Consists: Optic nerve, Optic chiasma, Optic tract, Lateral geniculate body, Optic radiation, and Visual cortex



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Visual Pathway

Consists:

Optic nerve, Optic chiasma, Optic tract, Lateral geniculate body, Optic radiation, and Visual cortex

Optic Nerve

The second cranial nerve. It contains the axons of the ganglion cells of the retina ( about one million axons). Optic nerve considered as part of the central nervous system and has no neurolemma and no ability to regenerate after damage due to trauma or inflammation.

Optic nerve has 4 parts:

Optic nerve head; anterior 1mm including intraocular part.

Intra-orbital 3 cm,

Intra-canlicular 0.6 cm

Intracranial 1cm

Blood supplies of the optic nerve head is mainly by short posterior ciliary arteries, while blood supplies of the rest of the optic nerve is mainly by ophthalmic artery .

Optic disc; the anterior end of the optic nerve. It is 1.5 mm in diameter and lies 3 mm nasal to the post pole of the eye. At the margin of the optic disc, the choroid and all the retinal layers stop except axons of the ganglion cells of the retina which pass through the lamina cribrosa of the sclera and emerge from the eye ball as the optic nerve. Optic cup is a central depression in the disc that dose not contain nerve fibers.


Optic nerve pass through optic canal to the middle cranial fossa, where it lies over the cavernous sinus. It converges with the optic nerve of the opposite side to form the optic chiasma where nasal fibers decussate to the opposite optic tract.

Optic Nerve Disorders


Optic Neuritis: optic nerve inflammation

Causes;


1-Demyelinating (MS): most common cause (70%), typically affects young adults, and women are affected more than men

2- Para-infectious. Viral infection or immunization. Children are affected much more common

3- Infectious, sinus related, syphilis, cat-scratch
Clinical features

Symptoms:

Sudden onset

Monocular visual loss (sometimes bilateral especially in young children)

Discomfort in or around the eye exacerbated by ocular movement

Signs:


Reduction in the visual acuity

Impairment of color vision

Diminished pupillary light reactions

Diminished light brightness sensitivity

Visual field defects, central scotoma, centrocaecal scotoma

Clinical classification:

Papillitis: 1/3 of cases, inflammation of the optic nerve head.

Ophthalmoscopic findings: hyperemic, edematous disc.


Retrobulbar neuritis: 2/3 of cases, inflammation behind the optic nerve head.

Ophthalmoscopic findings: normal optic disc.

Investigations to establish diagnosis of MS;

Visual Evoked Potential (VEP); which is EEG of the occipital cortex

MRI: peri-ventricular plaques
Neuro-retinitis: inflammation of the optic nerve head and the retina.

Least common is usually associated with viral infection

Ophthalmoscopic findings: Papillitis with macular star composed of hard exudates

Papilloedema

Bilateral optic disc swelling, secondary to increase intracranial pressure.

Causes:


  1. Space occupying lesions,( e.g. intracranial tumor).

  2. Blockage of the ventricular system

  3. Obstruction of CSF absorption,(e.g. damage to the arachoid villi post meningitis).

  4. Benign intra-cranial hypertension.

  5. Diffuse cerebral edema, (e.g. head trauma).

  6. Malignant hypertension

Clinical features:

Early papilloedema;

Visual acuity normal

Mild optic disc swelling

Hyperemic disc

Indistinct disc margins

Established papilloedema;

Transient visual obstruction lasting few seconds

Severe swelling,

Severe hyperemic, flam shaped hemorrhage

Blurring of optic disc margins

Engorged retinal veins

Hard exudates, cotton wool spots

Atrophic papilloedema;

Visual acuity impaired

Secondary optic disc atrophy

Pale disc, slightly elevated, indistinct margins.



Optic Disc Atrophy

Permanent damage to the neural tissue of the optic nerve



Primary optic atrophy; is caused by lesions affecting the visual pathways from behind the optic nerve head to the lateral geniculate body.

Causes: Hereditary

Compressive lesions on the optic nerve

Post retro-bulbar optic neuritis

Clinical appearance:

Pale, white and flat disc

Clear delineated margins

Attenuated blood vessels


Secondary optic atrophy; secondary to optic nerve head disease,

Causes: Papillitis

Papilloedema

Anterior ischemic optic neuropathy

Clinical appearance:

Pale, white and slightly elevated disc

Poorly delineated margins

Attenuated blood vessels




Anterior ischemic optic neuropathy
Infarction of the optic nerve head caused by occlusion of the short posterior ciliary arteries

Arteritic associated with Giant cell arteritis

Non- Arteritic; idiopathic, usually occurs in hypertensive patients between 40-60 years old.

Sudden monocular visual loss

Disc appears, pale and edematous

Pupil
Pupil; is the central aperture in the iris. Pupillary constriction (Miosis) is achieved by sphincter pupillae muscle, while dilatation of the pupil (Mydriasis) is achieved by dilator pupillae muscle.


Pupillary Reaction

A- Constriction of the pupil

Induced by Para-sympathetic innervations to the sphincter pupillae muscle of the iris

Light reflex; Constriction of the pupil when light is projected toward the eye.

Direct Light reflex; Constriction of the ipsilateral pupil

Consensual Light reflex; Constriction of the contra-lateral pupil (due to crossing of fibers at the optic chiasma, and mid-brain)

Light reflex; Consists of 4 neurones:



  1. from the retina to the pre-tectal nucleus in the mid-brain

  2. from pre-tectal nucleus to Edinger-Westphal nucleus

  3. from Edinger-Westphal nucleus to the ciliary ganglion with III n.

  4. from ciliary ganglion to sphincter pupillae and ciliary muscles with ciliary n.


Near reflex; constriction of the pupil on looking to a near object

Consist of

1- Increased Accommodation

2- Constriction of the pupil (Miosis)

3- Convergence of the visual axis
B- Dilatation of the pupil

Induced by sympathetic innervations to the dilator muscle of the iris

Consists of 3 neurones;

1- Starts from hypothalamus to ciliospinal center of Budge between C8 and T2

2- from ciliospinal center of Budge to the superior cervical ganglion in the neck

3- from the superior cervical ganglion pass along the internal carotid artery then

through the nasociliary nerve and long ciliary nerve to the dilator muscle of the iris
Abnormalities of Pupil
Amaurotic pupil; is caused by complete optic nerve damage.

Characterized by;

The involved eye is completely blind

Both pupils are equal in size

When the affected eye is stimulated neither pupil reacts, but

When the normal eye is stimulated both pupils react.



Marcus Gunn pupil (Relative afferent pupillary conductive defects, RAPD)

Caused by incomplete optic nerve lesion or severe retinal disease.

Characterized by;

Both pupils are equal in size

When the affected eye is stimulated both pupils react.

Swinging light test; in which each pupil is stimulated in rapid

succession by a flash light. When the affected eye is stimulated the

pupil dilates instead of constriction.




Argyll Robertson pupil

Caused by neuro-syphilis

Involvement is usually bilateral but asymmetrical

Pupil is small and irregular

The light reflex is absent, but near reflex is normal (light-near dissociation).

Adie pupil

Caused by denervation of the post ciliary ganglionic nerve supply to the eye (sphincter pupillae and ciliary muscles), which may follow viral infection.

Unilateral 80%

It typically affect young adult.

Affected pupil is large and regular

Light reflex is absent or very slow

Accommodation is slow

Horner Syndrome

Affect the sympathetic nerve supply to the eye (dilator pupillae muscle, and Muller muscle of the lid).



Cause;

Central; Brain stem, spinal cord diseases (tumor, vascular, demyelination)

Preganglionic; Pancoast tumor

Post ganglionic; Internal carotid aneurysm, idiopathic, congenital



Clinical feature

Mild ptosis

Miosis with normal papillary reaction to light and near

Disorders of the Chiasma

Pituitary adenoma, Craniopharyingioma, Meningioma

Visual field defects; Bitemporal hemianopia

Optic atrophy 50% of cases

General clinical features

Sings and symptoms of raised intracranial pressure (headache, vomiting, papilloedema)

Pituitary gland dysfunctions; hypopituitarism, acromegaly, Cushing disease

Disorders of Retro-Chiasma Visual pathways and Visual Cortex

Tumors, Vascular



Visual field defects; Homonymous hemianopia







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