Development of Ear and Congenital Anomalies of the External Ear

(University MS ENT PG Long Answer - 20 Marks)

INTRODUCTION

PART I: EMBRYOLOGICAL DEVELOPMENT OF THE EAR

A. Periods of Development (Scott-Brown's)

• Pre-embryonic: Days 1-21
• Embryonic: Days 22-56 (most organ formation occurs here)
• Foetal: Day 56 to birth (growth, shape refinement)

B. Branchial (Pharyngeal) Arches - The Foundation

• A bar of cartilage
• A group of muscles
• An artery
• A cranial nerve (post-trematic nerve)

C. Development of the External Ear

1. Development of the Auricle (Pinna)

• Hillocks 1, 2, 3 - from the 1st pharyngeal arch (mandibular arch)
• Hillocks 4, 5, 6 - from the 2nd pharyngeal arch (hyoid arch)

Development of the auricle: A - Auricular hillocks from 1st & 2nd arches at 6 weeks; B - 7-week embryo (The Developing Human, Moore)

• The 2nd arch hillocks form the bulk of the pinna (helix, antihelix, antitragus, lobule)
• The 1st arch contribution progressively reduces; it forms the tragus and part of the helical root
• Recent research (Cummings) indicates tissue caudal to the 2nd arch hillocks gives rise to the free ear fold (helix + scapha)
• The lobule is the last part of the auricle to develop and is the least vulnerable to teratogens

• Parts from 1st arch: mandibular branch of CN V
• Parts from 2nd arch: Greater auricular and lesser occipital nerves (cervical plexus)
• The facial nerve (nerve of 2nd arch) does not supply the skin of the 2nd arch derivatives - an exception explained by migration of cervical neural crest cells

2. Development of the External Auditory Meatus (EAM)

• Develops from the dorsal part of the first pharyngeal groove (ectoderm)
• Ectodermal cells at the base proliferate to form a solid epithelial plate: the meatal plug
• Late in foetal life (around week 28), the central cells of the meatal plug degenerate and the cavity formed becomes the internal (deep) part of the EAM
• The meatus is relatively short at birth and attains adult length by approximately 9 years of age
• Failure of meatal plug canalization causes congenital EAM atresia (meatal atresia)

3. Development of the Tympanic Membrane

D. Development of the Middle Ear

• The tubotympanic recess develops from the first pharyngeal pouch (endoderm)
• This recess expands to form:
  • Eustachian tube (medial part)
  • Tympanic cavity (lateral expanded part)
  • Mastoid antrum (further expansion)
• The Eustachian tube lumen and middle ear spaces are formed by 8 months' gestation
• The epitympanum and mastoid antrum develop by birth
• Mastoid air cells are mostly absent at birth (filled with amniotic fluid); 90% of air cell development is complete by age 6, with remainder by age 18

• Develop from the dorsal ends of arch cartilages, beginning at 4 weeks
• Malleus head + incus body: 1st arch (Meckel's cartilage)
• Stapes superstructure: 2nd arch (Reichert's cartilage)
• Stapes footplate: contributes from the otic capsule (lateral wall of the labyrinth)
• Adult shape, size, and ossification is present by 25 weeks' gestation
• Tensor tympani = 1st arch (CN V); Stapedius = 2nd arch (CN VII)

E. Development of the Internal Ear

• At day 22-23, ectodermal thickening forms on the sides of the head: the otic placode
• The placode deepens to form the otic pit, which closes off to become the otocyst (otic vesicle) by week 4
• The otocyst divides into:
  • Dorsal utricular part → utricle, semicircular ducts, endolymphatic duct
  • Ventral saccular part → saccule and cochlear duct; cochlear duct gives rise to the organ of Corti
• The bony labyrinth develops from the mesenchyme adjacent to the membranous labyrinth (otic capsule)
• Neural crest cells cluster near the otocyst to become the spiral (auditory), vestibular (Scarpa's), and geniculate (facial) ganglia
• Because the labyrinth is phylogenetically ancient, labyrinthine anomalies are rare

PART II: CONGENITAL ANOMALIES OF THE EXTERNAL EAR

Key Clinical Principle

"The auricle is formed early. Therefore, malformation of the auricle implies malformation of the middle ear, mastoid, and VII nerve. On the other hand, a normal auricle with canal atresia indicates abnormal development during the 28th week, by which time the ossicles and middle ear are already formed." - KJ Lee's Essential Otolaryngology

1. Microtia (Underdevelopment of the Auricle)

• Genetic factors: HOXA2, other genes
• Neural crest cell disturbance
• Vascular disruption (stapedial artery)
• Teratogens: retinoic acid (days 20-22), rubella, alcohol, thalidomide
• Living at altitude
• Associations: Hemifacial microsomia (Goldenhar/oculo-auricular-vertebral spectrum), Treacher Collins syndrome, Nager syndrome

Classification of Microtia (Marx/Modified - 4 grades, Cummings)

Fig. Grades of microtia: A-Grade I (constricted ear), B-Grade II (conchal type), C-Grade III (lobular type), D-Grade IV (anotia) - Cummings Otolaryngology

1. Observation
2. Prosthesis (adhesive or implant-retained)
3. Single-stage reconstruction with alloplastic framework (MedPor/SuPor with temporoparietal fascia flap)
4. Staged autologous costochondral reconstruction (gold standard) - Brent technique (3-4 stages) or Nagata technique (2 stages)
5. Future: Tissue engineering + 3D printing

• Age-appropriate hearing assessment is mandatory in all microtia patients
• EAC atresia repair is performed after auricular reconstruction
• Rib cartilage harvest is possible once ear reaches ~85% of adult size (around age 6-8)

2. Anotia

3. Congenital EAM Atresia (Meatal Atresia)

• Bony atresia (most common): solid bone occludes the canal
• Membranous/soft tissue atresia: soft tissue band occludes canal

• A normal auricle with canal atresia suggests the insult occurred around 28th week - ossicles and middle ear are likely normal
• Audiological evaluation and CT temporal bone (high-resolution) are essential
• Grading by Jahrsdoerfer score helps assess candidacy for surgical repair (canaloplasty)
• Usually managed by canaloplasty + meatoplasty or with bone-anchored hearing aids (BAHA)

4. Preauricular Sinus / Cyst / Fistula

• Preauricular sinus: blind-ended tract (most common), opens in front of the tragus
• Preauricular cyst: enclosed remnant, no external opening
• First branchial arch fistula (Collaural fistula): tract between the EAC and the skin of the cheek, runs through the parotid gland, often between branches of the facial nerve

• Small pit anterior to the tragus
• Intermittent discharge, recurrent infections
• 20% are bilateral
• May be asymptomatic for years

• Observe if asymptomatic
• Surgical excision (complete tract removal) - preferably when infection-free
• Collaural fistula: requires careful dissection around facial nerve

5. Accessory Auricle (Polyotia/Ear Tags)

• Small soft-tissue or cartilaginous tags anterior to the tragus or along the line from tragus to angle of mouth
• Represent supernumerary auricular hillocks or failure of regression of arch tissue
• Associated with other first and second arch anomalies
• Treatment: surgical excision

6. Prominent/Bat Ear (Prominent Auricle)

• Most common congenital auricle deformity
• Due to failure of normal folding of the antihelical fold
• The conchoscaphal angle is >90° (normal: 90°)
• Concha may be enlarged (conchal hypertrophy)
• Treatment: Pinnaplasty/Otoplasty - ideally before school age (5-6 years)
  • Mustardé suture technique (antihelical folding)
  • Converse technique
  • Furnas suture (conchomastoid fixation)

7. Darwin's Tubercle

• A small cartilaginous nodule on the helix
• Represents a vestigial remnant (phylogenetically, the tip of the pointed animal ear)
• Common, no clinical significance

8. Low-Set Ears

• Normal position: upper pole of pinna at the level of the lateral canthus of the eye
• Low-set ears: pinna positioned below this line
• Associated with chromosomal abnormalities, especially trisomy 13 (Patau) and trisomy 18 (Edwards)
• Also seen in Turner syndrome (45, XO)

9. Treacher Collins Syndrome (Mandibulofacial Dysostosis)

• Bony meatal atresia
• Malformed incus and malleus
• Short mandible
• Antimongoloid slant of eyes (outward-downward)
• Notched lower eyelid
• Fishmouth deformity
• Absent malar bones
• When maxilla is also malformed alongside the first arch features = Treacher Collins syndrome

10. Goldenhar Syndrome (Hemifacial Microsomia / OAV Spectrum)

• Unilateral facial underdevelopment
• Microtia + EAC atresia
• Epibulbar dermoids
• Vertebral anomalies
• Mandibular asymmetry
• Renal/cardiac anomalies

Summary Table: Embryological Origins and Associated Anomalies

Key Exam Points to Remember

1. Six hillocks of His → from 1st (hillocks 1-3) and 2nd (hillocks 4-6) arches
2. Tragus = only 1st arch contribution to adult pinna
3. Tympanic membrane = 3 layers from 3 sources (groove, pouch, mesenchyme)
4. Otic placode → otic pit → otocyst at day 22-23
5. Microtia + middle ear anomaly = insult before week 5-6 (early)
6. Normal auricle + canal atresia = insult around week 28 (late)
7. Preauricular sinus = improper fusion of hillocks; 20% bilateral
8. Collaural fistula = tract between EAC and cheek skin, traverses parotid, near facial nerve
9. Low-set ears = associated with trisomy 13 and trisomy 18
10. Labyrinthine anomalies are rare because the labyrinth is phylogenetically ancient
11. Mastoid process not formed until end of 2nd year; mastoid air cells mostly complete by age 6
12. Ossicles reach adult shape and ossification by 25 weeks' gestation