AN40.1-5 | Organs of hearing and equilibrium — Gate Quiz

Correct! The EAM has an outer cartilaginous third (containing ceruminous wax-secreting glands and hair follicles) and an inner bony two-thirds (formed by the tympanic plate of the temporal bone). This explains why otitis externa often begins in the outer part.

EAM: Outer 1/3 = cartilaginous → contains ceruminous glands + hair follicles (otitis externa here). Inner 2/3 = bony (tympanic plate). To straighten the meatus for otoscopy: adults — pull pinna upward and backward; children under 3 — pull pinna downward and backward.

Incorrect. The outer 1/3 is CARTILAGINOUS (with ceruminous glands + hair follicles) and the inner 2/3 is BONY. Not the reverse.

Correct! Arnold's nerve (auricular branch of CN X — vagus nerve) supplies the posterior inferior external auditory meatus. The vagus also supplies the larynx and hypopharynx. Hence laryngeal and hypopharyngeal cancers commonly cause referred otalgia (ear pain) via Arnold's nerve without any intrinsic ear pathology.

Referred otalgia in H&N cancer: (1) Larynx/hypopharynx → Arnold's nerve (CN X in EAM). (2) Tonsil/oropharynx → Jacobson's nerve (CN IX — tympanic nerve in middle ear). (3) TMJ/dental → auriculotemporal nerve (CN V3). RULE: Normal ear + otalgia = find the primary source, often malignancy.

Incorrect. The referred otalgia in laryngeal carcinoma is via Arnold's nerve — the auricular branch of the vagus (CN X) which supplies the posterior EAM. Otalgia with a normal ear always demands investigation for malignancy in head and neck.

Correct! Stapedius is supplied by the facial nerve (CN VII). In facial nerve palsy (Bell's palsy or other CN VII lesions above the stapedial branch), stapedius is paralysed → hyperacusis (abnormally loud sound perception due to uninhibited ossicular vibration).

Middle ear muscles: Tensor tympani (pulls malleus medially, stiffens TM, from anterior wall) = CN V3. Stapedius (pulls stapes posteriorly, dampens oscillations, from posterior wall pyramid) = CN VII. CN VII palsy → hyperacusis (stapedius paralysed). CN V3 branch lesion → tensor tympani paralysed (less clinically obvious).

Incorrect. Stapedius is supplied by CN VII (facial nerve). Tensor tympani is supplied by CN V3 (mandibular nerve via medial pterygoid nerve). Remember the pair: Tensor tympani = CN V3, Stapedius = CN VII.

Correct! In children, the auditory tube is shorter (about 18 mm vs 36 mm in adults), wider, and more horizontal (less oblique). This means nasopharyngeal secretions and bacteria can more easily ascend into the middle ear, predisposing children to acute otitis media.

Child vs Adult auditory tube: Child = shorter (~18 mm), wider, more horizontal → easier bacterial ascent from nasopharynx → acute otitis media. Adult = longer (~36 mm), narrower, more oblique (45° angle) → more protection. Adenoidal hypertrophy in children also mechanically obstructs the tube opening → glue ear.

Incorrect. In children, the tube is SHORTER, WIDER, and MORE HORIZONTAL — not longer or more oblique. This is the anatomical reason for the high incidence of acute otitis media in children under 5.

Correct! The posterosuperior quadrant is the most dangerous — it contains the chorda tympani (which runs between the malleus and incus, just medial to the tympanic membrane), the head of the malleus/incus ossicular chain, and above it the horizontal portion of the facial nerve canal. Incision here risks damaging taste (chorda tympani) and causing facial palsy.

Tympanic membrane quadrants and dangers: Posterosuperior = MOST DANGEROUS (chorda tympani, ossicles, facial nerve canal). Posteroinferior = round window beneath. Anteroinferior = SAFEST for myringotomy (furthest from vital structures). Anterosuperior = handle of malleus above. Cone of light reflex is in anteroinferior quadrant.

Incorrect. The main structures to avoid in myringotomy are the chorda tympani (taste) and the facial nerve canal — both located in the POSTEROSUPERIOR quadrant. The anteroinferior quadrant is chosen as it is furthest from these structures.

Correct! Endolymph has a unique composition resembling intracellular fluid: high K⁺ (~150 mEq/L) and low Na⁺ (~1 mEq/L). Perilymph resembles CSF/extracellular fluid: high Na⁺, low K⁺. This ionic difference is maintained by the stria vascularis (in the cochlea) and is essential for hair cell transduction.

Endolymph: High K⁺ (~150 mM), Low Na⁺ (~1 mM) — unique "intracellular-like" composition; produced by stria vascularis; fills membranous labyrinth. Perilymph: High Na⁺ (~140 mM), Low K⁺ (~5 mM) — like CSF; fills bony labyrinth around the membranous labyrinth. In Ménière's disease, excess endolymph (endolymphatic hydrops) → vertigo + tinnitus + fluctuating sensorineural hearing loss.

Incorrect. Endolymph = high K⁺, low Na⁺ (like intracellular fluid). Perilymph = high Na⁺, low K⁺ (like extracellular fluid/CSF). This is the OPPOSITE of what you might expect.

Correct! The cochlea is tonotopically organised: high frequencies are detected at the BASE (near the oval window) where the basilar membrane is narrow and stiff; low frequencies at the APEX (near the helicotrema) where the basilar membrane is wider and more compliant.

Cochlear tonotopy: Base = high frequency (narrow, stiff basilar membrane). Apex/helicotrema = low frequency (wide, floppy basilar membrane). Clinical application: In noise-induced hearing loss, high-frequency hair cells at the base are damaged first (the 4000 Hz notch on audiogram). Presbycusis also starts with high-frequency loss.

Incorrect. High frequencies → base (narrow stiff basilar membrane); Low frequencies → apex (wide compliant basilar membrane). This is tonotopy — the spatial arrangement of frequency detection along the cochlea.

Correct! The middle ear (tympanic cavity) communicates posteriorly with the mastoid antrum via the ADITUS AD ANTRUM — an opening in the posterior wall of the middle ear. Infection spreads through this route into the mastoid air cells, causing mastoiditis.

Middle ear posterior wall contains: (1) Aditus ad antrum — opening to mastoid antrum (route of spread → mastoiditis). (2) Pyramid — small bony eminence containing stapedius muscle. (3) Fossa incudis — holds the short process of the incus. Mastoiditis complication: subperiosteal abscess (Bezold's abscess), meningitis, sigmoid sinus thrombosis, facial palsy.

Incorrect. The route of spread from middle ear to mastoid is the ADITUS AD ANTRUM in the posterior wall of the tympanic cavity. The aditus connects the epitympanum (attic) to the mastoid antrum.

Correct! Malignant (necrotising) otitis externa is almost exclusively caused by Pseudomonas aeruginosa in immunocompromised patients (especially diabetics). It is "malignant" not because it is cancerous but because it spreads beyond the EAM to invade the skull base (temporal bone osteomyelitis), potentially causing facial nerve palsy (CN VII at the stylomastoid foramen) and other cranial nerve palsies.

Malignant otitis externa: Organism = Pseudomonas aeruginosa. Patient = elderly diabetic or immunocompromised. Route: EAM → fissures of Santorini → skull base → temporal bone osteomyelitis. CN VII most commonly affected. Radionuclide scan (Tc-99m) for diagnosis; prolonged (6-8 weeks) IV ciprofloxacin/piperacillin-tazobactam.

Incorrect. Malignant otitis externa = Pseudomonas aeruginosa in diabetics → skull base osteomyelitis → cranial nerve palsies (most commonly CN VII). High mortality if untreated; requires prolonged IV antipseudomonal antibiotics.

Correct! The anteroinferior quadrant is the standard site for myringotomy and grommet insertion. It is the safest quadrant because it is furthest from the chorda tympani (posterosuperior), the ossicles (central/anterosuperior), the facial nerve canal (posterosuperior), and the round window (posteroinferior).

Myringotomy site mnemonic: "AI = Always Incise anteroinferior." Reasons: away from chorda tympani (posterior), away from ossicles (superior), away from facial nerve (posterosuperior), away from round window (posteroinferior). The cone of light reflex (otoscopic landmark) is also in the anteroinferior quadrant.

Incorrect. The ANTEROINFERIOR quadrant is the standard and safest site for myringotomy. The posterosuperior quadrant (ossicles, facial nerve, chorda tympani) is the most dangerous.