Ophtha Neuro Ophthalmology

NeuroOphthalmology

Neuro-Ophthalmology • Eyes – intimately related to the brain • Give diagnostic clues to CNS disorders

• IC Disease - visual disturbance  CN III, IV, VI – EOM  CN V, VII – also associated with ocular function

Anatomy of the Optic Nerve • Trunk consisting of about 1 million axons arising from the ganglion cells of the retina  Ganglion Cells axons nerve fiber layer of the retina OPTIC NERVE

Anatomy of the Optic Nerve • Portions:  Orbital Portion: 25-30 mm long within the muscle cone  Intraosseous Portion: 4-9 mm  Intracranial Portion: 10 mm  Diameter: 1.5 mm (within the sclera) 3 mm (within the orbit)  80%: visual fibers  20%: pupillary fibers

Optic Nerve

Sheaths of the Optic Nerve Continous with the meninges • PIA – fibrous tissue with numerous small blood vessel – divides the nerve into bundles – continues to the sclera

Sheaths of the Optic Nerve Continous with the meninges • ARACHNOID – from the IC end of optic foramen to sclera – diaphanous connective tissue membrane with many septate connections with PIA

Sheaths of the Optic Nerve Continous with the meninges • DURA – splits into 2: periorbita (lining the orbital cavity) and outer dural covering of the optic nerve – continous with outer 2/3 of sclera

Sheaths of the Optic Nerve Continous with the meninges • SPACES – Subdural – Subarachnoid

Sheaths of the Optic Nerve

Visual Pathway • CN II – pathway for special sense of vision – at the chiasm, more than half of the fibers decussate; join uncrossed temporal fibers of the opposite side optic tract – all impulses from RVF projected to left cerebrum and vice versa

Visual Pathway • 80% of fibers to lateral geniculate body and 20% to pretectal area and superior colliculi geniculocalcarine tract optic radiation occipital cortex

Visual Pathway • Lesions: – anterior to chiasm – unilateral VF defect – posterior to chiasm – contralateral homonymous defect – chiasmal – bitemporal defect – more congrous homonomous defect – farther posterior lesion with macular sparing (maintenance of good visual acuity)

Visual Pathway

Lesions

Pupillary Pathways A. Light Reflex – Entirely subcortical – Affrent pupillary fibers within the optic nerve go to pretectal area of midbrain Edinger-Westphal nucleus – Efferent pathway is via CN III to the ciliary ganglion within the muscle cone short ciliary nerves sphincter muscle of the iris

Pupillary Pathways B. Near Reflex – 3 Reactions: • Accommodation • Convergence • Constriction of the pupil – Final common pathway – CN III – Marcus-Gunn Pupil • Relative Afferent Pupillary Defect (RAPD) – Swinging flashlight test

Pupillary Pathways

• Normal pupillary diameter: 3-4 mm – smaller in infancy – larger in childhood – progressively smaller with advancing age

• Physiologic Anisocoria – seen in 20-40% of normal patients – Difference in pupil size is about 0.5mm

• Argyll-Robertson Pupil – Usually bilateral – <3mm in diameter – No response to light but responds to neural stimulus – Dilate poorly with mydriatics – Suggestive of: CNS syphillis

Optic Neuritis • Inflammation, degeneration, demyelinization of optic nerve • Retrobulbar Neuritis

Clinical Features: • visual loss is subacute • impaired color vision and contrast sensitivity • pain in the region of the eye in 90% of cases • 50% exacerbated by eye movement • sluggish pupillary-light reflex • central scotoma • hyperemic optic disc w/ blurring of disc margin • distension of large veins

Treatment • Steroid therapy • Methylprednisolone by IV, 1g/day for 3 days OR • Oral prednisolone, 1mg/kilo/day tapered over 10-21 days

Optic Disc Edema • Papilledema/Choked Disk – non-inflammatory congestion of optic disk associated with increased ICP (due to cerebral tumors, abscesses, subdural hematoma, hydrocephalous, malignant hypertension) – Mechanism: obstruction of venous flow caused by pressure on the Central Retinal Vein where it leaves the optic nerve

Clinical Findings • enlarged blind spot • normal VA and normal VF • Fundus Findings: hyperemia of the disk, blurring of the margins, distention of retinal veins, absence of pulsation of CRV, disk swelling to 6-10 D, hemorrhages

Papilledema • Takes 24-48 hrs for early papilledema to occur, 1 wk to develop fully • Takes 6-8 wks to resolve following adequate treatment

Papilledema • Foster Kennedy’s syndrome – papilledema on one side w/ optic atrophy on the other eye

Papilledema • Can be mimicked by buried drusen of the optic nerve, small hyperopic disc, myelinated nerve fibers

Course and Prognosis • May or may not affect vision permanently • Papilledema of more than 5D, extensive retinal hemorrhages, macular stars – poor prognosis

Multiple Sclerosis • Demyelinating disorder of the CNS of unknown cause, chronic, relapsing, onset young adult • Involves the optic nerve, chiasm, brain stem, cerebellar peduncles, and spinal cord • Degeneration of myelin sheaths, glial tissue overgrowth, and complete nerve fiber destruction

MS in the Retinal Nerve Fiber Layer

MS • Retrobulbar/Optic Neuritis – blurring of vision, central scotoma, dilated pupil – diplopia due to EOM involvement from internuclear ophthalmoplegia – weakness of one rectus muscle/ptosis – nystagmus may become permanent (70%) – VER (visual evoked response) – abnormal in 80% of definite, 43% of probable, and 22% of suspected cases of MS

Anterior Ischemic Optic Neuropathy • characterized by pallid disc swelling with acute loss of vision • due to infarction of the optic nerve from occlusion or decreased perfusion of the short posterior ciliary arteries • occurs in 6th or 7th decade of age • associated with arteriosclerosis

Nutritional & Toxic Optic Neuropathies • Clinical Features: – Subacute progressive symmetrical visual loss w/ central visual field defects – Poor color vision and temporal disc pallor

Nutritional & Toxic Optic Neuropathies • Causes: – Vitamin B Complex deficiency – Tobacco-Alcohol Amblyopia – Heavy metal poisoning – Chemical-induced optic neuropathy • methanol poisoning (used in photocopier machines)

– Drug-induced optic neuropathy • ethambutol, quinine

Optic Nerve Trauma • Indirect Optic Nerve Trauma: – Optic nerve damage secondary to distant skull injury – Occurs in 1% of all head injuries – Due to transmission of shock waves through the orbital apex – Optic nerve avulsion usually results from an abrupt rotational injury to the globe

Optic Nerve Trauma • Direct Optic Nerve Trauma – Penetrating orbital trauma – e.g. fractures involving the optic canal

Optic Chiasm • Lesions of the chiasm cause bitemporal hemianopsia • Pituitary gland tumor

Internuclear Ophthalmoplegia • Conjugate horizontal eye movements are disrupted due to failure of coordination between the abducens nerve nucleus in the pons and the oculomotor nucleus in the midbrain • Results in slowing of saccades in the adducting eye producing transient diplopia on lateral gaze

Internuclear Ophthalmoplegia • Causes – Multiple sclerosis in young adults – Brain stem infarction in older patients – Tumors at arteriovenous malformations – Encephalitis

Syndromes Affecting CN III, IV, VI • Superior Orbital Fissure Syndrome – All extraocular peripheral nerves pass through the superior orbital fissure and can be involved by trauma or by tumor encroaching on the fissure

Syndromes Affecting CN III, IV, VI • Orbital Apex Syndrome • Similar to the superior orbital fissure syndrome with the addition of optic nerve signs and greater proptosis and less pain • Caused by orbital tumor, inflammation, trauma that damages the optic and extraocular nerves

Syndromes Affecting CN III, IV, VI • Complete Ophthalmoplegia (Sudden) – Can be due to brainstem vascular disease, pituitary apoplexy, myasthenia gravis, arteriosclerotic basilar aneurysm

Myasthenia Gravis • Abnormal fatigability of striated muscles after repetitive contraction • Improves after rest • Often is first manifested by weakness of the extraocular muscles • Diplopia is often an early symptom

Chronic Progressive External Ophthalmoplegia • Slowly progressive inability to move the eyes • Severe early ptosis • Normal pupillary reactions and accommodation • A form of mitochondrial myopathy

Color Vision

Electromagnetic Spectrum • Photons are classified according to their wavelength • Longest wavelength: radio and television waves • Shortest wavelength: gamma rays • Middle of the spectrum: visible light

Rods and Cones • Retinal photoreceptors that contain pigments that preferentially absorb photons with wavelengths 400-700 nm • Shortest wavelength: blue and green • Longer wavelengths: yellow, orange, red

Visual Pigments • Four visual pigments: – Rhodopsin: present in rods – 3 cone pigments • Erythrolabe (R cones): red, 570 nm • Cyanolabe (B cones): blue, 440 nm • Chlorolabe (G cones): green, 540 nm

3 Attributes of Color • Hue – “color” – Attribute of color perception denoted by blue, red, purple, etc – Depends largely on what the eye and brain perceive to be the predominant wavelength present in the incoming light

3 Attributes of Color • Saturation – “chroma” – purity or richness of a color – When all the light seen by the eye is the same wavelength, the color is fully saturated – e.g. pink is a desaturated red

3 Attributes of Color • Brightness – Luminance, value – Quantity of light coming from an object (the number of photons striking the eye)

Primary Colors • Red • Green • Blue

Relative Luminosity Curves • Illustrate the eye’s sensitivity to different wavelengths of light • Cones’ peak sensitivity: 555 nm • Rods’ peak sensitivity: 505 nm (blue)

Trichromats • 92% of the population who have “normal” color vision • Have all 3 different kinds of cones, normal concentration of cone pigments, normal retinal wiring

Congenital Dichromatism • Cones themselves are normal, but one of the 3 contains the wrong pigment • Deutranopes: – Lack green pigment

• Protanopes – Lack red pigment

• Tritanopes – Lack blue pigment

Congenital Dichromatism • Mode of inheritance: sex-linked recessive – Men almost exclusively manifest the disorder – Women are carriers

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