|CHAPTER||14||The Optic Nerve|
▃Anatomy and Physiology
Optic nerve consists of axons arising from ganglion cells of retina, terminating in the lateral geniculate body. It also contains afferent fibers of pupillary light reflex (pupillomotor fibers). Optic nerve (2nd cranial nerve) carries approximately 1.2 million afferent nerve fibers (axons) which are 2nd order neurons in visual pathway. Although termed as a nerve, optic nerve is actually a tract. It extends from optic disc to optic chiasma. It is approximately 50 mm long.
It is divided into four parts (Fig. 14.1):
•Intraocular (1 mm).
•Intraorbital (25–30 mm).
•Intracanalicular (5–9 mm).
•Intracranial (10–16 mm).
•Intraocular part (optic nerve head): It is visible as optic disc on fundus examination and extends to posterior scleral surface. Its vertical diameter is 1.5 mm. It is mostly transscleral. The optic nerve fibers pass through a sieve-like structure, lamina cribrosa, which bridges across the scleral canal. In relation to lamina cribrosa, it may be divided into:
•Intraorbital part: It extends from globe to optic foramen at the orbital apex. At the orbital apex, optic nerve is surrounded by annulus of Zinn from which four rectus muscles originate. Some fibers of superior and medial rectus muscles are adherent to nerve sheath, so retrobulbar neuritis may be associated with pain on ocular movement. This part is sinuous, allowing the eye movements without stretching of nerve.
•Intracanalicular part: It traverses the optic canal. Optic nerve is covered with piamater, arachnoid and duramater as soon as the nerve leaves the eyeball. This part is fixed to canal since duramater fuses with periosteum. Ophthalmic artery lies inferolateral to nerve. Sphenoidal and ethmoidal sinuses lie medial to it and are separated by a thin bone. Therefore, infection of these sinuses may spread to nerve causing optic neuritis.
•Intracranial part: It extends from posterior end of optic foramen to the optic chiasma. It lies above the cavernous sinus.
•Oligodendrocytes: Axonal myelination posterior to lamina cribrosa.
•Astrocytes: structural support.
Axons of optic nerve do not possess a neurilemma. Optic nerve has no power of regeneration.
All the three meninges of brain (piamater, arachnoid mater and duramater) are continuous with optic nerve. The duramater fuses with periosteum in the optic canal. The subdural and subarachnoid spaces are continuous with those of the brain. The subarachnoid space contains cerebrospinal fluid (CSF).
Ophthalmic artery (a branch of internal carotid artery) gives rise to the following branches:
•Long posterior ciliary arteries.
•Short posterior ciliary arteries.
After giving braches, it continues as central retinal artery along the center of the optic nerve after piercing the nerve sheath. Central retinal artery makes no contribution to lamina cribrosa. In sclera surrounding the optic nerve, short posterior ciliary arteries anastomose and give rise to intrascleral circle of Zinn. Circle of Zinn–Haller receives three major sources of blood: choroidal vessels, short posterior ciliary arteries (4 or 5), and small contribution from pial arterial network. It supplies the intraocular part of the optic nerve (Table 14.1).
Table 14.1 Blood supply of optic nerve
Part of optic nerve
Surface layer of optic disc
Supplied by capillaries derived from retinal arterioles.
Supplied mainly by branches from peripapillary choroidal vessels with small contribution from vessels in lamina cribrosa.
Supplied mainly by branches from posterior ciliary arteries and arterial circle of zinn.
Retrolaminar part of optic nerve
Supplied by pial plexus formed by pial branches from:
•Circle of zinn
•Central retinal artery
It is mainly drained by the central retinal vein. The prelaminar region also drains into the choroidal veins (Fig. 14.2).
Apart from neural signals, axons transfer the cytoplasmic organelles. Axoplasmic transport is the movement of cytoplasmic organelles within a neuron between cell body and terminal synapse. In case of optic nerve, the cell body of neuron is ganglion cell body in retina, and the synapse is at the lateral geniculate body (LGB). Rapid axoplasmic transport is an active mechanism, so it requires O2 and ATP (Flowchart 14.1).
▃Optic Nerve Dysfunction (OP8.5)
Clinical features of optic nerve dysfunction include:
•Decreased visual acuity.
•Afferent pupillary defect (APD).
•Decrease in brightness of objects.
•Dyschromatopsia (impairment of color vision)–mainly affects red and green.
•Diminished or loss of contrast sensitivity.
•Visual field defects which include central scotoma, centrocaecal scotoma, and nerve fiber bundle defects.
Clinically, optic nerve disease can be differentiated from macular disease (Table 14.2).
Table 14.2 Differentiation of optic nerve disease from macular diseases
Optic nerve disease
Sometimes with eye movements
By visual function
•Afferent pupillary defect (APD)
By special tests
•Photo stress test
Abbreviation: VER, visually evoked response.
Note: (+) denotes presence and (–) denotes absence.
In macular disease, a delay usually occurs in recovery of visual pigments that are bleached by a bright light, but bright light has no effect on optic nerve conduction. Hence, photostress testing may be very helpful in differentiation between maculopathies and optic nerve disorders. Penlight is used to stress each eye and recovery time is noted which is greater with maculopathy and not prolonged with optic neuropathies.
Disorders of optic nerve include:
1.Edema of optic disc.
4.Degeneration or atrophy.
■Swollen Optic Disc or Disc Edema (OP8.5)
Swelling of optic nerve head (optic disc) secondary to raised intracranial pressure is referred to as papilloedema. All other causes of disc swelling in absence of raised intracranial pressure are referred to as disc edema. Disc edema may be unilateral or bilateral. Causes of disc edema include the following:
•Central retinal vein occlusion (CRVO).
•Papillophlebitis (or optic disc vasculitis).
•Anterior ischemic optic neuropathy (AION).
•Meningioma of optic nerve.
•Thyroid ophthalmopathy (advanced Grave’s disease).
•Metastatic orbital masses.
6.Increased intracranial pressure (papilloedema).
•Ocular lesions include CRVO, ocular hypotony, papillitis, papillophlebitis, and anterior ischemic neuropathy.
•Orbital lesions include orbital tumors, orbital cellulitis, and meningioma of optic nerve.
•Foster–Kennedy syndrome: It is the atrophy of optic nerve on the side of the lesion due to direct pressure and papilloedema on the other side due to raised intracranial pressure. It is associated with frontal lobe tumors, particularly meningioma of olfactory groove.
In cases with raised intracranial pressure, disc swelling is bilateral (Fig. 14.3).
•Intracranial lesions include tumors, aneurysms, cavernous sinus thrombosis, and carotid-cavernous fistula.
•Systemic causes include malignant hypertension, diabetic papillopathy, advanced Grave’s disease, and anemia.
Papilloedema is the edema of optic disc due to raised intracranial pressure. Causes of papilloedema (or raised intracranial pressure) include:
•Intracranial space occupying lesions (ICSOLs): Tumors of anterior fossa tend to produce papilloedema later than those in the posterior fossa. Thus, tumors of midbrain, parieto-occipital region, and cerebellum produce papilloedema more rapidly.
•Other intracranial causes include the following:
◊Diffuse cerebral edema from blunt head trauma.
◊Cavernous sinus thrombosis.
◊Pseudotumor cerebri (idiopathic intracranial hypertension).
◊Disturbances with CSF circulation.
•Hypersecretion of CSF by choroidal plexus tumor.
•Obstruction to CSF flow in ventricular system or at exit foramina (foramina of Luschka and Magendie) of 4th ventricle, for example, stenosis of aqueduct of Sylvius.
•Impairment of CSF absorption via arachnoid villi which may be damaged by meningitis and subarachnoid hemorrhage.
•Tumors of spinal cord: The usual lower extremity symptoms are present.
When no intracranial reason for papilloedema can be found, the usual lower extremity symptoms should dismiss consideration of pseudotumor cerebri as diagnosis, and investigation for a spinal cord tumor should be considered.
Since meningeal sheaths covering the brain also enclose the optic nerve up to lamina cribrosa, the subdural and subarachnoid spaces around the optic nerve are also continuous with those of the brain. So, any rise in intracranial pressure becomes equally evident around the nerve resulting in edema at the optic nerve head. It is purely hydrostatic and noninflammatory. So, pathological changes in papilloedema show signs of passive edema without the evidence of inflammation.
Papilloedema is usually bilateral although it may be asymmetrical. However, unilateral papilloedema do occur with raised intracranial pressure as in Foster–Kennedy syndrome.
Foster–Kennedy syndrome: It is usually associated with an olfactory groove meningioma or a frontal lobe tumor (Flowchart 14.2).
If optic nerve sheath is opened surgically in patients with increased intracranial pressure, papilloedema is relieved.
In ocular hypotony, there is decrease in tissue pressure within the prelaminar region which results in disc edema.
•Visual symptoms: Vision remains normal for a long time. Transient attacks of blurred vision (“black outs”) lasting for few seconds may occur in the initial stages.
•Symptoms of raised intracranial pressure:
◊Headache early in the morning, which becomes worse in recumbent position.
◊Nausea and vomiting (often projectile).
◊Horizontal diplopia: Increased intracranial pressure causes stretching of VIth nerve over petrous tip, resulting in paresis of 6th nerve and diplopia.
•Mechanical signs appear due to disc edema and these are:
◊Blurring of optic disc margins.
◊Filling in of optic cup at disc.
◊Elevation of disc (optic nerve head [ONH]).
◊Edema of nerve fiber layer (NFL).
◊Retinal folds (Paton’s lines).
•Vascular signs appear due to secondary vascular changes and these are:
◊Hyperemia of ONH and loss of venous pulsations.
◊Peripapillary retinal hemorrhage.
◊Cotton wool (CW) spots (NFL infarcts).
•Blurring of optic disc margins: Blurring starts initially at nasal margin and later extends around superior, inferior, and temporal margins.
•Filling in of optic cup at disc: Progressive edema extends over the surface of the disc, reducing the size of physiological cup.
Because the swelling occurs first at the periphery of optic disc, the central optic cup is usually preserved until late. This is in distinction to pseudopapilloedema due to optic disc drusen in which optic cup is absent.
•Elevation of disc: Disc becomes elevated higher than the surrounding retina, so vessels bend sharply over its margins.
When measuring the elevation of disc (degree of disc edema) with direct ophthalmoscope, a difference in dioptric power is found between the focus of edematous disc and normal nonedematous retina. Three dioptres of disc elevation denotes 1 mm of elevation.
•Edema of NFL: Edema spreads into surrounding retina, leading to blurring of NFL, and retinal vessels at disc margin become hazy (as retinal vessels course in NFL).
•Paton’s lines: As the edema spreads, retinal folds concentric to disc may develop, especially temporally known as Paton’s lines. It is one of the surest signs of true disc edema. As the edema increases, Paton’s lines are no longer seen (Flowchart 14.4).
Any cause of edema can produce Paton’s lines, so their presence only signifies edema. These are not seen with drusen of ONH (pseudopapilloedema).
•Hyperemia of ONH and loss of venous pulsations: Hyperemia is due to capillary dilatation in ONH. There is loss of normal previous spontaneous venous pulsations.
However, in 20% of normal individuals, spontaneous venous pulsations are absent. So, loss of venous pulsations is helpful only if they are previously present.
•Venous congestion: Retinal veins become congested and tortuous.
•Peripapillary retinal hemorrhages: Vascular engorgement and stasis leads to hemorrhages within NFL which are flame-shaped and marked around the disc.
•CW spots: These are fluffy patches due to NFL infarcts.
•Hard exudates: Hard exudates appear on disc surface and in the retina itself. These may radiate from the center of fovea in the form of a “macular fan”: An incomplete star with temporal part missing.
1.Early papilloedema: This is characterized by the following (Fig. 14.5a):
•Blurring of disc margins and NFL.
•Mild elevation of disc.
•Visual symptoms are absent.
•Loss of venous pulsations, if present.
2.Established/fully developed papilloedema: This is characterized by the following (Fig. 14.5b):
•Transient visual obscurations.
•Gross elevation of ONH.
•Obliteration of physiological optic cup.
•Retinal folds (Paton’s lines).
•Hard exudates with CW spots.
•Enlargement of blind spot (on visual field examination).
3.Chronic papilloedema: This is characterized by the following (Fig. 14.5c):
•Optic cup remains obliterated.
•CW spots and hemorrhages resolve.
•ONH takes on the appearance of a “champagne cork.”
•Less disc hyperemia is seen.
•Visual fields begin to constrict.
•Drusen-like crystalline deposits (corpora amylacea) may be present on disc surface.
4.Atrophic papilloedema: This is characterized by the following (Fig. 14.5d):
•Optic disc appears dirty gray and blurred secondary to gliosis (secondary optic atrophy).
•Retinal arterioles are narrowed with white sheathing.
•Disc swelling subsides.
It is done by:
•Neuroimaging: MRI or CT scan.
•Lumbar puncture with manometry.
•Fluorescein angiography (FA): It is useful to distinguish true disc edema from pseudopapilloedema.
However, it does not distinguish among the different causes of disc edema.
Papilloedema must be distinguished from the following:
•Pseudopapilloedema: It occurs due to drusen of optic disc and hypermetropia
•Disc edema without raised intracranial pressure: It occurs due to the following causes:
◊Compressive optic neuropathies in orbital lesions.
◊Papillophlebitis (optic disc vasculitis).
◊Juvenile diabetic papillopathy.
◊Other causes: Hypotony and malignant hypertension.
•Drusen of optic disc: It may be confused with early papilloedema. Differentiating features of both are provided in Table 14.3.
•Hypermetropia: In hypermetropia, optic disc is small and nerve fibers from surrounding retina are heaped up on ONH. Therefore, margins appear swollen and blurred. In hypermetropia:
◊Swelling is never >2D.
◊No venous congestion.
◊No edema or exudates.
◊Blind spot not enlarged.
◊No leakage on FA.
•Papillitis: It is the inflammation of intraocular part of optic nerve that gives rise to disc edema, which is indistinguishable from that in papilloedema. In papillitis:
◊Impairment of vision is sudden and marked.
◊Features of raised intracranial pressure are absent.
◊Pain on ocular movements is present.
◊Afferent pupillary defect is present.
◊Cells in vitreous are present.
◊Visual field defects usually show central scotoma more for colors.
Table 14.3 Differentiating features of papilloedema and pseudopapilloedema
Drusen of optic disc (Pseudopapilloedema)
1. Color of optic disc
Hyperemic, so bright red
Not hyperemic, so pink or yellow
2. Spontaneous venous pulsations
Present (in 80% of cases)
3. Peripapillary NFL
Edematous and opacified
4. Peripapillary hemorrhages
5. Exudates or CW spots
6. Retinal veins
Dilated (venous engorgement)
7. Fluorescein angiography
Early leakage of dye
Show auto fluorescence and stain during late stages
8. Vessels over disc margin
Abbreviations: CW, cotton wool spots; NFL, nerve fiber layer.
Note: (+) denotes presence and (–) denotes absence.
It is defined as the presence of raised intracranial pressure in the absence of ICSOL or enlargement of ventricles due to hydrocephalus with normal CSF composition. It occurs in young, obese females of child-bearing age.
Clinical features are those of raised intracranial pressure. Symptoms include headache, transient blurring of vision, and diplopia. Characteristic signs include papilloedema, 6th nerve palsy (manifested as diplopia), and large blind spot on visual field examination.
Four criteria to diagnose pseudotumor cerebri are:
1.Increased intracranial pressure.
2.Normal-sized ventricles on neuroimaging (CT or MRI scan).
3.Normal CSF composition: Lumbar puncture shows CSF with no inflammatory cells, normal CSF glucose, and protein.
It is essentially the treatment of cause:
•Diuretics, especially carbonic anhydrase inhibitors (acetazolamide), are the first-line treatment for headache and visual field loss.
•Weight reduction in cases of pseudotumor.
•Optic nerve sheath decompression which involves incision of meningeal sheath around the optic nerve. The effect is remarkable if timely performed.
•Lumboperitoneal shunt may be performed.
■Inflammation of Optic Nerve (OP8.5)
Inflammation of optic nerve is known as optic neuritis. Depending upon the part affected by inflammation, optic neuritis can be categorized into:
•Papillitis: It is the inflammation of ONH.
•Neuroretinitis: It is the inflammation of ONH with adjacent retinal inflammation, that is, papillitis with retinitis.
•Retrobulbar neuritis is the inflammation of optic nerve behind the eyeball.
•Demyelinating disorders are the most common cause and include multiple sclerosis, neuromyelitis optica (Devic’s disease), and isolated optic neuritis.
•Associated with infections (infectious optic neuritis): The infection may be local or systemic.
Local infection may be intraocular (as in endophthalmitis) or contiguous spread from meninges (meningitis as in tuberculosis and syphilis), sinuses (sinusitis, particularly of sphenoid and ethmoid sinuses), or orbit (orbital cellulitis). Systemic (endogenous) infection may be:
•Measles, mumps, chickenpox, herpes zoster, and influenza.
•Tuberculosis, syphilis, and cat-scratch fever.
•Cryptococcosis and histoplasmosis.
•Toxocariasis and toxoplasmosis.
•Noninfectious optic neuritis: It includes systemic autoimmune diseases such as sarcoidosis, systemic lupus erythematosus (SLE), and polyarteritis nodosa.
White matter of central nervous system (CNS) (brain and spinal cord) consists of myelinated nerve fibers, and gray matter consists of nonmyelinated nerve fibers. In demyelination, myelinated nerve fibers lose their myelin sheath (myelin is phagocytosed by microglia and macrophages, subsequent to which astrocytes lay down fibrous tissue). Therefore, nervous conduction is disrupted within white matter tracts in brain and spinal cord but peripheral nerves are not involved. The involvement of visual pathway results in optic neuritis. Involvement of brain stem result in:
•Ocular motor cranial nerve palsies.
•V and VII nerve palsies.
Neuroretinitis is not seen in demyelinating disease (multiple sclerosis) as nerve fiber layer is nonmyelinated and is commonly due to an infectious etiology.