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Special Senses: Vision, Hearing & Vestibular

Name: Special Senses: Vision, Hearing & Vestibular
Creator: Veli Bakalov, MD

Reading field cuts, pupils, and the Weber-Rinne pattern from the wiring diagram

Neurology & Psychiatry·Special Senses·5 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Field cuts: optic nerve = ipsilateral monocular blindness; chiasm = bitemporal hemianopia (pituitary adenoma, craniopharyngioma); optic tract = contralateral homonymous hemianopia.
Meyer loop (temporal) lesions cause contralateral upper quadrantanopia ("pie in the sky"); an occipital PCA stroke causes homonymous hemianopia with macular sparing because the MCA also supplies the occipital pole.
Light reflex: CN II is afferent and CN III parasympathetics (Edinger-Westphal → ciliary ganglion → sphincter pupillae) are efferent; bilateral pretectal relay produces the consensual response.
Marcus Gunn pupil (optic neuritis in MS) = both pupils dilate when the swinging flashlight moves to the affected eye; Argyll Robertson pupil (neurosyphilis) accommodates but does not react to light.
Compressive CN III palsy (aneurysm, uncal herniation) = down-and-out eye, ptosis and a blown pupil; diabetic ischemic CN III palsy often spares the pupil; Horner = miosis, ptosis and anhidrosis.
Myopia = eye too long (concave lens); hyperopia = eye too short (convex lens); presbyopia = age-related lens stiffening.
The cochlear base encodes high frequencies and is damaged first by noise (4000 Hz notch) and aging; aminoglycosides, cisplatin, loop diuretics, salicylates and vancomycin are ototoxic.
Conductive loss: Weber lateralizes to the affected ear and Rinne shows BC greater than AC; sensorineural loss: Weber lateralizes to the normal ear and Rinne shows AC greater than BC.
Peripheral vertigo = horizontal or rotary, fatigable nystagmus suppressed by fixation; central = vertical, non-fatigable nystagmus with other neurologic signs; Meniere = vertigo, tinnitus, low-frequency hearing loss and aural fullness.

Optics and the visual pathway

Refractive errors. In myopia (nearsightedness) the eyeball is too long, so images focus in front of the retina; a concave (minus) lens corrects it. In hyperopia (farsightedness) the eye is too short and needs a convex (plus) lens. Presbyopia is age-related stiffening of the lens that removes accommodation, so near vision worsens after about age 40. It is a problem of lens hardening, not eye length. Astigmatism is uneven corneal curvature.

RetinaPhotoreceptors → bipolar cells → ganglion cells
Optic nerve (CN II)Carries one eye's whole field
Optic chiasmNasal fibers (temporal fields) cross
Optic tract → lateral geniculate nucleusNow carries the contralateral visual field
Optic radiations → calcarine (occipital) cortexMeyer loop (temporal) = upper field; parietal radiations = lower field

Field defects follow the wiring. Before the chiasm a lesion affects one eye. At the chiasm it affects the crossing fibers, and behind the chiasm it affects the same half of the field in both eyes (homonymous).

Optic nerve: ipsilateral monocular blindness.
Optic chiasm: bitemporal hemianopia, classically from a pituitary adenoma or craniopharyngioma compressing the crossing nasal fibers.
Optic tract: contralateral homonymous hemianopia.
Meyer loop (temporal lobe): contralateral upper quadrantanopia ("pie in the sky").
Parietal (dorsal) radiations: contralateral lower quadrantanopia ("pie on the floor").
Occipital cortex (PCA stroke): contralateral homonymous hemianopia with macular sparing.

Macular sparing in an occipital (PCA) stroke reflects collateral supply to the occipital pole from the MCA.

Pupillary reflexes and abnormal pupils

Light reflex: II in, III out. The afferent limb is CN II. Fibers relay in the pretectal nuclei, which project to the Edinger-Westphal nuclei on both sides. The efferent limb is the parasympathetic fibers of CN III, which synapse in the ciliary ganglion and constrict the sphincter pupillae. Because the pretectal relay is bilateral, light in one eye constricts both pupils (the consensual response).

Near (accommodation) reflex. Looking at a near object triggers a triad: convergence (medial recti), accommodation (the ciliary muscle contracts, zonules slacken and the lens rounds) and miosis.

Swinging-flashlight test. In a relative afferent pupillary defect (Marcus Gunn pupil), the optic nerve or retina on one side sends a weak signal, classically from optic neuritis in multiple sclerosis. When light swings from the healthy eye to the affected eye, both pupils paradoxically dilate. The efferent limb is intact, so light in the healthy eye still constricts both pupils.

CN III vs Horner. The pupillary parasympathetic fibers run on the outside of CN III. A compressive lesion such as a posterior communicating artery aneurysm or uncal herniation therefore blows the pupil early. Ischemic (diabetic) CN III palsy damages the core of the nerve and often spares the pupil. Horner syndrome interrupts the 3-neuron sympathetic chain (hypothalamus → C8–T2 cord → superior cervical ganglion → eye and face). Causes include a Pancoast tumor, carotid dissection and lateral medullary infarction.

Abnormal pupils

Finding	Lesion	Key features
Marcus Gunn pupil	Afferent: optic nerve or retina	Both pupils dilate when light swings to the affected eye
Argyll Robertson pupil	Neurosyphilis (tabes dorsalis)	Small pupils that accommodate but do not react to light (light-near dissociation)
CN III palsy	Efferent: oculomotor nerve	Eye down and out, ptosis, dilated ("blown") pupil if compressive
Horner syndrome	Sympathetic pathway	Miosis, ptosis (mild), anhidrosis

Both CN III palsy and Horner syndrome cause ptosis, but the pupil is large in CN III palsy and small in Horner syndrome.

Hearing

Auditory pathway. Sound travels from the organ of Corti in the cochlea to CN VIII, then the cochlear nuclei, superior olive, lateral lemniscus, inferior colliculus and medial geniculate nucleus, and finally the auditory cortex in the superior temporal gyrus. Input is represented bilaterally from the superior olive upward, so a unilateral cortical lesion rarely causes deafness.

Tonotopy and hair cells. The base of the cochlea is stiff and narrow and encodes high frequencies. The apex is wide and floppy and encodes low frequencies. Outer hair cells amplify the signal, and inner hair cells transduce it. Loud noise and aging damage the high-frequency base first, which is why presbycusis and noise-induced loss (a 4000 Hz notch) affect high pitches.

Weber and Rinne. In the Weber test, a tuning fork is placed on the midline of the skull. In the Rinne test, air conduction (AC) is compared with bone conduction (BC) in each ear. Normally AC is greater than BC.

Conductive vs sensorineural hearing loss

Feature	Conductive	Sensorineural
Weber	Lateralizes to the affected ear	Lateralizes to the normal ear
Rinne (affected ear)	BC greater than AC	AC greater than BC (normal pattern)
Site	External or middle ear	Cochlea or CN VIII
Causes	Cerumen impaction, otitis media, otosclerosis, cholesteatoma	Presbycusis, noise, ototoxic drugs, vestibular schwannoma

Weber goes to the bad ear in conductive loss and to the good ear in sensorineural loss.

Ototoxic drugs. Aminoglycosides, cisplatin, loop diuretics, high-dose salicylates and vancomycin damage hair cells and cause sensorineural loss.

Other causes. In otosclerosis, abnormal bone fixes the stapes at the oval window. A cholesteatoma is a mass of keratinizing squamous epithelium in the middle ear. A vestibular schwannoma of CN VIII at the cerebellopontine angle causes unilateral sensorineural loss and tinnitus, and bilateral tumors suggest NF2.

Vestibular system and vertigo

Canals sense spin, otoliths sense tilt. The three semicircular canals detect rotational (angular) acceleration. The otolith organs, the utricle and saccule (with their maculae), detect linear acceleration and head tilt relative to gravity. The vestibulo-ocular reflex runs from CN VIII to the vestibular nuclei and on to the eye-movement nuclei. It keeps gaze stable during head movement by moving the eyes opposite to the head.

Peripheral vs central vertigo

Feature	Peripheral	Central
Causes	BPPV, Meniere disease, vestibular neuritis, labyrinthitis	Brainstem or cerebellar stroke, MS, tumor
Nystagmus	Horizontal or rotary; fatigable; suppressed by fixation	Vertical (or direction-changing); not fatigable; not suppressed by fixation
Associated	Often positional; hearing symptoms common	Other neurologic signs (diplopia, dysarthria, ataxia)

BPPV: displaced otoliths (canaliths), usually in the posterior canal, cause brief attacks of vertigo triggered by head position. The Dix-Hallpike maneuver provokes the attack, and the Epley maneuver repositions the debris.
Meniere disease: endolymphatic hydrops causes attacks of vertigo lasting hours with tinnitus, fluctuating low-frequency sensorineural hearing loss and aural fullness.
Vestibular neuritis vs labyrinthitis: both cause acute, sustained vertigo, usually after a viral illness. Labyrinthitis also causes hearing loss, whereas vestibular neuritis spares hearing.