Detailed Anatomy of the Base of Skull, Ear, Nose, and Throat - Basic to Surgical

PART I: BASE OF SKULL

Overview

Lateral Skull Base - The Temporal Bone (Surgical Focus)

• Forms the lateral wall of the middle cranial fossa
• Contains the zygomatic process (roof of glenoid fossa)
• The temporal line is ~5 mm inferior to the middle fossa floor - a critical surgical landmark
• Temporalis muscle inserts on outer cortex; masseter on zygomatic process

• Bullous bony structure shaped by air cells; tip elongated by SCM and digastric pull
• MacEwen triangle (cribriform area): depressed area posterior to Spine of Henle - surgical gateway to mastoid antrum
• Stylomastoid foramen at anterior limit of digastric groove - exit point for CN VII
• Mastoid emissary vein communicates with sigmoid sinus

• Superior face (middle fossa floor): shows arcuate eminence (prominence of superior semicircular canal), groove for GSPN, depression for trigeminal ganglion (Meckel cave)
• Posterior face (anterior wall of posterior fossa): houses the internal auditory meatus; framed by sigmoid, superior petrosal, and inferior petrosal sinus sulci
• Petrous apex: between occipital and sphenoid bones; Meckel cave contains trigeminal ganglion here

• Forms anterior and inferior walls + part of posterior wall of the bony EAC
• Forms the posterior wall of the glenoid fossa (TMJ)
• Ends medially at the tympanic sulcus where the TM inserts
• Foramen of Huschke: defect in anterior bony canal from incomplete ossification - allows spread of disease to parotid gland deep lobe

Key Foramina of the Skull Base (Surgical Relevance)

Jugular Foramen Anatomy

• Pars nervosa (posterior): CN IX, Jacobson nerve; cochlear aqueduct opening is just medial/anterior to jugular spine, marking its most superior limit
• Pars venosa (anterior): jugular bulb, CN X and XI

Surgical Approaches to the Skull Base

• Cummings Otolaryngology, p. 3341-3360

PART II: THE EAR

External Ear

• Length: ~2.5 cm in adults
• Lateral 1/3: cartilaginous, contains sebaceous + apocrine (ceruminous) glands, hair follicles, lined by thick mobile skin
• Medial 2/3: bony (tympanic part of temporal bone), covered by thin skin tightly adherent to bone, no glands or hair
• Isthmus = bony-cartilaginous junction = narrowest point; fibrous interface = pathway for malignant spread (requires en bloc lateral temporal bone resection for EAC carcinoma)
• Fissures of Santorini: transverse slits in cartilaginous canal - allow spread of infection/neoplasm to surrounding soft tissues
• Foramen of Huschke: incomplete ossification defect in anterior bony canal - spreads disease to parotid deep lobe
• Cerumen: hydrophobic, slightly acidic (pH 6.0-6.5); formed by glandular secretions + sloughed epithelium; self-cleansing by centrifugal migration from TM surface outward

Tympanic Membrane (TM)

1. EAC skin (outer squamous layer)
2. Outer radiate fibrous layer - inserts on malleus manubrium
3. Inner circular fibrous layer - circumferential at periphery
4. Middle ear mucosa (inner layer)

• Umbo: tip of malleus manubrium attachment (most depressed point)
• Mallear stria: runs superiorly from umbo along manubrium
• Mallear prominence: formed by lateral process of malleus
• Pars flaccida (Shrapnell membrane): superior, lacks fibrous layer; anchored to notch of Rivinus - site of acquired cholesteatoma formation
• Pars tensa: rest of TM (inferior); anchored by annular ligament to tympanic sulcus (fibrocartilaginous ring)
• Anterior and posterior mallear folds demarcate pars tensa from pars flaccida

Middle Ear

• Mesotympanum: at the level of the TM
• Epitympanum (attic): above the level of the TM; contains malleus head and incus body
• Hypotympanum: below the level of the TM

• Malleus: consists of handle (manubrium), neck, lateral process, and head; the long process of the malleus is the manubrium; head articulates with incus in the attic
• Incus: body (in attic) + short process (posterior) + long process (descending toward stapes); the long process of the incus has a single nutrient vessel and no collateral circulation - the most vulnerable portion of the ossicular chain to avascular necrosis
• Stapes: head, anterior and posterior crura, oval window footplate; footplate is held in the oval window by the annular ligament

• Promontory: bulge of basal turn of cochlea
• Oval window (fenestra vestibuli): covered by stapes footplate
• Round window (fenestra cochleae): separated from oval window by round window niche; covered by secondary TM
• Cochleariform process: bony landmark where tensor tympani tendon turns laterally to insert on malleus; facial nerve runs over its superior aspect (key surgical landmark)
• Pyramidal eminence: small bony eminence posterior to oval window; stapedius tendon emerges from its apex
• Facial nerve (tympanic/horizontal segment): runs in fallopian canal over the oval window; may be dehiscent in 15-40% of ears

• Contains heads of malleus and incus
• Supratubal recess (STR): anterior extreme of attic, superior to ET opening; geniculate ganglion lies just deep to its medial wall (may be dehiscent); site of CN VII injury in cholesteatoma surgery or blind dissection

• ~35 mm long; bony portion (medial 1/3) + cartilaginous portion (lateral 2/3)
• Opens into nasopharynx; functions in pressure equalization and drainage
• The semicanal of the tensor tympani lies superior to the ET semicanal in the musculotubal canal

• 2.5 turns spiraling around the modiolus
• Tonotopic organization: high frequencies at base, low frequencies at apex
• Three scalae: scala vestibuli (perilymph), scala media / cochlear duct (endolymph), scala tympani (perilymph); helicotrema connects vestibuli and tympani at apex
• Organ of Corti: sits on basilar membrane within scala media; contains inner hair cells (IHC, 3500) and outer hair cells (OHC, 12,000)
• Afferent neurons in the spiral ganglion (bipolar); project centrally as the cochlear nerve

• 3 semicircular canals (superior/anterior, posterior, lateral/horizontal) - sense angular acceleration
• Utricle and saccule (otolithic organs) - sense linear acceleration and gravity
• Superior SCC creates the arcuate eminence on the middle fossa floor - key landmark for middle fossa surgery

• Cummings Otolaryngology, Ch. 126-127, pp. 2443-2454

Mastoid Pneumatization

PART III: THE NOSE

External Nose and Nasal Framework

• Upper lateral (triangular) cartilages: form the middle vault; articulate with the caudal edge of the nasal bones; the internal nasal valve (angle between upper lateral cartilage and septum, ~15°) is the narrowest part of the nasal airway
• Lower lateral (alar/greater alar) cartilages: medial crura (form columella base) + lateral crura (form alar rims); govern nasal tip projection and rotation
• Nasal septum: quadrilateral cartilage (anterior), perpendicular plate of ethmoid (posterior-superior), vomer (posterior-inferior), maxillary crest + palatine bone (floor)

Nasal Cavity

• Floor: hard palate
• Roof: nasal bones, cribriform plate of ethmoid (very thin, ~0.5 mm - risk of CSF leak in surgery), sphenoid body
• Medial wall: nasal septum
• Lateral wall: most complex - carries the turbinates

• Inferior turbinate (largest, independent bone): inferior meatus contains the nasolacrimal duct (opens ~6 mm posterior to the anterior end of the inferior turbinate)
• Middle turbinate (ethmoid bone): middle meatus contains the ostiomeatal complex (OMC) - the drainage pathway for frontal, anterior ethmoid, and maxillary sinuses
• Superior turbinate (ethmoid bone): superior meatus drains posterior ethmoid cells and sphenoid sinus via sphenoethmoidal recess

• Key structures: uncinate process, hiatus semilunaris, ethmoid bulla, infundibulum
• Maxillary sinus ostium opens into infundibulum (ethmoid infundibulum)
• Frontal recess drains above/lateral to uncinate process
• Site of obstruction in chronic rhinosinusitis - target of FESS (functional endoscopic sinus surgery)

• Kiesselbach plexus (Little's area): anterior septum, anastomosis of 5 arteries:
  • Anterior ethmoidal artery (from ophthalmic artery → ICA)
  • Posterior ethmoidal artery (from ophthalmic artery)
  • Sphenopalatine artery (largest - from internal maxillary artery → ECA) - enters nasal cavity through sphenopalatine foramen (posterior to middle turbinate)
  • Greater palatine artery
  • Superior labial branch of facial artery
• Anterior epistaxis: >90% at Little's area (Kiesselbach plexus); managed conservatively or by cautery
• Posterior epistaxis: sphenopalatine artery territory; requires endoscopic sphenopalatine artery ligation or embolization

• Upper nasal cavity: olfactory epithelium (pseudostratified columnar with olfactory receptor neurons)
• Fila olfactoria pass through cribriform plate foramina → olfactory bulb
• Cribriform plate = thinnest skull base bone; fractured in anterior skull base trauma (CSF rhinorrhea); traversed in endoscopic skull base approaches

Paranasal Sinuses

• Lamina papyracea: paper-thin medial orbital wall; penetration during ethmoidectomy can herniate orbital fat or injure medial rectus
• Optic nerve: may bulge into sphenoid sinus or be dehiscent in 4-6% of people - endangered during posterior ethmoidectomy and sphenoidotomy
• Anterior ethmoidal artery: crosses the orbit to enter the skull base at the junction of the ethmoid roof and the anterior skull base; ligation used in epistaxis management; landmark in endoscopic skull base surgery
• FESS (Functional Endoscopic Sinus Surgery): based on middle meatus surgery - uncinectomy, ethmoid bulla opening, frontal recess widening, maxillary ostium enlargement

PART IV: THE THROAT (Pharynx and Larynx)

Pharynx

• Roof = body of sphenoid + basioccipital
• Posterior wall = C1-C2 vertebrae
• Lateral wall = Eustachian tube orifice (torus tubarius) with the Fossa of Rosenmüller (lateral pharyngeal recess) - most common site of nasopharyngeal carcinoma
• Adenoid (pharyngeal tonsil) on posterior wall
• Passavant ridge: formed by superior pharyngeal constrictor during swallowing/speech

• Contains: soft palate, uvula, anterior and posterior tonsillar pillars (palatoglossus + palatopharyngeus), palatine tonsils (between pillars in tonsillar fossa), posterior pharyngeal wall, base of tongue (lingual tonsil, circumvallate papillae)
• Waldeyer ring: lymphoid tissue ring - adenoids (nasopharynx), tubal tonsils (ET orifice), palatine tonsils, lingual tonsil
• Peritonsillar space: potential space between tonsillar capsule and superior pharyngeal constrictor - site of peritonsillar abscess; surgical access by needle aspiration or incision and drainage

• Piriform fossae (sinuses): lateral recesses on either side of the laryngeal inlet; lined by mucosa over inner surface of thyroid cartilage; common site of hypopharyngeal cancer and foreign body impaction
• Posterior pharyngeal wall: C3-C6 vertebrae
• Postcricoid region: posterior to the cricoid

• Superior, middle, inferior pharyngeal constrictors (overlap like stacked cups; innervated by pharyngeal plexus - CN IX, X)
• Stylopharyngeus (CN IX alone): elevates pharynx
• Salpingopharyngeus (CN X): opens ET during swallowing
• Palatopharyngeus (CN X): elevates and closes nasopharynx

Larynx

• Laryngeal inlet: tilted posteriorly; bounded by epiglottis (anterior), aryepiglottic folds (lateral), interarytenoid notch (posterior)
• Vestibule: from inlet to false (vestibular) folds
• False vocal folds (vestibular folds): mucosa over vestibular ligament; no phonation function
• Laryngeal ventricle (sinus of Morgagni): recess between true and false cords; the saccule extends superiorly from its anterior end
• True vocal folds (cords): from vocal process of arytenoid to inner surface of thyroid cartilage; consist of epithelium + Reinke's space + vocal ligament + vocalis muscle; vibrate at fundamental frequency during phonation
• Subglottis: below true cords to lower border of cricoid; narrowest part of infant airway (ring of cricoid)

• Supraglottis: from laryngeal inlet to plane through laryngeal ventricle
• Glottis: true vocal folds + anterior and posterior commissures (1 cm above and below free edge)
• Subglottis: from lower border of glottis to lower border of cricoid

• Superior laryngeal nerve (SLN) from vagus:
  • Internal branch: sensory above cords (laryngeal inlet, epiglottis, piriform fossa) - pierces thyrohyoid membrane
  • External branch: motor to cricothyroid muscle; at risk during thyroid surgery (affects pitch)
• Recurrent laryngeal nerve (RLN):
  • Right: hooks under right subclavian artery
  • Left: hooks under arch of aorta, runs in tracheoesophageal groove
  • Enters larynx just posterior to cricothyroid joint under lower border of inferior pharyngeal constrictor
  • Supplies all intrinsic laryngeal muscles except cricothyroid + sensory below vocal cords
  • At major risk during thyroid, parathyroid, and anterior cervical spine surgery

• Superior laryngeal artery (from superior thyroid artery): enters via thyrohyoid membrane with internal SLN
• Inferior laryngeal artery (from inferior thyroid artery): with RLN

Fascial Spaces and Deep Neck

• Retropharyngeal space: between posterior pharyngeal wall and prevertebral fascia; from skull base to T1-T2; conduit for infection spread from neck to mediastinum
• Parapharyngeal space: prestyloid (contains ICA, IJV, CN IX-XII, sympathetic chain); poststyloid fat space
• Danger space (space 4): between alar and prevertebral fascia; extends from skull base to diaphragm - rapid spread of deep neck infection to mediastinum

Surgical Advances Summary

• Cummings Otolaryngology, Chapters 126-127, 175; Gray's Anatomy for Students, Chapters 8 and related; Scott-Brown's Otorhinolaryngology

1. Only abductor of the vocal cord = Posterior CricoArytenoid (PCA) - bilateral RLN palsy = adducted cords = stridor/airway emergency
2. Most vulnerable ossicle = Long process of incus (single blood supply)
3. MacEwen triangle = guide to mastoid antrum (posterior to Spine of Henle, inferior to temporal line)
4. Kiesselbach plexus (Little's area, anterior septum) = site of 90% of epistaxis
5. Cribriform plate = thinnest skull base bone; torn in anterior head trauma → CSF rhinorrhea + anosmia
6. Geniculate ganglion = dehiscent in 25% of ears; site of viral tropism (Ramsay Hunt syndrome = herpes zoster oticus)
7. Killian dehiscence = weak posterior pharynx between thyropharyngeus and cricopharyngeus = Zenker's diverticulum
8. Pharyngeal plexus innervation: all pharyngeal muscles except stylopharyngeus (CN IX alone)
9. Left RLN longer course (hooks aortic arch) = more commonly injured/compressed by mediastinal lesions/thyroid goiter
10. Sphenopalatine artery = terminal branch of internal maxillary artery = workhorse of posterior epistaxis surgery