Optic (ii) Nerve

Leber hereditary optic neuropathy (LHON)

Hereditary, toxic, and nutritional deficiency optic neuropathies commonly present with bilateral, insidious vision loss. In some cases, there can be asymmetry in visual acuity and visual fields. LHON often presents as acute or subacute unilateral vision loss that is  followed within several  months by vision loss in  the  fellow eye.  The appearance of the optic nerve in LHON is characterized by a triad of circumpapillary telangiectasia, swelling of the nerve fiber layer around the disc, and absence of leakage on fluorescein angiography. LHON should be included in the diagnosis of acute unilateral vision loss from such disorders as optic neuritis and ischemic optic neuropathy.

An Onodi cell (or spheno ethmoidal air cell) is defined as an ethmoidal air cell that lies posteriorly, and sometimes, superiorly to the sphenoidal sinus. As a result of its location the optic nerve, and less commonly, the internal carotid artery, are very closely related with as little a 0.03mm (median 0.08mm) of bone separating them. 

This person is blind in the right eye. The afferent limb of the pupillary light reflex is carried by the optic nerve (CN II), and the efferent limb is via the oculomotor nerve (CN III), which carries parasympathetic fibers from the Edinger-Westphal nucleus. Thus, shining a light in the affected eye will not elicit any pupillary response. On the other hand, shining the light in the left eye will result in simultaneous constriction of both pupils (assuming an intact right CN III), since the left optic nerve is intact.

Ref: Greenberg D.A., Aminoff M.J., Simon R.P. (2012). Chapter 7. Neuro-Ophthalmic Disorders. In D.A. Greenberg, M.J. Aminoff, R.P. Simon (Eds), Clinical Neurology, 8e.

B. i.e. Internal corotid artery

A. i.e. Optic nerve

Optic nerve & ophthalmic artery pass through optic canalQ not through superior orbital fissure

Ans. is ‘a’ i.e., Formaldyhyde and formic acid

o Methyl alcohol (methanol) is metabolized to formaldehyde by alcohol dehydrogenase and then to formic acid by aldehyde dehydrogenase. These compounds are responsible for the toxicity. Formic acid can lead to coma and blindness also. Therefore, inhibitor of alcohol dehydrogenase, Femepizole is used for treatment of methanol over dose.

B,C, i.e. 4 cm long ;  Covered by 3 layers continuous with meninges.

Optic Nerve

Origin

Optic nerve consists of 1.2 million axons (afferent nerve -fibers) which originate in retinal ganglion cellsQ and project to the 8 primary visual nuclei. Most of these synapse in lateral geniculate body/nucleus, although some reach other centers like pre tectal nucleus in

midbrain (mainly), nucleus of accessory optic

system/superior colliculus/ suprachiasmatic/ supraoptic/ paraventricular and pulvinar nucleus.

80% of optic nerve fibers originate from macular region, which represents 90% of retinal ganglion cells. Hence, diseases of optic nerve and macula can mimic each other. -Nearly 1/3^rd of the fibers subserve the central 5° of visual field.

–  > 90% of retinal ganglion cells are relatively small, concentrated in macula, and contribute axons of small caliber that project to the parvocellular layers of dorsal lateral geniculate nucleus (P-cell – system). P cells have color-opponent physiology and subserve high contrast, high spatial frequency resolution. Whereas M cells are larger cells that contain large, fast conducting axons and make 5-10% of retinal ganglion cells. M cells are primarily involved with noncolor information of high temporal and low spatial frequency.

Component

The axons of 2^nd cranial nerve are 2^nd order neurones and though termed as a nerve the optic nerve is actually a white matter tract (not a peripheral nerve) identical to other white matter tracts of the brain. Unlike peripheral nerve it is not covered by neurilemma

(and so has no powers of regeneration i.e. does not regenerate when cut). Optic nerve possesses oligodendrocytes, microglia and astrocytes, whereas true peripheral nerves possess Schwann cells, fibroblasts and macrophages. The axons of optic nerve acquire myelin sheaths proximal to lamina cribrosa. Like other parts of CNS it is covered with pia, arachnoic and duramaterQ.

Optic nerve is about 50mm (47-50mm) long from eye globe to chiasma and can be divided into 4 portions: 1) intraocular (optic disc, optic nerve head) is shortest (1 mm length) and has 3 anatomical zones i.e. retinal, choroidal / laminar and sclera] / retrolaminar; 2) intra-orbital segment (25-30 mm long) and extends from globe to optic foramen at the orbital apex, where it is surrounded by tough fibrous annulus of zinn; 3) intracanalicular segment (5-9 mm) within the optic canal is closely related to opthalmic arteryQ which lies infero-medial to it and crosses obliquely over it, as it enters the orbit to lie on its medial side. Posterior ethmoidal and sphenoid sinuses lie medial to it; 4) intracranial segment (5-16 mm) joins the chiasma.

C i.e. Ptosis

Ptosis (i.e. drooping of upper eyelid) is due to paralysis of levator palpebrae superiorisQ and is seen in occulomotor (3^rd) nerve injury not optic nerve injuryQ

Clinical features of Nerve Injury

2nd (optic) nerve

• Loss of visionQ / Decreased visual acuity
• Loss of direct light reflexQ (retained consensual)
• Dilation of pupilQ
• RAPD (Relative afferent pupillary defect)
• Dyschromatopsia (impairment of colour vision)
• Diminished light brightness sensitivity (may be present even with normal acuity)
• Diminished contrast sensitivity
• Visual field defects 

3^rd (occulomotor) nerve

• Down & Out eyeQ (due to unopposed LR & S.0.)
• Upward gaze & medial rotation (adduction) is not possibleQ
• PtosisQ due to LPS paralysis
• Loss of accommodationQ
• Loss of pupillary / light reflexQ (direct & consensual bothQ).
• Dilation of pupil (mydriasis) Q

C i.e. Afferent pupillary defect 

Marcuss Gunn pupil or RAPD (relative afferent papillary defect) is the earliest indication of optic nerve disease even in presence of normal visual acuity.Q

D i.e. Perimetry

Investigations for Optic Nerve Disease

• Perimetry : Visual field defects are best detected by Goldmann kinetic perimetry (full-field) & Humphery automated threshold perimetry
• MRI: Best investigation for imaging of optic nerves
• Visually evoked potential
• Fluorescein angiography: Occasionally helpful in differentiating papilloedema(showsdisc leakage) from optic disc drusen (no leakage).
• Ophthalmoscopic appearance of the disc may vary considerably from normal to elevated to arophied, so they may not help in diagnosis, though may be useful in differential diagnosis.
• USG is not used for evaluation of optic nerve but for retinal function and structure.

D i.e. Pilocytic

Optic nerve glioma is slow-growing pilocytic astrocytoma Q which typically affects young girls in 1^st decade. Neurofibromatosis-1 is a common association.

Ans. Ganglion cells

Ans. Neurilemma

Ans. Optic nerve sheaths

Ans. Kingking of optic nerve

Ans. Perimetry

Ans. Ptosis

Cranial Nerve I: Olfactory

• Arises from the olfactory epithelium.
• Functions solely by carrying afferent impulses for the sense of smell.

Cranial Nerve II: Optic

• Arises from the retina of the eye.
• Optic nerves pass through the optic canals and converge at the optic chiasm.
• Functions solely by carrying afferent impulses for vision.

Cranial Nerve III: Oculomotor

• Fibers extend from the ventral midbrain, pass through the superior orbital fissure, and go to the extrinsic
• eye muscles.
• Functions in raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shape.
• Parasympathetic cell bodies are in the ciliary ganglia.

Cranial Nerve IV: Trochlear

• Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle.
• Primarily a motor nerve that directs the eyeball.

Cranial Nerve V: Trigeminal

• Three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3).
• Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3).
• Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication.

Cranial Nerve VI: Abdcuens

• Fibers leave the inferior pons and enter the orbit via the superior orbital fissure.
• Primarily a motor nerve innervating the lateral rectus muscle.

Cranial Nerve VII: Facial

• Fibers leave the pons, travel through the internal acoustic meatus, and emerge through the stylomastoid foramen to the lateral aspect of the face.
• Mixed nerve with five major branches.
• Motor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glands.
• Sensory function is taste from the anterior two-thirds of the tongue.

Cranial Nerve VIII: Vestibulocochlear

• Two divisions – cochlear (hearing) and vestibular (balance).
• Functions are solely sensory – equilibrium and hearing.

Cranial Nerve IX: Glossopharyngeal

• Fibers emerge from the medulla, leave the skull via the jugular foramen, and run to the throat.
• Nerve IX is a mixed nerve with motor and sensory functions.
• Motor – innervates part of the tongue and pharynx and provides motor fibers to the parotid salivary gland.
• Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx.

Cranial Nerve X: Vagus

• Fibers emerge from the medulla via the jugular foramen.
• The vagus is a mixed nerve.
• Its sensory function is in taste.

Cranial Nerve XI: Accessory

• The spinal root passes upward into the cranium via the foramen magnum.
• The accessory nerve leaves the cranium via the jugular foramen.
• Primarily a motor nerve.
• Supplies fibers to the larynx, pharynx, and soft palate.

Cranial Nerve XII: Hypoglossal

• Fibers arise from the medulla and exit the skull via the hypoglossal canal.
• Innervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech.

Ans. B: 2nd order neuron

In the case of the optic nerve and tracts there are three separate neurons linked together, extending from the retina to the occipital cortex.

The sensory end-organ consists of the rods and cones.

• The first neuron begins in the outer plexiform or granular layer, and ends in the inner plexiform or granular layer. The cell-body governing the nutrition of this fiber is the bipolar cell, which cells forms the inner nuclear layer. This neuron, although extremely short is the true optic nerve in the physiological sense, and corresponds to the long fiber which runs from the sole of the foot up into the spinal cord.
• The second neuron in the retina begins in the inner nuclear layer and runs from here up to the primary optic ganglia, i.e., chiefly the external geniculate body.

This is the anatomical optic nerve.

The cell governing the nutrition of this fiber is the ganglion cell.

Ans. is ‘d’ i.e., Arachnoid

Optic Nerve Meningioma :‑

• These are invasive tumoursarising from the arachnoidal villi.
• Meningiomas invading the orbit are of two types :

a) Primary intraorbitalmeningiomas

• These are also known as ‘optic nerve sheath meningiomas’.
• These produce early visual loss associated with limitation of ocular movements, optic disc oedema or atrophy, and a slowly progressive unilateral proptosis.

b) Secondary orbital meningiomas

• These intracranial meningiomas, secondarily invade the orbit, either arise from the sphenoid bone or involve it en route to the orbit.
• Orbital invasion may occur through: floor of anterior cranial fossa, superior orbital fissure and optic canal.
• Meningioma affecting the greater and lesser wings of sphenoid and taking origin in the region of pterion, is the most common variety affecting the orbit secondarily.

Ans. is ‘d’ i.e., Intact contrast sensitivity

Signs of optic nerve damage are as follows:

• Visual acuity is usually reduced markedly.
• Colour vision is often severely impaired.
• Pupil shows ill-sustained constriction to light.
• Marcus Gunn pupil which indicates relative afferent pupillary defect (RAPD) is a diagnostic sign. It is detected by the swinging flash light test.
• Visual field changes. The most common field defect in optic neuritis is a relative central or centrocaecal scotoma.
• Contrast sensitivity is impaired.
• Visually evoked response (VER) shows reduced amplitude and delay in the transmission time.

Ans. is ‘a’ i.e., Neurotibromatosis type I

Optic nerve glioma

Optic nerve glioma (astrocytoma) is the most common intrinsic tumor of the optic nerve. Most common type of optic nerve glioma is juvenile pilocytic astrocytoma. The majority of optic nerve gliomas are of astrocytic origin. However, a few rare optic nerve gliomas arise from oligodendrocytes. 70% of optic gliomas occur during first decade of life. Optic nerve gliomas are associated with Neurofibromatosis-1 (Von Recklinghausen’s disease) in 55% of cases. 75% of optic gliomas arise from optic chiasma and adjacent optic nerve. 25% are confined to optic nerve alone (orbital optic nerve glioma).

Patients present with some degree of unilateral visual dysfunction, visual field loss, afferent pupillary defect, decreased ocular motility, optic atrophy, pain headache and nystagmus. Proptosis is often the chief complaint of an orbital glioma. Nystagmus may also occur. Other late features of optic nerve glioma are neovascular glaucoma, anterior segment ischemia and hemorrhagic glaucoma from retinal vascular occlusion.

In chiasmal gliomas there may be visual field defects, raised ICT, papilloedema and endocrine abnormalities like diabetes insipidus, hypopituitarism, dwarfism and precocious puberty.

. Ans. is ‘c’ i.e., Intraorbital 

Optic nerve

• Each optic nerve (second cranial nerve) starts from the optic disc and extends up to optic chiasma, where the two nerves meet.
• It is the backward continuation of the nerve fibre layer of the retina, which consists of the second order neurons i.e., axon originating from the ganglion cells.
• It also contains the afferent fibres of the papillary light reflex.
• Morphologically and embryologically, the optic nerve is comparable to a sensory tract.
• Unlike peripheral nerves it is not covered by neurilemma (so it does not regenerate when cut).
• The fibres of optic nerve, numbering about a million, are very fine (2-10 m in diameter as compared to 20 mm of sensory nerves).
• Optic nerve is about 47-50 mm in length and can be divided into 4 parts : ‑  
• Intraocular part (1 mm) : – It begins at optic disc (optic nerve head) and exits the nerve through a hole in sclera that is occupied by a mesh like structure called the lamina cribrosa. The nerve fibres from the retina leave the eye through pores (holes) in lamina cribrosa, a sieve-like structure made up of collagen meshwork.
• Intraorbital part (30 mm) : – Extends from back of the eyeball (at lamina cribrosa) to the optic foramina.
• Intracanalicular part (6-9 mm) : – It lies within the optic canal and closely related to ophthalmic artery which crosses obliquely over it.
• Intracranial part (10 mm) : – It lies above the cavernous sinus and converges with its fellow from contralateral side to form optic chiasma.
• Like other parts of CNS, the optic nerve is covered by meningeal sheaths (Pia, archnoid and dura mater) as soon as the nerve leaves the eyeball.