Neuro-ophthalmology
Optic nerve
Applied anatomy:
The optic nerve carries about 1.2 million afferent nerve fibers, which
are represent the axons of retinal ganglion cells. Most of these (90%)
synapse in the lateral geniculate body "LGB" (carrying visual
stimulation), and the rest (10%) reach other centers, notably the pre-
tectal nuclei in the mid brain carrying light pupillary reflex. The
afferent pathway for light pupillary reflex is the optic nerve while the
efferent pathway is the oculomotor nerve.
The optic nerve is approximately 5 cm long from globe to optic
chiasm, where decussation occurs, the temporal fibers for each nerve
pass to ipsilateral optic tract, while nasal fiber cross to contralateral
optic tract (crossing fibers).
Any injury to the axons of ganglion cells before LGB causes optic
disc atrophy seen by fundoscopy (ipsilateral if pre-chiasmal and
bilateral if post-chiasmal), while the injury that occurs after LGB
(optic radiation and occipital cortex), will not cause optic disc
atrophy.
Visual center lies mainly on the medial surface of the occipital cortex
(Broadman's area no. 17).
Optic nerve can be subdivided into four segments:
1- Intra-ocular segment (optic disc, nerve head): about 1mm depth
& 1.5mm in diameter.
2- Intra-orbital segment: 3.0 cm, this part has S shape allowing the
eye for movement without nerve stretching (i.e. the distance from the
apex of orbit to the posterior part of eyeball is less than 3.0 cm).
3- Intra-canalicular segment: 1cm.
4- Intra-cranial segment: 6-8 mm, joins the chiasm.
* The optic nerve is surrounded by pia, arachnoid and dura mater, so
the CSF reaches up to the posterior sclera around the optic nerve.
Axoplasmic transport:
It is the movement of cytoplasmic organelles within a neuron between
the cell body and the terminal synapse of Ganglion cells.
Retinal cotton-wool spots are the result of accumulation of
cytoplasmic organelles due to interruption of axoplasmic flow
between the retinal ganglion cells and their terminal synapses.
Papilloedema is similarly caused by hold-up of axoplasmic flow at the
lamina cribrosa (Small pores present at posterior sclera for exit of
optic nerve fibers).
Signs of optic nerve dysfunction:
1- Decreased visual acuity.
2- Diminish light pupillary reflex.
3- Dyschromatopsia (impairment of color vision): affected eye sees
the colors less bright.
4- Diminished light brightness sensitivity.
5- Visual field defect: depends on the type of the pathology, e.g.
central scotomas, centrocaecal scotomas and altitudinal.
Special investigations:
1- Manual kinetic Perimetry (Goldmann) for assessment of peripheral
VF.
2- Automated Perimetry for assessment of peripheral and central VF.
3- MRI: detect tumors or degenerative diseases like multiple sclerosis.
4- Visual Evoked Potential (VEP): is a recording of the electrical
activity of the visual cortex by stimulation of the retina (diagnose any
damage from ganglion cell to occipital cortex), while for diseases
from receptors to ganglion cell we use ERG (Electro-Retinography).
5- Fluorescein angiography: to differentiate between optic nerve
diseases and papilloedema, e.g. optic disc drusen and papilloedema,
as drusen do not leaking fluorescein dye while papilloedema leaking
the dye.
Optic neuritis
It is an inflammatory or demyelinating process affecting the optic
nerve.
1- Ophthalmoscopic classification:
a- Retrobulbar neuritis: in which the optic disc appearance is
normal, at least initially, because the optic nerve head is not
involved. It is the most frequent type in adult and is frequently
associated with multiple sclerosis.
b- Papillitis: in which the pathological process affects the optic
nerve head. It is characterized by variable hyperemia and edema
of the optic disc, which may be associated with parapapillary
flame-shaped hemorrhages. Papillitis is the most common type
of optic neuritis in children, although can also affect adults.
2- Etiological classification:
a- Demyelinating: which is by far most common cause usually
young females with Multiple Sclerosis (MS).
b- Para-infectious: it is follow a viral infection or immunization.
c- Infectious: which may be sinus-related or associated with
syphilis, lyme disease, cat-scratch fever and cryptococcal
meningitis. patients with AIDS or Herpes zoster can be
presented with optic neuritis.
d- Autoimmune: may be associated with systemic autoimmune
disease.
Treatment of optic neuritis:
is according to the etiology.
if the cause is demyelination in MS, the patient need urgent IV
methylprednisolone, then oral prednisolone because the vision is
severely affected.
Optic atrophy
It is an important sign of advanced optic nerve disease. It is of two
types:
1- Primary optic atrophy:
It is occurs without antecedent swelling of the optic nerve head. It
may be caused by lesions affecting the visual pathways from the retro
laminar (behind lamina cribrosa) portion of the optic nerve to the
lateral geniculate body. Lesions anterior to the optic chiasm result in
unilateral optic disc atrophy, whereas those involving the chiasm and
optic tract will cause bilateral optic disc atrophy.
Causes:
- Retro bulbar neuritis (but not Papillitis, as it is preceded by disc
swelling)
- Compressive lesions, such as tumors and aneurysms.
- Hereditary optic neuropathies.
- Toxic and nutritional optic neuropathies.
Signs:
- Pale, flat disc with clearly delineated margin.
- Reduction in number of small blood vessels on the disc surface.
2- Secondary optic atrophy:
It is preceded by swelling of the optic nerve head.
Causes:
- Papillitis.
- Chronic papilledema.
- AION (Anterior Ischemic Optic Neuropathy): usually occurs in old
age patients, it is of two types; non arteritic [in diabetes, hypertension]
and arteritic .e.g. Giant cell arteritis.
Signs:
- White or dirty grey, slightly raised disc with poorly delineated
margins due to gliosis.
- Reduction in number of small blood vessels on the disc surface.
Papilledema
It is swelling of the optic nerve head secondary to raised intracranial
pressure. It is nearly always bilateral, although it may be
asymmetrical.
All other causes of disc edema in the absence of raised ICP are
referred to as "disc swelling" and usually produce visual impairment.
All patients with bilateral discs swelling should be suspected of
having an intracranial mass until proved otherwise. However, not all
patients with raised ICP (intra cranial pressure) have necessarily
developed papilledema.
1- Early features of papilledema:
- Visual symptoms are absent and visual acuity is normal.
- Optic disc shows hyperemia and mild elevation.
2- Established papilledema:
- Transient visual obscurations lasting a few seconds.
- Visual acuity is normal or reduced.
- Optic disc shows severe hyperemia, moderate elevation and
indistinct margin.
3- Atrophic papilledema:
- Visual acuity is severely impaired.
- Optic discs are dirty grey color, slightly elevated and indistinct
margin.
Other differential diagnosis of bilateral discs swelling:
1- Malignant hypertension.
2- Bilateral simultaneous Papillitis.
3- Bilateral compressive thyroid ophthalmopathy.
4- Bilateral simultaneous AION.
5- Bilateral compromised venous drainage in central retinal vein
occlusion or carotid-cavernous fistula.
Abnormal pupillary reaction
Applied anatomy
LIGHT REFLEX
The pupillary light reflex consists of four neurons.
1. The first connects the retina with the pre-tectal nucleus in the mid-
brain at the level of the superior colliculus. The reflex is mediated by
the retinal photoreceptors. Impulses originating from the nasal retina
are conducted by fibers which decussate in the chiasm and pass up the
optic tract to terminate in the contralateral pre tectal nucleus. Impulses
originating in the temporal retina are conducted by uncrossed fibers
which terminate in the ipsilateral pre tectal nucleus.
2. The second connects the pre tectal nucleus to both Edinger-
Westphal nuclei by internuncial fibers. This is why a unilateral light
stimulus evokes a bilateral and symmetrical pupillary constriction.
Damage to these internuncial neurons is responsible for light-near
dissociation in neurosyphilis and pinealomas.
3. The third connects the Edinger-Westphal nucleus to the ciliary
ganglion inside the orbit. In the orbit, these parasympathetic fibers
pass in the inferior division of the third cranial nerve and reach the
ciliary ganglion via the nerve to the inferior oblique muscle.
4. The fourth leaves the ciliary ganglion and passes with the short
ciliary nerves to innervate the sphincter pupillae. The ciliary ganglion
is located within the muscle cone, just behind the globe. It should be
noted that, although the ciliary ganglion contains other nerve fibers
(sensory and sympathetic), only the parasympathetic fibers synapse
there.
NEAR REFLEX
The near reflex triad consists of: (1) increased accommodation, (2)
convergence of the visual axes and (3) constriction of the pupils. The
term 'light-near dissociation' refers to a condition in which the light
reflex is absent or abnormal, although the near response is intact.
Vision is not a prerequisite for the near reflex, and there is no clinical
condition in which the light reflex is present but the near response
absent. Although the final pathways for the near and light reflexes are
the same (i.e. third nerve, ciliary ganglion, short ciliary nerves), the
center for the near reflex is ill-defined. There are probably two supra
nuclear influences: the frontal and occipital lobes. The mid-brain
center for the near reflex is probably located in a more ventral
location than the light reflex (in pre-tectal nucleus) and this may be
one of the reasons why compressive lesions such as pinealomas
preferentially involve the dorsal pupillomotor fibers, sparing the
ventral fibers until late.
SYMPATHETIC SUPPLY
The sympathetic supply consists of three neurons:
1. The first starts in the posterior hypothalamus and descends,
uncrossed, down the brain stem to terminate in the ciliospinal center
of Budge located between C8 and T2.
2. The second passes from the ciliospinal center of Budge to the
superior cervical ganglion in the neck. During its long course, it is
closely related to the apical pleura where it may be damaged by
bronchial carcinoma (Pancoast's tumor) or during surgery on the neck.
3. The third ascends along the internal carotid artery to enter the
skull, where it joins the ophthalmic division of the trigeminal nerve.
The sympathetic fibers are also passing through ciliary ganglia but
without relay and it are reaching the ciliary body and the dilator
pupillae muscle via the nasociliary nerve and the long ciliary nerves.
Afferent pupillary conduction defects
A total afferent pupillary defect (TAPD, Amaurotic pupil) is caused
by a complete optic nerve lesion and is characterized by the
following:
1. The involved eye is completely blind (i.e. no light perception).
2. Both pupils are equal.
3. When the affected eye is stimulated neither pupil reacts but when
the normal eye is stimulated both pupils react normally.
4. The near reflex is normal in both eyes.
A relative afferent pupillary defect (RAPD, Marcus Gunn pupil) is
caused by an incomplete optic nerve lesion or severe retinal disease,
but not by a dense cataract or vitreous hemorrhage. The clinical
features are those of an Amaurotic pupil but more subtle. The
difference between the pupillary reactions is enhanced by the
'swinging-flashlight test' in which each pupil is stimulated in rapid
succession. When the abnormal pupil is stimulated it dilates instead of
constricting. This paradoxical reaction of the pupil to light occurs
because the dilatation of the pupil, by withdrawing the light from the
normal eye, outweighs the constriction produced by stimulating the
abnormal eye.