to form the inner ear  

More specifically:

• Hillocks 1 to 3: tragus, helical root, and helical crus.
• Hillocks 4 to 6: antihelix, antitragus, and lobule.

The other parts of the ear (remaining helix, superior crus, scaphoid fossa) are derived from the 'free ear fold' posterior to the posterior hillocks.  

Microtia severity depends on the timing. An earlier embryological issue results in a more severe presentation. Important dates to consider are:

• Week 5: External ear begins to form.
• Week 6: anterior and posterior hillocks form.
• Week 9: ear becomes recognisable.

Clinical Features of Microtia

Microtia is a clinical spectrum of severity that can be syndromic or occur in isolation (more common)¹. It is graded in accordance with the number of vestigial ear structures present.

• Anotia: the complete absence of the external ear.
• Lobule-type: remnant ear & lobule without concha, acoustic meatus & tragus.
• Concha-type: lobule, concha, acoustic meatus, tragus & incisura tragica are present to variable degrees.
• Small concha-type: remnant lobule, small indentation representing concha.
• Atypical microtia: patients who cannot be classified above.  

This is illustrated in the diagram below.

Microtia is just one aspect of congenital ear deformities. The entire spectrum of "congenital ear deformities" (which includes microtia) was published by Tanzer.

• Type 1: Anotia
• Type 2: Complete hypoplasia (microtia)
• Type 2A: Microtia with atresia of the external auditory meatus
• Type 2B: Microtia without atresia of the external auditory meatus
• Type 3: Hypoplasia of the middle-third of the ear
• Type 4: Hypoplasia of the superior-third of the ear.
• Type 4A: Constricted ear – lop/cup
• Type 4B: Cryptotia
• Type 4C: Hypoplasia of the entire upper third of the ear
• Type 5: Prominent Ear

Secondary Features of Microtia

Hearing Loss

Hearing loss in microtia is ~15% sensorineural and ~85% conductive. There can also be mixed (sensorineural and conductive)⁷.  It is associated with:

• Ossicles: disrupted or absent
• External Auditory Canal: stenosis (mixed hearing loss and risk of cholesteatoma) or atresia (conductive hearing loss) ⁵
• Loss of tympanic cavity

Bone

The severity of external deformities correlates with the severity of temporal bone abormalities¹. Bone pathology can include:

• Mandibular hypoplasia (commonly the condyle)
• Temporal bone
• Vertebra.
• Limb reduction defects

Other

• Head and Neck: Facial nerve pathology, Cleft lip and palate
• Cardiac Defects
• Eye: Anophthalmia and microphthalmia
• Renal: syndromic ear anomalies are associated with brachio-oto-renal syndrome and Townes-Brocks syndrome. Ultrasound of the kidneys should be considered.

Syndromes

It is suggested to be the mildest form of hemifacial microsomia^2-3. It is also linked to other syndromes such as:

• Treacher Collins Syndrome
• Craniofacial (Hemifacial) Microsomia
• Goldenhar Syndrome

Investigations for Microtia

Radiology

• CT: show temporal bone & middle ear, ossicles, cholesteatoma (squamous epithelium trapped in the middle ear)
• MRI: show facial nerve (often displaced in middle ear malformations^6,7)

Audiology

In terms of audiometric testing, this will determine conductive vs sensorineural hearing loss.

Management of Microtia

The management of microtia should focus on the patient's goals, the timing of intervention, and surgical options in a multi-disciplinary team. Reconstruction is usually around the age of 7 - at this stage, the patient's ear size is suitable, donor site cartilage is suitable and the patient can often help make an informed decision.  

The primary goal of microtia management is to improve acoustic function. Other goals to consider are:

• Preservation of the normal ear
• Improve the affected one
• Enable speech development
• Improve aesthetics.

Timing of Microtia Surgery

The exact time for microtia surgery is reflective of the patient and surgical preference. Issues to take into consideration are:

• Is the ear developed? Most ears are developed by the age of 4 or 5. The width of the ear continues to grow until 10. It's also important to consider if there is adequate donor-site rib cartilage.
• What surgery is required? This is discussed in more detail in the next section. The Brent Technique is performed earlier than the Nagata Technique.
• Is the kid being bullied? Emotion and psychological factors, particularly in young school kids, are important considerations.
• Is Middle Ear Surgery required? Ideally, the external ear should be treated first. Middle ear reconstruction is not indicated for unilateral microtia with normal hearing in the contralateral ear⁸. There is a risk of injury to the facial nerve, but it can improve hearing by 70% in some cases^9-10.

Hearing

Hearing loss and atresia can be treated through different techniques. The majority of hearing issues are conductive hearing loss. Patients with stenosis of EAM can have mixed sensorineural and conductive; atresia patients have conductive hearing loss.

Bone-anchored hearing aid (BAHA)

• Two-stage radiologically-guided osseointegration, discovered by Brånemark.
• Abutment placed >65mm posterior to the external auditory meatus to avoid scars on the non-hair-bearing mastoid skin.
• It can be placed after external ear reconstruction in unilateral cases.
• Ideally performed by age of 4 or 5.

Other considerations:

• Meatoplasty/Canaloplasty: not routinely performed due to mild hearing improvement and risk of facial nerve injury.  
• Middle ear implants

External Ear Reconstruction

Staged autogenous cartilage reconstruction remains the gold standard for correcting microtia. Two techniques that have popularised autologous rib cartilage graft frameworks are Brent (4 stages) and Nagata (2 stages).

Brent Technique¹¹

This technique can be performed earlier than the Nagata technique as less rib cartilage is required. It is usually performed by age of 6.

• Stage I: Plan, harvest from the contralateral chest, and create a subcutaneous pocket by incising anterior to the auricular vestige and removing any residual cartilage.
• Stage II: Lobule created
• Stage III: Post-auricular sulcus is created by incising outside the helical rim, advancing a post-auricular flap and resurfacing with a split-thickness skin graft.
• Stage IV: Tragus created by using normal ear conchal cartilage.

After a 5-year follow-up, Brent concluded¹¹ that 48% grow at the same rate as normal ears, 46% grow slightly bigger, and 10% grow slightly smaller.

Nagata Technique¹²

This technique is usually performed by the age of 9. Its main other difference to the Brent technique is that the creation and elevation are done at the same time.  

• Stage I: cartilage framework with a tragal component and lobule are inset into a subcutaneous pocket created by a posterior incision.
• Stage II: post-auricular sulcus is created by a cartilage wedge, a local flap (temporoparietal fascial flap or galeal flap), and a split-thickness skin graft.

Alloplastic Reconstruction:

• Single-stage subcutaneous implantation, often Medpor
• Reinisch technique
• Less commonly used due to the higher rates of extrusion.

Tissue expansion is less commonly used because expanded skin is less supple than unexpanded skin. It is less able to conform to the anatomic details of an autologous or prosthetic ear cartilage framework.

Other Auricular Abnormalities

Stahl Ear

• An abnormal third crus traverses the superior third of the ear (from the crus antihelix to the edge of the helix).
• The appearance of a "flattened" helix
• It can be treated with neonatal moulding.

Cryptotia

• The upper pole of the helix is buried beneath the temporal skin
• Less common findings can include missing upper sulcus, underdeveloped scapha or antihelical crura.
• Reconstruction aims to create a posterior sulcus with a skin graft or flap.
• Cause is due to an anomaly of intrinsic oblique and transverse auricular muscles.

Constricted Ear

• A deficiency in the "circumference of the helix"
• Lop ear: folding of the superior aspect of the helix
• Cup ear: folding of the superior aspect of the helix and a prominent ear.

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