Microtia (MI) is a congenital condition characterized by an underdeveloped auricle, often accompanied by a narrowed or completely absent external auditory canal (atresia). MI can present as a unilateral and bilateral disorder potentially leading to significant hearing loss. Computer tomography will reveal potential atresia and possible further occult malformations of the middle ear. Prescription of hearing aids is strongly advised in patients with impaired hearing.
Due to evident auricle deformations, most patients present as infants or children with significant hearing loss (40-60 dB). Microtia (MI) usually affects one ear and manifests as a non-syndromic condition in up to eighty percent of cases. Ten percent of diagnosed patients suffer from bilateral MI .
MI manifestations have been categorized in four grades. Grade I MI features a normal but small ear anatomy with a narrowed or absent external auditory canal. Grade II MI presents with an underdeveloped auricle, particularly in upper auricle parts, and usually with atresia. In Grade III MI, the auricle is generally underdeveloped and the external auditory canal is absent. Grade IV MI patients do not present with residual ear structure (anotia) and always suffer from atresia .
In twenty up to sixty percent of diagnosed cases, MI can also be found in a syndromic context, e.g. in hemifacial or craniofacial microsomia, Goldenhar Syndrome (GS), Townes-Brocks Syndrome, Nager Syndrome and in mandibulofacial dysostosis. MI occurs more often in males as a unilateral condition    . In a syndromic MI manifestation, additional symptoms may include renal, cardiac and vertebral anomalies .
MI often presents as a mild form of GS and share the following characteristics: variable phenotypic expression, asymmetric facial anatomy, right side preponderance, male predilection, and familial occurrence within an sporadic or autosomal dominant (rarely recessive) inheritance mechanism    .
Initial clinical evaluation requires a correct categorization of MI based on presented auricle underdevelopment and atresia, which are both possible unilaterally or bilaterally. If patient history reveals a possible syndromic manifestation within a familial accumulation of MI, it is reasonable to examine for renal, cardiac, cervical, lumbar spine or thoracic anomalies using ultrasound or radiographic procedures, respectively .
External auditory canal status can be diagnosed within an otoscopic examination and found to be normal, stenotic, blindly ending or atretic. In any case, standard audiologic tests are the strongly advised for both ears to appropriately assess hearing function. More elaborate audiologic tests like auditory brainstem response (ABR), pure tone audiometry (PTR) may also be necessary in selected cases .
A non-contrast temporal bone computer tomography (CT) scan is almost always advisable, since it resolves the grade of atresia and potential further malformations best and guides. If a patient presents with craniofacial microsomia, three-dimensional CT scans may provide good insights for subsequent surgical measures .
An optimal treatment of MI depends on the diagnosed grade and nature (unilateral, bilateral). External auricle underdevelopment can be corrected with cosmetic surgery, while anatomic anomalies of the external auditory canal and the middle ear must be tackled within auditory surgery and typically involves canaloplasty as well as ossicle and tympanic reconstruction, if necessary. These procedures will require up to three surgical interventions under general anaesthesia, which may be too much of a health strain for the MI patient .
Pinna reconstruction can start in children older than six but may be postponed in selected cases, if the child is still too small at this age, since the surgeon will have to harvest a rib of appropriate size for framework grafting in the context of the natural cartilage technique .
The use of alloplastic implants made of porous polyethylene or of prosthetics may also be considered  .
In infants with pronounced hearing loss as well as MI patients who refuse to undergo above-mentioned reconstruction surgery the use of hearing aids will help to partly recover hearing function. For pre-lingual infants the prescription of such hearing aids is most advisable in order to allow for proper speech development .
Since hearing function can be significantly impaired (40-60 dB) in the affected ear, bilateral MI is a serious diagnosis which requires hearing aids in infants to guarantee speech development. If MI is unilateral, speech development in children should be normal.
The main impact of untreated unilateral MI is psychological. Children older than four and particularly teenagers suffering from untreated MI may suffer from low self-esteem, anxiety, increased peer pressure and social isolation due to the evident anatomic deformations. This effect may be even more pronounced in syndromic MI manifestations like GS.
Teenagers are usually more enthusiastic about reconstructive surgery than younger children but may be unrealistic in their expectations. Proper professional counselling and parental care are advised in any case.
Surgical reconstruction typically provides excellent perspectives for recovered hearing and satisfying auricle aesthetics.
The cause of MI can be genetic in syndromic MI manifestations, related to irregularities during pregnancy   or possibly related to neural crest cells disturbance, vascular disruption or the altitude of maternal residence during pregnancy .
Genetic causes are more likely to be Mendelian in syndromic and familial MI manifestations, whereas sporadic cases are typically polygenetic and/or multifactorial .
Specific pregnancy-related risk factors are a maternal rubella infection during the first trimester of pregnancy  . Further risk factors include low birth weight, high maternal parity, maternal use of medication, advanced maternal age, multiple births and maternal diabetes mellitus. Maternal intake of folic-acid-containing supplements may also promote MI manifestation .
Reported MI prevalence ranges from 0.83 up to 17.4 in 10 000 births and has been shown to be higher in Hispanics, Asians, Native Americans and Andeans .
MI is more frequent in males, who are at an increased risk of twenty to forty percent to suffer from MI. 77-93% of diagnosed MI is unilateral, 60% of these cases concern the right ear. Bilateral MI is more common in syndromic cases        .
The anatomy of the affected ear in infant patients resembles the ear of an embryo after six weeks of development. The exact molecular reasons for a stalled development are unknown and currently in the focus of research.
To date, a disturbance of neural crest cells has been shown to be related to numerous craniofacial syndromes . In mandibulofacial dysostosis, TCOF1 mutations are suspected to cause insufficient cell proliferation and increased neuroepithelial apoptosis resulting in a reduced migration of cells into the first and second pharyngeal arches, which is conducive to a craniofacial phenotype including bilateral MI .
Vascular disruption is also a possible cause for microtia. Early perturbations of development of the vascular system of the head and neck may lead to tissue necrosis .
Increased MI prevalence at high altitudes  still poses a major conundrum for researchers.
Hereditary and sporadic MI cannot be prevented. Maternal caution during pregnancy can, however, reduce the odds of MI manifestation in the progeny. Avoiding infections with rubella   and drug abuse as well as maintaining a healthy diet should have a positive effect. Women should not suffer from diabetes mellitus before and during pregnancy to prevent MI manifestation . Maternal diet during pregnancy should not be low in carbohydrates and folic acid. Intake of folic acid may, however, reverse the effect .
Microtia is a congenital condition presenting with underdeveloped or absent auricle as well as malformed or absent external auditory canal. Patients are usually infants or children. MI can have genetic or pregnancy-related causes and mostly appears as an isolated symptom affecting the right ear in males. Otoscopic examinations, audiometric checks and CT scans of the temporal bone should be performed. Reconstructive surgery can be taken in consideration for children older than six. Prescription of hearing aids is imperative for pre-lingual infants with diagnosed hearing loss to ascertain proper speech development.
Microtia is an innate condition which is characterized by malformations or the complete absence of one or both ears. The inner parts of the ear can also be affected. In this case, substantial hearing loss is a major symptom. Patients should seek professional help in any case because the psychological effects of a malformed ear can be detrimental. Doctors will recommend reconstructive surgery and may prescribe hearing aids, particularly in infants with diagnosed hearing loss.