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Neurobiology of the Eye and Ear

2-Hour Medical Student Session Skeleton

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SESSION AT A GLANCE

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BLOCK 0 - HOOK & OBJECTIVES (5 min)

• Show a patient with sudden unilateral vision loss (central scotoma) alongside a patient with sudden sensorineural hearing loss - "same mechanism, different organ - why?"
• Pose: "What does a photon and a 1000 Hz sound wave have in common by the time they reach your cortex? They're both action potentials."

1. Trace light from cornea to occipital cortex and name every transduction step
2. Explain the molecular cascade of phototransduction in rods and cones
3. Describe the structure of the retina and its five cell types with functional roles
4. Explain how the cochlea performs frequency analysis (place coding + rate coding)
5. Describe hair cell mechanotransduction including the role of tip links and stereocilia
6. Trace the auditory pathway from CN VIII to primary auditory cortex (A1)
7. Apply these principles to clinical deficits (visual field cuts, conductive vs. sensorineural deafness)

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BLOCK 1 - THE EYE: FROM OPTICS TO PHOTOTRANSDUCTION (35 min)

1A. Gross Anatomy & Optics of the Eye (8 min)

• Outer coat: cornea (provides ~70% of refractive power) + sclera
• Iris/pupil: the aperture - miosis (parasympathetic via CN III, sphincter pupillae) vs. mydriasis (sympathetic, dilator pupillae)
• Lens: crystalline, biconvex, suspended by zonule fibers from ciliary body
  • Accommodation: ciliary muscle contracts → zonules relax → lens rounds up → increases power (near focus)
  • Presbyopia: loss of lens elasticity with age
• Vitreous humor (posterior) vs. aqueous humor (anterior): refractive media
• Retina: fovea centralis (highest acuity, cone-dense), optic disc (blind spot, no photoreceptors)

• Myopia: eyeball too long or cornea too curved → image falls anterior to retina → concave lens corrects
• Hyperopia: eyeball too short → image behind retina → convex lens corrects
• Astigmatism: non-spherical corneal curvature → cylindrical lens corrects

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1B. Retinal Architecture - The 5 Cell Types (10 min)

• Müller cells act as fiber optic guides, channeling light to the photoreceptors
• RPE (retinal pigment epithelium): absorbs stray photons, regenerates retinal (visual pigment cycle)

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1C. Phototransduction in Rods - The Molecular Cascade (12 min)

• Rod outer segment: cGMP-gated cation channels are OPEN
• Constant influx of Na⁺ and Ca²⁺ → "dark current" depolarizes the rod to ~-40 mV
• Rod is continuously releasing glutamate onto bipolar cells

1. Photon absorption → retinal isomerizes from 11-cis-retinal to all-trans-retinal
2. Rhodopsin (opsin + retinal) is activated → becomes metarhodopsin II
3. Activated rhodopsin activates transducin (G protein, Gαt) → GTP replaces GDP on Gαt
4. Gαt-GTP activates phosphodiesterase (PDE)
5. PDE hydrolyzes cGMP → 5'-GMP (cGMP levels fall)
6. cGMP-gated channels CLOSE → Na⁺/Ca²⁺ influx stops
7. Rod hyperpolarizes to ~-70 mV
8. Glutamate release decreases → signals change at bipolar cells

• Arrestin binds metarhodopsin II → quenches signal
• Rhodopsin kinase phosphorylates rhodopsin (inactivation)
• Guanylyl cyclase regenerates cGMP (Ca²⁺-dependent; as Ca²⁺ falls when channels close, GC is disinhibited)
• RPE: all-trans-retinal → 11-cis-retinal recycled (visual cycle; takes minutes - hence dark adaptation lag)

• Same cascade but with different opsins: Short (S, ~420nm/blue), Medium (M, ~530nm/green), Long (L, ~560nm/red)
• Young-Helmholtz trichromacy: color perception arises from comparing relative activation of S, M, L cones
• Opponent-color processing: red-green and blue-yellow opponent channels in ganglion cells and LGN

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1D. ON/OFF Bipolar Cells - The First Processing Step (5 min)

• ON bipolar cells: use mGluR6 (metabotropic) → paradoxically, less glutamate (in light) → DEPOLARIZES them. They detect light onset.
• OFF bipolar cells: use AMPA/kainate (ionotropic) → less glutamate (in light) → hyperpolarizes them. They detect light offset.
• This center-surround architecture (mediated by horizontal cells) forms the basis of contrast detection
• Mach band phenomenon - lateral inhibition sharpens edges

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BLOCK 2 - RETINAL OUTPUT & CENTRAL VISUAL PATHWAYS (15 min)

2A. Retinal Ganglion Cells and the Optic Nerve (5 min)

• M-type (Magno): Large; respond to motion, low spatial freq, coarse contrast; project to LGN layers 1-2
• P-type (Parvo): Small; respond to fine detail, color, high spatial freq; project to LGN layers 3-6
• K-type (Konio): Chromatic info; project to blob regions of V1
• ipRGC (intrinsically photosensitive): Contain melanopsin; pupillary light reflex + circadian rhythm entrainment (project to suprachiasmatic nucleus and pretectal area)

2B. The Retinofugal Projection - Visual Pathway (7 min)

Retina → Optic nerve → Optic chiasm → Optic tract → LGN → Optic radiation → V1 (calcarine cortex)

• Nasal fibers (from nasal hemiretina = temporal visual field) cross to contralateral side
• Temporal fibers (from temporal hemiretina = nasal visual field) stay ipsilateral
• Net result: Each hemisphere receives the contralateral visual hemifield

• 6 layers: Layers 1-2 = Magnocellular (M-pathway); Layers 3-6 = Parvocellular (P-pathway)
• Layers 1, 4, 6: contralateral eye; Layers 2, 3, 5: ipsilateral eye
• Only relay; minimal processing here (~80% input is feedback from cortex, not retina)

• Located in calcarine sulcus (occipital lobe)
• Retinotopically organized; fovea = disproportionately large representation (cortical magnification)
• Simple cells (respond to oriented edges), Complex cells (motion + orientation), Hypercomplex cells
• Ocular dominance columns; orientation columns; cytochrome oxidase blobs (color)

• Ventral stream (V1 → V2 → V4 → IT): "What pathway" - object recognition, color, shape
• Dorsal stream (V1 → V2 → MT/V5 → posterior parietal): "Where/How pathway" - motion, spatial location, visuomotor guidance

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BLOCK 3 - BREAK + MCQ CHECKPOINT (10 min)

1. A patient has a bitemporal hemianopia. Where is the lesion? (Optic chiasm)
2. cGMP levels fall in a photoreceptor when light is applied. What happens to membrane potential? (Hyperpolarization - channels close)
3. Which cell type mediates the pupillary light reflex? (ipRGC via pretectal nucleus)

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BLOCK 4 - THE EAR: FROM SOUND WAVES TO HAIR CELL TRANSDUCTION (30 min)

4A. Anatomy of the Ear - Three Compartments (8 min)

• Pinna: collects and filters sound; provides directional cues (especially vertical)
• External auditory meatus: ~2.5 cm canal; resonates at ~3-4 kHz (amplifies speech frequencies)
• Tympanic membrane (eardrum): converts pressure waves to mechanical vibration

• Three ossicles: Malleus → Incus → Stapes (mnemonic: "My Incredibly Smart")
• Stapes footplate sits on oval window → transmits vibration to cochlea
• Impedance matching function: Air (low impedance) → fluid (high impedance). Without ossicles, 99.9% of energy would reflect. Ossicles amplify pressure ~22x (area ratio of tympanic membrane to oval window ~17:1 + lever action)
• Tensor tympani (CN V₃) + stapedius (CN VII): acoustic reflex - contract in response to loud sounds, dampen ossicular chain, protect cochlea (~50 dB protection but too slow for impulse noise)
• Eustachian tube: equalizes pressure; opens during swallowing/yawning

• Bony labyrinth (filled with perilymph, similar to ECF: high Na⁺, low K⁺)
• Membranous labyrinth inside it (filled with endolymph: high K⁺, low Na⁺ - like ICF)
• Contains: cochlea (auditory) + vestibular apparatus (semicircular canals + utricle/saccule)

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4B. The Cochlea - Frequency Analysis (10 min)

• Scala vestibuli (top): perilymph; connected to oval window
• Scala media (middle, = cochlear duct): endolymph; contains organ of Corti
• Scala tympani (bottom): perilymph; connected to round window
• Scala vestibuli + scala tympani are continuous at the helicotrema (apex)
• Reissner's membrane separates scala vestibuli from scala media
• Basilar membrane separates scala media from scala tympani

• Base: narrow, stiff → responds best to HIGH frequencies (~20,000 Hz)
• Apex: wide, floppy → responds best to LOW frequencies (~20 Hz)
• This is the basis of the place code for frequency (Bekesy's traveling wave theory - Nobel Prize 1961)
• Sound enters at oval window → pressure wave travels up scala vestibuli → around helicotrema → down scala tympani → dissipated at round window (round window bulges out as oval window is pushed in)

• Inner hair cells (IHC): ~3,500; the primary sensory transducers (95% of afferent fibers)
• Outer hair cells (OHC): ~12,000-15,000; act as mechanical amplifiers (electromotility via prestin)
• Tectorial membrane: gelatinous membrane overhanging hair cells; stereocilia of OHC are embedded in it; IHC stereocilia are NOT embedded (deflected by fluid movement)
• Supporting cells: Deiters' cells, pillar cells, Hensen's cells

• OHC express prestin (SLC26A5): voltage-sensitive motor protein
• When depolarized: OHC shorten; when hyperpolarized: elongate
• This active electromotility amplifies basilar membrane motion ~40-100x, allowing detection of sounds as quiet as 0 dB SPL
• Otoacoustic emissions (OAEs) are a clinical consequence - OHCs generate measurable sounds back out the ear; used in newborn hearing screening

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4C. Hair Cell Mechanotransduction (12 min)

• Each hair cell has a bundle of stereocilia (actin-filled, NOT true cilia) arranged in a staircase pattern (tallest → shortest)
• Tip links: extracellular filaments connecting the tip of a shorter stereocilium to the side of the next taller one
• Tip links are made of cadherin-23 and protocadherin-15 (CDH23, PCDH15)

1. Basilar membrane deflects upward (toward scala vestibuli) in response to sound
2. This shears the hair cell bundle against the tectorial membrane
3. Stereocilia deflect toward the tallest stereocilium (excitatory direction)
4. Tip links are pulled taut → mechanically gate MET channels (mechanoelectrical transduction channels, likely TMC1/TMC2)
5. K⁺ and Ca²⁺ rush in from endolymph (high K⁺, +80 mV endocochlear potential drives K⁺ in)
6. Hair cell depolarizes
7. Depolarization → opens voltage-gated Ca²⁺ channels at the basolateral membrane
8. Ca²⁺ influx triggers glutamate exocytosis from ribbon synapses onto spiral ganglion neuron dendrites (CN VIII)

• K⁺ leaves through basolateral channels → taken up by supporting cells → gap junctions → lateral wall → stria vascularis → re-secreted into endolymph. Loop is essential for maintaining endocochlear potential.
• Stria vascularis generates and maintains the +80 mV endocochlear potential (EP) - acts as a battery that drives K⁺ into hair cells. EP is the energy source for transduction.

• Deflection TOWARD tallest stereocilium → depolarization → increased firing
• Deflection AWAY from tallest stereocilium → hyperpolarization → decreased/silenced firing

• Place code (Tonotopy): which IHC is activated (position on basilar membrane) → encodes frequency
• Rate code: firing rate of spiral ganglion neuron → encodes intensity
• Phase locking: for low frequencies (<4 kHz), neurons fire at a specific phase of the sound wave → temporal code supplements place code

• Aminoglycoside ototoxicity: damages OHC (basal turn first → high-frequency loss first)
• Noise-induced hearing loss: OHC damage at the 4 kHz notch (resonance of ear canal + OHC vulnerability)
• Connexin 26 (GJB2) mutations: most common cause of congenital SNHL; disrupts K⁺ recycling
• Ménière's disease: endolymphatic hydrops → fluctuating low-frequency SNHL + vertigo

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BLOCK 5 - CENTRAL AUDITORY PATHWAYS (15 min)

5A. Ascending Auditory Pathway (8 min)

Organ of Corti
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Spiral ganglion neurons (bipolar, CN VIII) - cell bodies in modiolus
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Cochlear nuclei (dorsal + ventral) - FIRST RELAY - all ipsilateral
    ↓ (bilateral projections - multiple crossings)
Superior olivary complex (SOC) - FIRST BINAURAL CONVERGENCE
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Lateral lemniscus
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Inferior colliculus (midbrain tectum) - MANDATORY RELAY for all fibers
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Medial geniculate nucleus (MGN) of thalamus
    ↓
Primary auditory cortex (A1) - Heschl's gyri, superior temporal plane (Brodmann areas 41/42)

• All auditory input arrives here first
• Ventral cochlear nucleus → projects bilaterally to SOC
• Dorsal cochlear nucleus → projects contralaterally → bypasses SOC → lateral lemniscus

• Medial superior olive (MSO): detects interaural time differences (ITD) - horizontal plane; low-frequency sounds; neurons are coincidence detectors (Jeffress model)
• Lateral superior olive (LSO): detects interaural level differences (ILD) - intensity differences between ears; high-frequency sounds

• All ascending fibers converge here - mandatory integrating hub
• Tonotopically organized
• Also receives descending cortical input (corticofugal projection)
• Projects to superior colliculus (auditory-visual integration for orienting reflexes)

• Thalamic relay; tonotopically organized
• Receives input from IC
• Projects to A1 via auditory radiation

• Heschl's gyri on the superior temporal plane (planum temporale)
• Tonotopically organized: low frequencies anterolaterally, high frequencies posteromedially
• Core (A1) → Belt (A2) → Parabelt → association cortex
• Left hemisphere dominant for language processing (Wernicke's area - posterior superior temporal gyrus)

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5B. Auditory Reflexes (3 min)

• Loud sound → CN VIII → cochlear nucleus → facial motor nucleus (CN VII) → stapedius muscle contraction
• Bilateral (both ears stiffen even when only one ear stimulated)
• Clinical use: tympanometry + acoustic reflex testing localizes lesions in CN VII pathway

• Medial olivocochlear (MOC) neurons → directly suppress OHC gain
• Function: improves speech-in-noise detection; protects from acoustic trauma

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5C. Conductive vs. Sensorineural Hearing Loss (4 min)

• Normal / SNHL: Air conduction (AC) > Bone conduction (BC) = Rinne POSITIVE
• Conductive hearing loss: BC > AC = Rinne NEGATIVE

• Sound lateralizes to WORSE ear in conductive loss (BC bypasses blocked outer/middle ear)
• Sound lateralizes to BETTER ear in sensorineural loss

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BLOCK 6 - CLINICAL INTEGRATION & WRAP-UP (10 min)

Case 1 (3 min): The Pituitary Tumor

• 52-year-old presents with progressive peripheral vision loss and headaches. Visual field testing shows temporal field loss bilaterally.
• Localize: Optic chiasm (decussating nasal fibers)
• Cause: Pituitary adenoma pressing up on chiasm from below
• Expected MRI finding: sellar mass + chiasmal compression

Case 2 (3 min): The Factory Worker

• 45-year-old metalworker with 20 years of exposure to machinery noise. Audiogram shows a 4 kHz notch bilaterally. OAEs are absent.
• Localize: OHC damage at basal turn of cochlea (4 kHz resonance)
• Why no OAEs: OHC prestin-dependent amplification is gone
• Rinne/Weber: Rinne positive bilaterally; Weber doesn't lateralize (symmetric loss)

Case 3 (2 min): Quick rapid-fire

• Patient can't localize sounds in horizontal plane but hears pure tones normally → SOC (MSO) lesion
• Patient has night blindness and tunnel vision → Rod degeneration (retinitis pigmentosa)
• Patient has tinnitus, fullness, fluctuating low-frequency hearing loss, episodic vertigo → Ménière's disease (endolymphatic hydrops)

Closing summary (2 min) - "Principles to take home":

1. Both systems convert a physical stimulus into a graded receptor potential → AP via ion channel gating (cGMP channels in photoreceptors; MET channels in hair cells)
2. Both systems extract features at the receptor level before sending output centrally (retinal center-surround; cochlear tonotopy)
3. Both systems have hardwired amplifiers (OHC prestin; lateral inhibition in retina)
4. Both pathways have a mandatory thalamic relay (LGN for vision; MGN for hearing)
5. Both primary cortices are topographically organized (retinotopy; tonotopy)
6. Clinical localization depends on understanding the pathway anatomy

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TEACHING RESOURCES & PREP NOTES

Required Visuals/Slides:

• Cross-section of eye with labeled structures
• Retinal cell layer diagram (inverted retina + 5 cell types)
• Phototransduction cascade flowchart (dark → light)
• Visual pathway diagram with clinical lesion table
• Ear anatomy diagram (outer/middle/inner)
• Cochlear cross-section (3 scalae + organ of Corti)
• Basilar membrane tonotopic map (base to apex)
• Hair cell stereocilia bundle + tip link diagram
• Ascending auditory pathway schematic
• Visual field defect localization graphic (HIGH YIELD)
• Audiogram showing 4 kHz notch

Recommended Pre-Session Reading (assign before class):

• Neuroscience: Exploring the Brain (5th Ed) - Ch. 9 (The Eye) + Ch. 10 (Central Visual System) + Ch. 11 (The Auditory and Vestibular Systems)
• Costanzo Physiology Ch. (Special Senses)
• Ganong's Review - Vision + Hearing sections

Interactive Elements:

• Phototransduction cascade "fill in the blanks" worksheet (hand out blank diagram)
• Visual field defect localization MCQ at break
• Audiogram interpretation at the end
• Pair-and-share: "Rinne and Weber" clinical vignette

Common Student Pitfalls to Pre-empt:

1. Confusing hyperpolarization (rods in light) with depolarization - emphasize rods are UNUSUAL (depolarized in dark)
2. Optic chiasm anatomy - draw it slowly; many students flip nasal/temporal retina vs. visual field
3. Cochlear base = high frequency (counterintuitive since "base" sounds foundational/low)
4. Tip links pull open, not compress shut - mechanics of stereocilia deflection
5. Endolymph vs. perilymph ionic composition - endolymph is K⁺-rich like ICF (made by stria vascularis)

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Important Eye and Ear Disorders

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EYE DISORDERS

1. Glaucoma

• Open-angle glaucoma (most common): Trabecular meshwork resistance increases IOP gradually. Insidious onset - patients lose peripheral vision first and are often unaware until late.
• Angle-closure glaucoma: The iris physically occludes the anterior chamber angle. Can present as an acute emergency - sudden painful red eye, halos around lights, nausea/vomiting. More common in hyperopes (shallow anterior chamber).
• Normal-tension glaucoma: Optic nerve damage despite normal IOP - vascular insufficiency is implicated.
• Secondary glaucoma: Caused by neovascularization (from diabetic retinopathy/CRVO), uveitis, steroids, or trauma.
• Pathology: Diffuse retinal ganglion cell loss + thinning of the retinal nerve fiber layer + cupped, atrophic optic nerve.
• Robbins & Kumar Basic Pathology, p. 869

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2. Cataract

• Age-related (senile): Most common - nuclear sclerosis (yellowing/hardening of lens nucleus) from oxidative damage to lens proteins.
• Secondary causes: Diabetes (sorbitol accumulation via aldose reductase), corticosteroids (posterior subcapsular), galactosemia, Wilson's disease, radiation, trauma, uveitis.
• Congenital cataracts: Rubella infection (classic TORCH association), Down syndrome, metabolic disorders.
• Symptoms: Painless progressive blurring, glare, myopic shift ("second sight"), monocular diplopia.
• Treatment: Phacoemulsification with IOL (intraocular lens) implantation.
• Robbins & Kumar Basic Pathology, p. 867

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3. Age-Related Macular Degeneration (AMD)

• Dry AMD (~85%): Drusen deposits (extracellular debris between RPE and Bruch's membrane), geographic atrophy of RPE and photoreceptors. Gradual central vision loss.
• Wet AMD (~15%, but responsible for most severe vision loss): Choroidal neovascularization (CNV) - abnormal blood vessels grow through Bruch's membrane under the retina, leak fluid and blood → rapid central vision distortion/loss.
• Risk factors: Age, smoking (strongest modifiable risk), family history, complement factor H polymorphism (CFH Y402H).
• Symptoms: Metamorphopsia (distorted central vision), central scotoma. Peripheral vision spared.
• Treatment: Anti-VEGF intravitreal injections (ranibizumab, bevacizumab, aflibercept) for wet AMD; no proven therapy for dry AMD (AREDS supplements slow progression).

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4. Diabetic Retinopathy

• Nonproliferative diabetic retinopathy (NPDR): Microaneurysms (earliest sign - small red dots on fundoscopy), intraretinal hemorrhages (dot-blot), hard exudates (lipid deposits in outer plexiform layer), cotton-wool spots (nerve fiber layer infarcts from arteriole occlusion), macular edema (most common cause of visual loss in diabetics).
• Proliferative diabetic retinopathy (PDR): Retinal ischemia → VEGF upregulation → neovascularization on retinal surface and into vitreous. New vessels bleed easily → vitreous hemorrhage, tractional retinal detachment, neovascular glaucoma.
• Mechanism: Pericyte loss → capillary weakness → microaneurysms; basement membrane thickening; blood-retinal barrier breakdown.
• Screening: Annual dilated fundoscopy in all diabetics; OCT for macular edema.
• Treatment: Glycemic + BP control (prevention); laser photocoagulation (panretinal for PDR); anti-VEGF for macular edema/PDR; vitrectomy for complications.
• Robbins & Kumar Basic Pathology, p. 872

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5. Retinal Detachment

• Rhegmatogenous (most common): Full-thickness retinal tear/break; liquefied vitreous seeps behind retina. Risk factors: high myopia, posterior vitreous detachment, trauma, prior cataract surgery.
• Tractional: Fibrovascular membranes (from proliferative DR, sickle cell, ROP) pull retina off RPE - no tear.
• Exudative/Serous: Fluid accumulates under retina from choroidal tumors, severe hypertension, inflammatory conditions - no tear.
• Symptoms: Floaters, photopsia (flashes of light), then a "curtain/shadow" advancing across the visual field. Painless.
• Macula-on vs. macula-off: Macula-off detachment = central vision affected, worse prognosis for recovery.
• Treatment: Pneumatic retinopexy, scleral buckle, vitrectomy - urgency depends on macular involvement.
• Robbins & Kumar Basic Pathology, p. 871

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6. Retinitis Pigmentosa (RP)

• Genetics: Autosomal dominant (most common, e.g., rhodopsin mutations), autosomal recessive, X-linked; over 60 causative genes.
• Pathology: Rod degeneration starts in mid-peripheral retina → progressive peripheral vision loss → tunnel vision → eventual central vision loss (late).
• Hallmark findings: Bone-spicule pigmentation in mid-periphery on fundoscopy, attenuated retinal vessels, waxy pale optic disc, reduced/absent ERG (electroretinogram).
• Symptoms: Night blindness (nyctalopia) - EARLIEST symptom; progressive peripheral field constriction; eventual blindness.
• Associated syndromes: Usher syndrome (RP + deafness), Bardet-Biedl syndrome (RP + obesity + polydactyly), Kearns-Sayre (RP + CPEO + heart block).

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7. Central Retinal Artery Occlusion (CRAO)

• Cause: Embolus (carotid atherosclerosis, cardiac source), thrombosis, vasospasm (in young: cocaine, GCA in elderly).
• Fundoscopy: Pale/milky retinal oedema with cherry-red spot at fovea (choroidal circulation is intact under fovea, which has no inner layers, while surrounding ischemic retina whitens).
• Treatment: Must act within 90 minutes - ocular massage, IOP-lowering, anterior chamber paracentesis, consider thrombolytics; workup for embolic source mandatory.

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8. Central Retinal Vein Occlusion (CRVO)

• Cause: Hypertension, atherosclerosis, hypercoagulability, glaucoma.
• Fundoscopy: Diffuse flame-shaped hemorrhages in all 4 quadrants, disc oedema, dilated tortuous veins, cotton-wool spots.
• Complications: Macular oedema (most common cause of vision loss), neovascular glaucoma (100-day glaucoma - VEGF-driven ~3 months after ischemic CRVO).
• Treatment: Anti-VEGF for macular oedema; treat underlying cause.

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9. Uveitis

• Anterior uveitis (iritis/iridocyclitis): Most common; associated with HLA-B27 spondyloarthropathies (AS, reactive arthritis, psoriatic arthritis), JIA, sarcoidosis. Presents with painful red eye, photophobia, reduced vision, perilimbal flush, keratic precipitates, cells and flare in anterior chamber.
• Posterior uveitis (chorioretinitis): Toxoplasmosis (most common infectious cause), CMV retinitis (AIDS), TB, sarcoidosis. Painless floaters/visual loss.
• Panuveitis: Sarcoidosis, VKH syndrome, sympathetic ophthalmia (autoimmune response after penetrating eye injury - both eyes affected).
• Treatment: Topical/systemic steroids; treat underlying cause; cycloplegics for anterior.
• Robbins & Kumar Basic Pathology, p. 870

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10. Optic Neuritis

• Classic association: Multiple sclerosis (50% of optic neuritis patients have MS; 50% of MS patients have optic neuritis at some point). Also: NMO (neuromyelitis optica - anti-AQP4 antibodies, severe bilateral).
• Presentation: Unilateral painful vision loss, worse with eye movement; reduced color vision (especially red desaturation); afferent pupillary defect (Marcus Gunn pupil); often no disc swelling (retrobulbar neuritis: "patient sees nothing, doctor sees nothing").
• Uhthoff's phenomenon: Vision worsens with heat/exercise.
• Diagnosis: MRI with gadolinium (T2 hyperintense optic nerve); VEP (prolonged P100 latency).
• Treatment: IV methylprednisolone speeds recovery but doesn't change final outcome.

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11. Keratoconus

• Epidemiology: Usually bilateral (asymmetric), presents in adolescence/young adulthood.
• Associations: Eye rubbing, atopic disease, Down syndrome, Marfan syndrome.
• Symptoms: Progressive myopia and irregular astigmatism, ghosting; contact lens intolerance.
• Signs: Scissor reflex on retinoscopy, Vogt's striae, Fleischer ring (iron deposits at cone base), Munson's sign.
• Treatment: Spectacles/RGP contacts; corneal collagen cross-linking (CXL) to halt progression; keratoplasty (PKP or DALK) in advanced cases.

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EAR DISORDERS

12. Otitis Media (OM)

• Acute OM (AOM): Bacterial (S. pneumoniae, H. influenzae, M. catarrhalis) or viral. Presents with ear pain, fever, conductive hearing loss, bulging erythematous TM.
• OM with effusion ("glue ear"): Chronic middle ear effusion without acute signs; most common cause of conductive hearing loss in children. Often follows AOM or Eustachian tube dysfunction.
• Chronic suppurative OM (CSOM): Chronic TM perforation with persistent purulent discharge. Risk of cholesteatoma.
• Complications: Mastoiditis, meningitis, labyrinthitis, facial nerve palsy, brain abscess.
• Treatment: Watchful waiting for mild AOM; amoxicillin for moderate-severe or persistent; myringotomy + grommets for recurrent OM/glue ear.

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13. Cholesteatoma

• Types: Congenital (behind intact TM, often found incidentally) vs. acquired (from retraction pocket of TM, most common).
• Mechanism: Retraction pocket → traps keratin debris → expands → enzymatic and pressure erosion of ossicles, mastoid bone, tegmen, facial canal, labyrinth.
• Symptoms: Painless, foul-smelling otorrhoea; progressive conductive hearing loss; may develop SNHL if labyrinth eroded; facial nerve palsy (advanced).
• Complications: Facial nerve palsy, labyrinthitis, meningitis, sigmoid sinus thrombosis, brain abscess.
• Treatment: Surgery (mastoidectomy) - only definitive treatment.
• Cummings Otolaryngology

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14. Otosclerosis

• Pathology: Abnormal spongy bone replaces normal otic capsule bone → fixation of the stapes footplate at oval window → impaired sound transmission.
• Epidemiology: Autosomal dominant with variable penetrance; more common in white females; bilateral in ~70%.
• Symptoms: Progressive bilateral conductive hearing loss (starting 20s-30s), paracusis Willisii (hear better in noisy environments - Lombard effect), normal-looking TM.
• Audiogram: Conductive loss with Carhart's notch (dip at 2 kHz on BC), normal tympanogram but absent acoustic reflexes.
• Cochlear otosclerosis: If lesion extends to cochlea → mixed or sensorineural component.
• Treatment: Stapedectomy/stapedotomy (surgical gold standard) or hearing aids; sodium fluoride may slow progression.

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15. Ménière's Disease

• Classic tetrad: Episodic vertigo (20 min - 12 hours), fluctuating low-frequency sensorineural hearing loss, tinnitus, aural fullness.
• Pathophysiology: Excess endolymph distends the scala media → rupture of Reissner's membrane → perilymph-endolymph mixing → potassium toxicity to hair cells and spiral ganglion neurons.
• Diagnosis: Clinical (Barany Society 2015 criteria); low-frequency SNHL on audiogram; glycerol dehydration test; MRI to exclude acoustic neuroma.
• Treatment: Low-salt diet + diuretics (first-line); betahistine; intratympanic gentamicin (ablative) or steroids; endolymphatic sac surgery; labyrinthectomy/vestibular nerve section (last resort).

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16. Benign Paroxysmal Positional Vertigo (BPPV)

• Mechanism: Otoliths (calcium carbonate crystals = canaliths) dislodge from utricle → fall into posterior semicircular canal (canalolithiasis) → abnormal endolymph deflection → episodic positional vertigo.
• Symptoms: Brief (seconds to <1 minute) intense vertigo triggered by specific head movements (lying down, rolling over, looking up). No hearing loss or tinnitus.
• Diagnosis: Dix-Hallpike test → upbeat-torsional nystagmus with latency and fatigability.
• Treatment: Epley maneuver (canalith repositioning) - ~80% effective in a single session.
• K.J. Lee's Essential Otolaryngology

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17. Acoustic Neuroma (Vestibular Schwannoma)

• Origin: Usually arises from the vestibular division of CN VIII (superior or inferior vestibular nerve) in the internal auditory canal (IAC).
• Bilateral: Pathognomonic of Neurofibromatosis type 2 (NF2 - merlin/schwannomin gene mutation, Chr 22).
• Symptoms: Unilateral progressive SNHL + poor speech discrimination (disproportionate to pure tone loss), unilateral tinnitus, unsteadiness (rather than episodic vertigo - slow compensation). Facial numbness (CN V compression) in larger tumors. CN VII palsy is rare and a late sign.
• Diagnosis: MRI with gadolinium (gold standard) - enhancing mass in IAC/CPA; ABR (most sensitive screening test - abnormal in 82% of small tumors); absent stapedial reflexes with rollover on PI function.
• Treatment: Surveillance (small tumors in elderly), stereotactic radiosurgery (Gamma Knife), or microsurgical resection (translabyrinthine, retrosigmoid, or middle fossa approaches).
• K.J. Lee's Essential Otolaryngology, p. 75; Adams & Victor's Principles of Neurology, 12th Ed

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18. Noise-Induced Hearing Loss (NIHL)

• Mechanism: Excessive noise → metabolic exhaustion + mechanical damage to OHC stereocilia (basal turn of cochlea, corresponding to 4 kHz) → OHC death → permanent threshold shift.
• Audiogram hallmark: Bilateral symmetric SNHL with a characteristic 4 kHz notch (notch recovers slightly at 8 kHz). Progression with continued exposure eventually flattens the notch into a downward-sloping curve.
• Absent OAEs: OHC damage eliminates otoacoustic emissions - useful diagnostic marker.
• Prevention: Only effective treatment - hearing protection (earplugs, earmuffs) in hazardous noise environments (>85 dB for 8 hr/day = OSHA limit).

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19. Presbycusis

• Mechanism: Multifactorial - cumulative OHC loss, stria vascularis atrophy (energy failure), spiral ganglion degeneration, basilar membrane stiffening.
• Pattern: Bilateral symmetric high-frequency SNHL (downward-sloping audiogram); disproportionate difficulty with speech discrimination, especially in noise.
• Types (Schuknecht classification): Sensory (OHC loss, abrupt high-freq cutoff), neural (spiral ganglion loss, poor discrimination), strial/metabolic (stria vascularis atrophy, flat loss), cochlear conductive (basilar membrane changes).
• Treatment: Hearing aids; cochlear implants in severe-profound cases.

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20. Sudden Sensorineural Hearing Loss (SSHL)

• Causes: Idiopathic (~85-90%); viral (herpes simplex, mumps); vascular (microvascular ischemia of cochlea); autoimmune; acoustic neuroma (must exclude).
• Workup: MRI with gadolinium (exclude retrocochlear lesion); FBC, ESR, VDRL, FTA-ABS; autoimmune screen.
• Treatment: Oral corticosteroids (prednisolone) within 2 weeks of onset (standard of care); intratympanic steroid injection as salvage therapy; hyperbaric oxygen (adjunct).
• Prognosis: Spontaneous recovery in ~60-65%; best prognosis with mild loss, young age, early treatment.

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21. Labyrinthitis vs. Vestibular Neuritis

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Quick-Reference Summary Table