Possible Cause & Incidence (Etiology)

Incidence of Microtia

According to an extensive study conducted by Dr. Grabb in 1965, microtia occurs once in every 6,000 births [53]. The occurrence is estimated at one in 4,000 in Japanese, and as high as one in 900 to 1,200 births in Navajo Indians [1].

Evaluation of my first 1,000 microtia patients reveals that 52.8 percent of the ears are right; 32.4 percent are left; and 9.4 percent are bilateral. 63.1 percent of the patients are males, and 36.9 percent are females (see Table 1).

Table 1 - Microtia: Case Material
Author's Series of 1,000 Microtia Patients
 
  Cases %Total # Ears     Cases %Total
Right: 582 (58.2%) 582   Males: 631 (63.1%)
Left: 324 (32.4%) 324   Females: 369 (36.9%)
Bilateral: 94 (9.4%) 188     1,000 100%
  1,000 100.0% 1,094        

Hereditary Factors

In a superb study conducted by Rogers, structural, anatomic, and genetic inter-relationships were shown to exist between microtia, constricted, and protruding ears. In this thorough investigation, he demonstrated that these deformities are inter-related and can be hereditary [90].

Preauricular pits and sinuses, and a combination of pits, preauricular appendages (skin tags), cupping deformity, and deafness are all hereditarily dominant [74, 106]. Both dominant and recessive characteristics have been revealed in deafness associated with several auricular abnormalities [62]. Ear deformities frequently recur in families with Treacher-Collins Syndrome  [89]. In my own experience, these are occasionally constricted ear deformities, an abnormality which is known to be hereditary [44, 86].

In a thorough, intensive survey of 96 families of their 171 microtic patients, Takahashi and Maeda ruled out chromosomal aberrations and concluded that inheritance must be multifactorial and that the recurrence risk is 5.7 percent [95]. In previous studies, others have found multifactorial inheritance between 3-8 percent in first degree relatives. Although Tanzer found that approximately 25 percent of his 43 microtia patients had relatives with evidence of branchial arch syndrome, microtia was present only in four instances [98].

In examining my own volume of cases, I found that 4.9 percent of the 1,000 patients had major auricular deformities recurring within the immediate family, i.e., parents, siblings, aunts, uncles, or grandparents. When "distant" relatives were included, the percentage jumped to 10.5 percent. If a couple has two children with microtia, the risk of recurrence is thought to be as high as 15 percent.

In 6 percent of my patients, preauricular skin tags or minor auricular defects were observed in the immediate family. This number rose to 10.3 percent when all relatives were included. Immediate family members with normal auricles but underdeveloped jaws or facial nerves were seen in 1.2 percent of my 1,000 patients.

Specific Factors

McKenzie and Craig [72] and Poswillo [85] theorize that tissue ischemia (decreased blood flow) resulting from an obliterated artery is the cause of developmental auricular abnormalities. This speaks to the deformity being caused by a problem arising during the fetus' development rather than a hereditary problem. Interestingly, and in support of this theory, I have treated 10-12 patients with microtia who have a proven, identical twin with normal ears; I only have one set of identical twin patients where both twins have ear deformities. I also have more than 40 microtia patients in my practice with normal fraternal twin siblings, one with normal triplet siblings, and one with normal quintuplet siblings. Pediatric dysmorphologists state that many types of birth deformities are more common in multiple births and that this may be due to a phenomenon called "placental steal" syndrome, i.e., the placenta of the normal twin is often larger than the placenta of the abnormal one and that perhaps the normal twin is thus "stealing" circulation from the other twin who then develops abnormally because of it.

The occurrence of deafness and occasional microtia resulting from rubella (German measles) during the first trimester of pregnancy is well known. Also, use of certain drugs during this critical period may be causative; I have seen at least three cases of microtia that resulted from the mother's ingestion of the tranquilizer Thalidomide [61a]. Accutane has also been cited as causing ear deformities when ingested during the first trimester [4, 55]. Several mothers of my microtia patients had been unaware of this problem and had used Accutane to control acne. Other medications that reportedly cause microtia are clomid [61] and retinoic acid [66].

Although one can occasionally link an actual causative event during early development, medical history analysis of my 1,000 patients reveals no consistent, unusual contributing factors. When asked if anything unusual occurred during the first trimester of pregnancy, 69.5 percent of the mothers said "no"; 2.9 percent had suffered influenza; 1.5 percent had contracted or been exposed to measles; 1.2 percent had suffered some "traumatic injury"; 1.2 percent had experienced radiation exposure; 3.7 percent had menstruated during the first trimester; 1.0 percent had hyperemesis; 0.8 percent were diabetic; 0.6 percent had "severe emotional problems"; 0.8 percent were exposed to tree sprays; and one mother mentioned excess alcohol intake. Use of various medications were also mentioned, the most common being "morning sickness drugs" (4.8 percent of patients' mothers).

In conclusion, the occurrence of microtia is usually a random, sporadic event, and it is important for the parents to understand that the deformity was not caused by anything the mother did before or during the pregnancy; and the risk of recurrence within the family is about five percent, or, put another way, one in twenty.