1st Branchial Arch Disorder Ashnr Poster

1.  To review the normal CT anatomy of the 1st branchial arch (BA) and associated structures with embryological correlation. 2.  To illustrate the imaging findings in classic syndromes affecting the first BA and associated structures. 3.  To discuss the list of differential diagnosis when examining malformations of the first BA and associated structures.

The 1st BA, also called the first pharyngeal arch and mandibular arch, is the first of six BAs that develops in fetal life. The BAs develop in the 4th and 5th weeks of development. The 1st BA is located between the stomodeum and the first pharyngeal groove. This arch divides into a maxillary and a mandibular process. The maxillary process becomes the maxilla and palate.

Treacher Collins syndrome is a disorder of craniofacial development. The features include antimongoloid slant of the eyes, coloboma of the lid, micrognathia, microtia and other deformity of the ears, hypoplastic zygomatic arches, and macrostomia. Conductive hearing loss and cleft palate are often present.

12a

The 1st BA consists of a maxillary process and a mandibular process. The mesenchyme of the maxillary process gives rise to the maxilla, zygomatic bone and part of the temporal bone through membranous ossification. The mandible is also formed by membranous ossification of mesenchymal tissue surrounding Meckel’s cartilage. The 1st BA also contributed to formation of the ossicles.

15d

Angle of mandible Mandibular symphysis

Mental tubercle

Mental foramen

10d

6

7

10e

10f

10g

HFM is a common birth defect involving 1st and 2nd BA derivatives. The phenotype is highly variable. There may be cardiac, vertebral, and CNS defects, in addition to craniofacial anomalies. The features of HFM include unilateral deformity of the external ear and small ipsilateral half of the face with epibulbar dermoid and vertebral anomalies. Coloboma of the upper eyelid is frequent. The ear deformities range from preauricular tags of cartilagenous masses, to atresia of the external auditory canal, anomalies in the size and shape of the external auricle, and even to anotia. This syndrome is sometimes called oculoauriculovertebral dysplasia or Goldenhar Syndrome.

Orbitomeatal plane

Failure of fusion between any of the facial structures results in a cleft which may be unilateral or bilateral. The two most common types of facial cleft are cleft lip which results from failure of the maxillary swellings to fuse with the intermaxillary process, and cleft palate, which results from the failure of the palatine shelves to fuse in the midline.

ACS is characterized by prominent malformed ears with auricular clefts, mandibular condyle aplasia or hypoplasia, and a number of other features that are secondary to the auricular and oral abnormalities. In its most severe form, there is severe micrognathia and a characteristically round facial appearance with prominent cheeks.

13a

13d

Coronoid process 11a 9a 2

14e

9c

11a

9d

11b

11c

14f

16a

16b

16c

16d

Condylar process

Body of mandible

A 2nd BCC is the most common BCC accounting for >90% of all BCC anomalies. It is considered a failure of closure of the cervical sinus. Typically painless mass along the sternocleidomastoid (SCM), but may become infected.

14c

13c

13b

9b

Angle of mandible

14b

14d

Zygomatic arch

Ramus of mandible

14a

8

1

TMJ

15c

10c

10b

10a

Intermaxillary suture

Body of mandible

15b

5

Maxilla

Ramus of mandible

12d

12c

A 1st BCC is a cystic mass that arise in the parotid, posterior submandibular space or preauricular region. It is a remnant of the 1st branchial cleft due to incomplete closure in utero.

15a 4

3

TMJ

12b

Also called DiGeorge syndrome, VCFS is caused by a 1.5 to 3.0-Mb hemizygous deletion of chromosome 22q11.2. The syndrome consists of cleft palate, cardiac anomalies, typical facies, and learning disabilities. Less frequent features included microcephaly, mental retardation, short stature, slender hands and digits, minor auricular anomalies, and inguinal hernia. VCFS is the most frequent clefting syndrome, accounting for 8.1% of children with palatal clefts seen in their center. Cardiac anomalies were found in 82% including isolated ventricular septal defect and tetrology of Fallot.

13e

13f

13g

Clinical Finding

Affected individuals

Facial anomaly

100%

Congenital heart defect

82%

Speech / Learning difficulties

80%

Neonatal hypocalcemia

73%

T-Cell deficiency

69%

Skeletal anomaly

32%

Palatal anomaly

31%

Asymmetric crying face

21%

Renal malformations

15%

Genital anomaly

11%

The first branchial arch is the first of six branchial arches and is the embryological origin of most of the structures of the face. A wide variety of congenital conditions may arise from its contents. Understanding of the anatomic formation of this region is important in understanding abnormalities in development which aids in formation of a precise diagnoses and lists of differentials.

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