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DNB Otorhinolaryngology - ENT Paper 1 | Model Answers

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Question 1 [10 Marks]

a) What are Cochlear Microphonics? [2]

• Generated primarily by OHCs (mainly the basal turn)
• They reflect the mechanical displacement of the basilar membrane
• Preserved in auditory neuropathy spectrum disorder (ANSD), distinguishing it from cochlear hair cell loss
• Recorded on ABR as a response that alternates phase when the stimulus polarity is alternated (rarefaction vs condensation clicks)
• CM is absent/reduced in ototoxicity and noise-induced damage (OHC loss)

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b) Role of Otoacoustic Emissions in Neonatal Hearing Screening [3]

1. Transient Evoked OAE (TEOAE) - most commonly used for screening; evoked by click stimuli; tests 1-4 kHz range
2. Distortion Product OAE (DPOAE) - uses two pure tones (f1, f2); tests a wider frequency range (0.5-8 kHz); useful for monitoring ototoxicity

• OAEs are generated by functioning OHCs and are absent when hearing loss exceeds 25-30 dB HL
• Screening test of choice in NICU and well-baby nurseries due to ease of use, no subjective response needed, quick (< 5 min), non-invasive
• A pass result indicates OHC function is intact; a refer result triggers follow-up with AABR (Automated Auditory Brainstem Response)
• OAEs cannot detect auditory neuropathy alone - AABR is required for NICU babies
• Combined TEOAE + AABR protocol is used in NICU infants (two-stage screening)

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c) High-Risk Neonatal Hearing Screening vs Universal Neonatal Hearing Screening, and why UNHS should be in NPPCD [5]

• NICU stay > 5 days
• Family history of permanent childhood hearing loss
• Craniofacial anomalies
• TORCH infections (CMV, rubella, herpes, toxoplasmosis)
• Hyperbilirubinemia requiring exchange transfusion
• Ototoxic medications (aminoglycosides, loop diuretics)
• Birth asphyxia / low APGAR scores
• Syndromes associated with hearing loss (Down, Waardenburg, Pendred, etc.)
• Bacterial meningitis

• Only 50% of children born with hearing impairment have identifiable risk factors - the other 50% are missed by targeted programs
• UNHS ensures mean age of identification is < 3 months (vs 14-30 months in non-screened populations)
• Allows intervention (hearing aids, cochlear implants) before the critical period of language development (0-3 years)
• The 1-3-6 principle: diagnose by 1 month, confirm by 3 months, intervene by 6 months

1. Prevalence of congenital hearing loss in India is 1-6 per 1000 live births
2. Universal screening identifies 100% of affected infants vs ~50% with targeted programs
3. Early amplification and auditory-verbal therapy leads to near-normal speech and language outcomes
4. Cost-effectiveness: early intervention is far cheaper than lifetime special education
5. Infrastructure for screening (OAE devices) is now available even at secondary-level hospitals
6. UNHS aligns with the WHO global disability action plan 2014-2021

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Question 2 [10 Marks]

a) Indications for Intraoperative Facial Nerve Monitoring [3]

• Surgery in a field where the facial nerve has been displaced from its normal anatomy (revision mastoid surgery, previously operated parotid)
• Cerebellopontine angle (CPA) tumors - vestibular schwannoma, meningioma
• Any surgery where the nerve cannot be directly visualized throughout (e.g., large acoustic neuroma)

• Parotid surgery - parotidectomy, especially deep lobe or revision; parotid malignancies
• Middle ear/mastoid surgery - revision tympanomastoidectomy, canal wall down mastoidectomy for cholesteatoma (where nerve may be dehiscent or displaced by disease)
• Temporal bone resection for cancer
• Surgery for skull base tumors (glomus jugulare, glomus tympanicum)
• Facial nerve decompression or repair surgery
• Second-look surgery after previous facial nerve trauma
• Large acoustic neuroma surgery (where NF-2 is suspected)
• Cochlear implantation in cases of anatomical distortion
• Pediatric cases (difficult anatomy)

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b) Anaesthesia Modifications Required During Intraoperative Facial Nerve Monitoring [3]

1. Avoidance of long-acting neuromuscular blocking agents (NMBAs): Total neuromuscular blockade abolishes EMG responses. Succinylcholine may be used for intubation only (short-acting). No vecuronium, rocuronium, or pancuronium after intubation.
2. TIVA (Total Intravenous Anaesthesia): Preferred - uses propofol + remifentanil infusion. Volatile agents are acceptable but muscle relaxants must be avoided.
3. Train-of-four (TOF) monitoring: Anesthesiologist must confirm TOF ratio of 3-4 twitches (≥75%) before neuromonitoring begins. Some centers require TOF = 4/4 (complete reversal).
4. Communication: Surgeon and anesthesiologist must coordinate - the surgeon must be notified before any muscle relaxants are given, and anesthesiologist should be notified when monitoring is active.
5. Temperature and hemodynamic stability: Hypothermia and severe hypotension impair nerve conduction and monitoring accuracy.
6. Electrode placement: Facial EMG electrodes placed in orbicularis oculi and orbicularis oris before draping; no electrical interference from diathermy during monitoring.

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c) Management of Surgical Facial Nerve Trauma with Loss of Signal on Monitor Intraoperatively [4]

1. Stop surgical manipulation in the area
2. Check for technical failure first (electrode displacement, interference) - re-test by direct nerve stimulation proximal to injury site
3. Assess type of injury: stretch/compression vs partial transaction vs complete transaction

• Grade I (neuropraxia): conduction block, recovers fully
• Grade II (axonotmesis): axon disrupted, good recovery
• Grades III-V: increasing severity, poor spontaneous recovery

• Document the event clearly in the operative note
• Inform patient and family about the injury
• Start high-dose steroids (dexamethasone 8-16 mg IV initially)
• Eye care: lubricating drops, taping at night, moisture chamber to prevent corneal exposure
• Refer to neuro-rehabilitation / physiotherapy
• EMG at 3 weeks to assess Wallerian degeneration
• Follow-up nerve conduction studies and ENOG to guide prognosis

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Question 3 [10 Marks]

a) Keratosis Obturans Presenting with Facial Nerve Palsy [4]

• Severe deep-seated otalgia (acute, severe - distinguishes from EAC cholesteatoma)
• Progressive conductive hearing loss
• Mass of white keratin in a widened bony EAC
• Associated with bronchiectasis and sinusitis (in ~50% of cases - aetiology unclear, possibly abnormal epithelial migration)
• Age: usually young adults

• Accumulating keratin plug expands circumferentially, eroding the bony EAC
• The tympanic segment of the facial nerve, which lies in the floor of the middle ear and can be dehiscent (10-15% normal population), may be exposed by progressive erosion
• Expanded keratin mass can directly compress the facial nerve through a dehiscent fallopian canal, particularly in the tympanic segment
• Unlike EAC cholesteatoma, keratosis obturans more commonly erodes circumferentially (posterosuperior canal wall) rather than along a localized front
• Presents as progressive lower motor neuron facial palsy

• Removal of keratin plug under general anaesthesia (if large/impacted)
• Regular aural toilet and follow-up
• If facial palsy: urgent CT temporal bone to assess extent of erosion
• Surgical decompression if nerve compressed
• Hearing aids if residual CHL

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b) Recurrent Preauricular Sinus Infection After Surgery [3]

1. Incomplete excision of the sinus tract (most common cause)
2. Failure to excise medial extensions or ramifications of the tract
3. Intraoperative entry into the sinus (contamination of wound)
4. Failure to excise surrounding fibrosis and infected tissue from previous attacks
5. Inadequate identification of sinus tract anatomy (the tract can be longer than anticipated)
6. Not using supra-auricular approach, which provides better visualization

• "Sinectomy" - complete excision of the entire tract with a cuff of surrounding tissue
• Supra-auricular approach (Prasad/Satish Prasad technique) - most recommended; provides wide exposure, superior to conventional approaches in recurrence rates
• Methylene blue injection into the sinus helps trace the tract intraoperatively
• Excision should extend to the cartilage of the helix superiorly and the tragus anteriorly
• Never excise during an acute infection - treat with antibiotics first, then elective surgery after 6-8 weeks

1. Treat acute infection with antibiotics (Amoxicillin-clavulanate or co-amoxiclav)
2. Incision and drainage if abscess formed
3. Definitive re-excision after complete resolution using supra-auricular approach with wider excision margins
4. Inform patient: re-excision has higher chance of bleeding, nerve damage (great auricular)

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c) Tympanosclerosis of the Middle Ear [3]

• Begins as subepithelial hyalinization of the fibrous layer of the tympanic membrane (myringosclerosis) or middle ear mucosa
• Calcification and ossification of the deposits
• Can involve: tympanic membrane (myringosclerosis), promontory, ossicular joints, round window niche, oval window niche, tensor tympani muscle tendon

• Mild to moderate conductive hearing loss (CHL)
• Tympanic membrane may appear chalky-white (plaques)
• Variable tympanogram (As - stiffened, or normal)
• Ossicular fixation leads to maximum CHL (~60-65 dB)
• Middle ear mucosa may appear white and thickened on otoscopy

• CHL, AB gap
• Type As (stiff) tympanogram
• Absent or reduced stapedial reflexes

• Tympanotomy and removal of tympanosclerotic deposits
• Oval window involvement is the most serious and surgically challenging - removal risks perilymph fistula and profound SNHL
• Ossicular reconstruction after clearance: incus interposition, partial or total ossicular replacement prosthesis (PORP/TORP)
• When oval window is involved: stapes surgery risks (similar to stapedectomy) must be consented
• Post-surgical results: fair to good when oval window not involved; guarded when it is

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Question 4 [10 Marks]

a) Neural Plasticity and Neural Scavenging [2]

• Critical period: The central auditory system is most plastic in the first 3-5 years of life
• Use-it-or-lose-it principle: Prolonged auditory deprivation leads to tonotopic reorganization in the auditory cortex - neurons previously responding to deprived frequencies are "taken over" by adjacent frequencies
• Clinical relevance in cochlear implantation: Longer duration of deafness leads to greater cortical reorganization and poorer speech outcomes post-implant; hence early implantation (before 12-18 months) is vital
• Cross-modal plasticity: In profound deafness, auditory cortex neurons may be recruited for visual/tactile processing - this can reduce benefit from cochlear implantation in late-implanted patients

• The extent of cross-modal scavenging can predict cochlear implant outcome
• Auditory deprivation for > 5 years significantly reduces cortical auditory responsiveness
• High levels of neural scavenging = poor speech perception outcomes post-implant

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b) Diagnosis and Management of Auditory Neuropathy [2+6 = 8 Marks]

• Inner hair cell dysfunction
• Synaptic pathology (ribbon synapse) - most common
• Auditory nerve demyelination/pathology (e.g., CHARGE, NICU babies with hyperbilirubinemia)

• Neonatal hyperbilirubinemia (kernicterus)
• Prematurity, hypoxia
• Genetic: OTOF gene mutation (otoferlin), PJVK, DFNB9
• Auditory nerve hypoplasia
• Immune-mediated
• Infectious (CMV)

1. OAE present - confirms OHC function
2. CM present on ABR testing (identified by alternating stimulus polarity)
3. ABR absent or grossly abnormal
4. CT/MRI temporal bone - rule out cochlear nerve hypoplasia/aplasia (important before implantation)
5. Genetic testing (OTOF mutation - good CI candidate)

1. Mild to moderate ANSD:
   • Trial of hearing aids - benefit is unpredictable (aided sound reaches cochlea but neural synchrony is disrupted)
   • FM systems to improve signal-to-noise ratio
   • Auditory verbal therapy
2. Severe to profound ANSD:
   • Cochlear implantation is the treatment of choice
   • Excellent outcomes in OTOF-related ANSD (pre-neural pathology) - CI bypasses the dysfunctional inner hair cell/synapse and directly stimulates the auditory nerve
   • Poor outcomes if lesion is in the auditory nerve itself (e.g., cochlear nerve hypoplasia, demyelination)
   • Pre-implant MRI is mandatory to confirm cochlear nerve presence
3. Monitoring and counseling:
   • Regular audiological follow-up
   • Speech-language therapy
   • School support and FM systems
   • Genetic counseling if OTOF mutation confirmed
4. Contraindication to CI:
   • Absent/hypoplastic cochlear nerve on MRI (auditory brainstem implant may be considered instead)

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Question 5 [10 Marks]

Hidden Areas of the Middle Ear Cleft [3]

• CWD converts the mastoid and EAC into a single cavity, providing wide exposure
• Allows direct visualization of sinus tympani, retrofacial cells, and hypotympanum
• Lower residual/recurrence rate for cholesteatoma (especially sinus tympani disease)
• Easier to monitor post-operatively and spot recurrences
• Disadvantage: cavity maintenance required (regular syringing/cleaning), hearing rehabilitation more difficult

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Management of Sinus Tympani Cholesteatoma Going Beyond the Vertical Facial Nerve [4]

• High-resolution CT temporal bone (1 mm cuts): assess extent, erosion of facial canal, labyrinthine fistula, sigmoid sinus exposure
• MRI (DWI - diffusion-weighted) to detect non-echo planar DWI positive signal indicating residual cholesteatoma matrix
• Assess hearing: pre-op audiogram (PTA + tympanometry)

1. Canal Wall Down (Modified Radical) Mastoidectomy - preferred approach:
   • Provides the widest access to the posterior tympanum, sinus tympani, and retrofacial cells
   • Identifies facial nerve throughout its tympanic and mastoid segments
   • Allows anterior extension through the facial recess
   • Direct access to disease beyond the vertical facial nerve
2. Transcanal Endoscopic Ear Surgery (TEES) - Endoscopic approach:
   • 0-degree and 45-degree rigid endoscopes provide panoramic visualization of sinus tympani medial to facial nerve
   • Combined endoscopic-microscopic approach: microscope for mastoid, endoscope for sinus tympani residual disease
   • Preferred by modern endoscopic ear surgeons for disease in the sinus tympani
3. Infra-cochlear approach / Retrofacial approach:
   • Used when disease extends into the infralabyrinthine compartment below the cochlea
   • Requires careful dissection along the stylomastoid foramen

• Identify facial nerve (tympanic and mastoid segments)
• Preserve chorda tympani if possible
• Follow disease into sinus tympani with angled instruments (45-degree hook)
• Endoscopes (45-degree) are invaluable to "look around the corner" behind the facial nerve
• Ensure complete matrix removal - residual matrix = recurrence
• Consider second-look surgery at 9-12 months (or non-echo planar DWI MRI)

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Question 6 [10 Marks]

a) Auditory Effects of Noise Pollution [4]

• Transient elevation of hearing threshold after noise exposure, recovering within 16-24 hours
• Represents reversible metabolic exhaustion of OHCs (metabolic fatigue, ischemia, depletion of glycogen)
• Maximal at 4 kHz (c-5 notch or 4 kHz notch)
• Complete recovery after adequate rest from noise

• Irreversible SNHL from permanent destruction of OHCs (especially basal turn)
• Hallmark: 4 kHz notch on audiogram (corresponds to the 3 kHz region of maximal basilar membrane movement in response to 4 kHz stimulation; the notch may extend to 3 and 6 kHz)
• Initially insidious, bilateral, symmetrical
• Progress: 4 kHz affected first, then extends to 2 kHz and 8 kHz
• Associated tinnitus (high-pitched, persistent)
• No pain (unlike acoustic trauma)

• Single exposure to extremely loud impulse noise (blast injury, firearms)
• Can cause tympanic membrane rupture, ossicular disruption, and inner ear hemorrhage
• Immediate and often severe SNHL

• Cardiovascular: hypertension, tachycardia, raised cortisol
• Psychological: stress, anxiety, sleep disturbance
• Cognitive: reduced concentration, reduced productivity

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b) Hearing Conservation Programme (HCP) for Occupational NIHL [4]

• Identify high-noise areas using sound level meters and dosimeters
• Calculate Time-Weighted Average (TWA) - threshold for action: 85 dB(A) over 8 hours

• Machine enclosures, acoustic insulation, damping
• Substitution with less noisy equipment
• Distance and barriers (increasing distance from source by doubling = -6 dB reduction)

• Job rotation to limit individual exposure time
• Limit working hours in noisy environments
• Schedule noisy tasks during low-occupancy periods

• Earplugs (foam, pre-molded, custom): attenuation 20-35 dB
• Earmuffs: attenuation 25-40 dB
• Combined (plug + muff): for very high noise environments
• NRR (Noise Reduction Rating) must be checked; real-world attenuation = NRR/2

• Baseline audiogram before employment
• Annual monitoring audiograms (PTA at 0.5, 1, 2, 3, 4, 6 kHz)
• Identification of Standard Threshold Shifts (STS): ≥10 dB change at 2, 3, or 4 kHz (OSHA)
• Workers with STS: re-evaluate HPD use, consider job reassignment

• Training on NIHL risks and proper use of HPDs
• Regular reinforcement sessions

• Maintain noise exposure records, audiograms, and medical records for at least 30 years

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c) 60% Rule [2]

• Listen at no more than 60% of maximum volume
• For no more than 60 minutes per day
• This limits the risk of NIHL from recreational noise (headphones, earbuds)

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Question 7 [10 Marks]

a) Difference Between Meniere's Disease and Meniere's Syndrome [3]

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b) Aetiopathogenesis of Meniere's Disease [3]

1. Overproduction of endolymph:
   • Increased secretion by the stria vascularis
   • Impaired reabsorption by the endolymphatic sac
2. Endolymphatic sac dysfunction:
   • The endolymphatic sac (in the posterior fossa dura) is responsible for endolymph reabsorption and immune function
   • Genetic hypoplasia or functional insufficiency of the sac leads to accumulation
   • Histologically: a smaller endolymphatic sac is found in some Meniere's patients
   • Viral infection (CMV, herpes simplex) may damage the sac
3. Autoimmune theory:
   • Evidence of antibodies against type II collagen and endolymphatic sac antigens
   • Response to steroids supports immune-mediated component
4. Membranous rupture hypothesis (Schuknecht):
   • Distended membranes rupture periodically
   • K+-rich endolymph floods the perilymph space
   • Depolarizes hair cells and vestibular neurons - precipitating an acute attack
   • Rupture heals, explaining the episodic nature
5. Genetic factors:
   • Family history in 10-15% cases
   • Associations with HLA-Cw7, aquaporin gene variants (AQP2, AQP4 regulate water transport in inner ear)
6. Migraine association:
   • Strong epidemiological link with migraine; shared vasospastic mechanism proposed
7. Ion transport dysregulation:
   • Abnormal Na+/K+ ATPase activity in stria vascularis affects endolymph composition

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c) Role of Intratympanic Drug Therapy in Meniere's Disease [4]

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• Mechanism: Aminoglycoside preferentially toxic to type I vestibular hair cells of the saccule and cristae ampullaris - ablates vestibular function
• Goal: Vestibular ablation to control episodic vertigo
• Titration protocols:
  • Fixed-dose protocol: 1-3 injections on a fixed schedule (e.g., weekly x 3)
  • Titration protocol: Single injection; repeat only if vertigo persists after 4-6 weeks (lower risk of SNHL)
• Efficacy: 70-90% control of episodic vertigo
• Risk: SNHL (5-35% depending on protocol), imbalance (managed with vestibular rehabilitation)
• Indication: Unilateral Meniere's with disabling vertigo, failed medical therapy, serviceable hearing (though hearing not preserved)
• Not for bilateral disease (risk of bilateral vestibular loss)

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• Mechanism: Anti-inflammatory, immunosuppressive; may reduce endolymphatic hydrops; improves stria vascularis function; upregulates ion channels
• Goal: Control of vertigo + potential hearing preservation
• Protocol: 4 mg/mL or 16 mg/mL; 0.5-1 mL injected through tympanic membrane; patient supine with head tilted 45 degrees, round window down for 30 min; weekly x 4-5 injections
• Efficacy: Vertigo control ~60-70%; some evidence for hearing improvement and tinnitus reduction
• Advantages over gentamicin: No risk of hearing loss, safe in bilateral disease
• Limitations: Short-lived effect, may require repeat courses; response variable; no definitive evidence it alters long-term course of disease (Cummings, Otolaryngology)
• Indication: Patients with serviceable hearing wishing to preserve it; bilateral Meniere's; failed dietary/medical management

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1. Topical anesthetic to TM (EMLA cream/phenol)
2. 25G spinal needle through posteroinferior quadrant of TM
3. Inject drug with patient supine, head turned 45 degrees
4. Patient instructed not to swallow for 30 minutes (prevents drug drainage via Eustachian tube)

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Question 8 [10 Marks]

a) Superior Semicircular Canal Dehiscence Syndrome (SSCDS) [5]

• Minor bony thinning over the SSC is found in ~13% of the population; symptomatic dehiscence is much rarer
• Proposed aetiology: incomplete pneumatization of the petrous bone / developmental failure

• Sound entering via oval window can escape through both round window AND the dehiscence
• This explains the auditory symptoms
• Pressure changes (Valsalva, jugular compression) transmitted to the dehiscence, stimulating the SSC ampulla - explains vestibular symptoms

• Autophony (hearing own voice, heartbeat abnormally loud in affected ear)
• Low-frequency apparent conductive hearing loss (pseudo-CHL)
• Hyperacusis to bone-conducted sounds
• Pulsatile tinnitus

• Tullio phenomenon: vertigo and oscillopsia evoked by loud sounds
• Hennebert's sign: vertigo evoked by pressure changes (Valsalva, nose blowing, pressing on tragus)
• Chronic disequilibrium and oscillopsia

1. Middle fossa approach (transmiddle cranial fossa):
   • Standard approach
   • Identifies and plugs the dehiscence with bone wax, fascia, or bone chips
   • Alternatively: resurfacing the dehiscence
2. Transmastoid approach:
   • Used when middle fossa approach is not feasible
   • Occludes the canal through mastoid
3. Conservative: Avoidance of triggers (nose blowing, heavy lifting); reassurance if symptoms mild

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b) Tuberculosis of the Temporal Bone [5]

• Haematogenous spread from primary pulmonary/lymph node TB
• Direct extension from nasopharyngeal TB via Eustachian tube
• Rarely: direct inoculation

1. Multiple perforations of tympanic membrane (later coalesce into large central perforation)
2. Pale, "watery" (non-offensive) aural discharge - profuse
3. Early and severe facial nerve palsy (more common than in CSOM)

• Young patient (children, young adults)
• Painless, profuse, watery aural discharge
• Pale granulations in middle ear (not vascular red granulations of CSOM)
• Facial palsy disproportionate to the degree of middle ear disease
• Rapidly progressive, severe hearing loss
• Failure to respond to conventional antibiotics
• Multiple TM perforations (pathognomonic)
• Evidence of systemic TB (lymphadenopathy, pulmonary TB on CXR)

• Mastoiditis (coalescent, Bezold's abscess can occur)
• Labyrinthitis - severe SNHL, vertigo
• Facial nerve paralysis (most feared complication) - due to granulomatous involvement of facial canal; occurs earlier than in CSOM
• Meningitis (transcanal/haematogenous)
• Osteomyelitis of petrous bone

1. Biopsy of granulation tissue from middle ear - caseating granulomas with Langhans giant cells (gold standard)
2. ZN stain / Auramine stain for AFB (low sensitivity due to paucibacillary disease)
3. Culture on LJ medium (takes 6-8 weeks)
4. CBNAAT/GeneXpert - rapid molecular diagnosis, also detects rifampicin resistance
5. Mantoux test / IGRA (Quantiferon-TB Gold)
6. CT temporal bone - mastoid sclerosis, bone destruction, multiple erosions; no specific radiological features
7. CXR / CT chest - pulmonary focus

1. Anti-tubercular therapy (ATT) is the mainstay:
   • Intensive phase: Isoniazid (H) + Rifampicin (R) + Pyrazinamide (Z) + Ethambutol (E) - 2 months
   • Continuation phase: H + R - 4 months (total 6 months for uncomplicated cases)
   • Duration may be extended to 9-12 months for severe bone involvement
2. Surgery: Only adjunctive - rarely needed primarily for TB
   • Mastoidectomy if coalescent mastoiditis, abscess formation
   • Facial nerve decompression if facial palsy fails to resolve on ATT
   • Myringoplasty after ATT is completed (at least 6 months post-treatment) for residual perforation
3. Facial palsy in TB otitis: May resolve with ATT alone; decompression reserved for non-recovery after adequate ATT

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Question 9 [10 Marks]

a) Surgical Anatomy of the Petrous Apex (with Diagram) [3]

• Anterolateral: otic capsule (labyrinth)
• Anterior: carotid canal (internal carotid artery)
• Posterior: posterior cranial fossa (dura)
• Superior: trigeminal impression (Meckel's cave - V ganglion)
• Medial: clivus / petro-occipital suture
• Inferior: jugular fossa, eustachian tube

1. Internal carotid artery (petrous segment) - anteromedial
2. Internal auditory canal (IAC) - posterolateral
3. Eustachian tube - inferior and medial
4. VI (abducens) nerve - crosses the petrous apex at Dorello's canal (under Gruber's ligament)
5. Trigeminal ganglion (Meckel's cave) - superior
6. Cochlea - lateral
7. Endolymphatic duct - runs to the posterior fossa

         [Middle Cranial Fossa Dura]
              CN V (trigeminal)
[IAC - CN VII, VIII] ← Petrous Apex → [ICA]
         [Cochlea]        [ET tube]
              [Posterior Fossa]

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b) Clinical Features of Petrous Apex Cholesteatoma and Surgical Approaches [3+4]

• Primary (congenital): From embryonic epithelial rests
• Secondary (acquired): Extension from cholesteatoma of the middle ear/mastoid via infralabyrinthine or translabyrinthine route

1. Otitis media / otorrhea
2. VI nerve palsy (abducens paralysis = diplopia)
3. Retro-orbital/periorbital pain (trigeminal - V1 and V2)

• CT temporal bone: expansile, non-enhancing lytic lesion with smooth scalloped bony margins
• MRI: T1 hypointense, T2 hyperintense; DWI (diffusion-weighted imaging) positive - pathognomonic for cholesteatoma (restricted diffusion - high signal on DWI, low ADC)
• Audiogram: SNHL or mixed HL
• CN examination

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• Indications: Disease inferior to the cochlea (infralabyrinthine compartment), serviceable hearing
• Access: Through mastoid, below the posterior semicircular canal, between the labyrinth (superior) and jugular bulb (inferior)
• Landmarks: Facial nerve (posterior), jugular bulb (inferior), cochlea (anterosuperior)
• Can access petrous apex cells without disturbing cochlea/labyrinth
• Hearing preservation possible

• Similar to infralabyrinthine but goes below the cochlea
• Requires careful dissection near the ICA
• Good for anterior-inferior petrous apex
• Hearing preservation possible

• Indications: Superior petrous apex, good hearing, no labyrinthine involvement
• Requires temporal craniotomy
• High risk to CN VII, greater petrosal nerve, ICA
• Good visualization of the tegmen and superior petrous apex

• Indication: Non-serviceable hearing (profound SNHL) or large posterior extension
• Sacrifices hearing (removes labyrinth)
• Excellent access to posterior petrous apex and IAC
• Low risk to ICA; good facial nerve exposure

• For very anterior, large lesions
• Sacrifices hearing and cochlea
• Wide anterior access including ICA

• Combines elements of middle fossa and translabyrinthine

• Hearing present + inferior lesion: Infralabyrinthine or infracochlear
• Hearing present + superior lesion: Middle fossa
• No hearing + large posterior lesion: Translabyrinthine
• Very anterior lesion: Transcochlear or infracochlear

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Question 10 [10 Marks]

a) Overall Survival Rate and How It Is Calculated [3]

OS at time T = (Number of patients alive at T) / (Total number of patients at start) × 100%

• Divides follow-up into intervals (usually years)
• Calculates probability of surviving each interval
• Accounts for patients "lost to follow-up" (censored observations)
• Formula per interval: p = (alive at start - died during interval) / (alive at start - ½ censored)
• Cumulative OS = product of all interval survival probabilities

• Non-parametric method that calculates survival probability at each event time (death)
• Handles censored data (patients lost to follow-up or alive at end of study) correctly
• Produces the characteristic step-down Kaplan-Meier survival curve
• Median survival = time at which the curve crosses 50%
• Curves compared between groups using log-rank test

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b) Case-Control Study with a Suitable Example [3]

                    Exposed → [Cases] ← Unexposed
Time → Backwards   
                    Exposed → [Controls] ← Unexposed

• Retrospective (looks back in time)
• Starts with outcome, searches for exposure
• Relatively quick and inexpensive
• Good for rare diseases and diseases with long latency

• Cannot calculate relative risk (incidence not known - sampled design)
• OR ≈ RR when disease is rare (< 10% prevalence)

• Cases: 100 newly diagnosed oral cavity SCC patients
• Controls: 100 age and sex-matched patients without oral cancer (e.g., from orthopedic OPD)
• Interview both groups for past tobacco smoke/chewing exposure
• Calculate OR:

• Good for rare diseases (e.g., acoustic neuroma, petrous apex cholesteatoma)
• Time and cost efficient
• Can study multiple exposures simultaneously

• Selection bias (choice of controls)
• Recall bias (cases may recall exposure differently from controls)
• Cannot establish incidence or absolute risk
• Temporality may be difficult to establish

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c) Difference Between Incidence and Prevalence [4]

Prevalence ≈ Incidence × Average Duration of Disease
P = I × D

• A disease with short duration (acute, rapidly fatal, or quickly cured) will have low prevalence relative to incidence
• A chronic disease (e.g., presbycusis, chronic otitis media) will have high prevalence relative to incidence
• A new effective treatment that CURES disease rapidly reduces prevalence but not incidence
• A new treatment that PROLONGS LIFE without curing increases prevalence

• NIHL: Incidence = 2 new cases per 1000 workers per year; Prevalence = 150 per 1000 workers (because it is permanent and accumulates over a lifetime of work)

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• Q1 (Pages 1-5): Write CM + OAE parts first, then UNHS vs HRNS with the 1-3-6 principle highlighted
• Q2 (Pages 6-10): The intraoperative management table is exam-friendly - draw it out with scenarios
• Q3 (Pages 11-15): For keratosis obturans, include the comparison table with EAC cholesteatoma - examiners love differentials
• Q4 (Pages 16-20): Neural plasticity answer ties directly into why early cochlear implantation matters - make that link explicit
• Q5 (Pages 21-25): Hidden areas question - list all 6 recesses in a structured format; use the endoscope point for sinus tympani
• Q6 (Pages 26-30): HCP components can be written as a numbered list with bold headings for clarity
• Q7 (Pages 31-35): The IT injection table (gentamicin vs dexamethasone) is the scoring part - present it clearly
• Q8 (Pages 36-40): SSCDS - always include the VEMP threshold finding (< 70 dB nHL) and Tullio/Hennebert signs; TB - the multiple perforations and facial palsy association are high-value points
• Q9 (Pages 41-45): Draw the petrous apex diagram as a simple box/schematic showing ICA, IAC, cochlea, CN V relationships; include Gradenigo's triad for cholesteatoma features
• Q10 (Pages 46-50): The incidence-prevalence relationship formula (P = I × D) and the Kaplan-Meier censoring concept are standard scoring points