11.7.3 Neuro-opthalmology And Optic Nerve

Question 1

Afferent visual pathways consists of

Answer 1

• Retina
• Optic nerve
• Optic chiasm
• Optic tract
• Lateral geniculate nucleus
• Optic radiation
• Visual Cortex in the occipital lobe

Question 2

Detecting pathology in afferent visual pathways

Answer 2

• Patient history
• Visual acuity (best corrected)
• Relative afferent pupillary defect (RAPD) Ocular examination
• Visual field defects
• Adjunctive tests: colour vision, contrast sensitivity, photostress recovery test

Question 3

Examination of pt with reduced visual acuity: RAPD
General
Examination
Interpretation

Answer 3

Relative afferent pupillary defect
- after ⬇️ visual acuity is established = check for RAPD
- ambient light & pt fixate in distance

Examination
- Stimulate the one eye for 2-3 seconds and quickly move across the bridge of the nose to stimulate the other eye for 2-3 seconds.
- Make several alterations and mentally average the pupil responses.

Interpretations
- dense RAPD is easily detected when the affected eye’s pupil dilates in response to direct light stimulation.
- magnitude of the RAPD correlates with degree of damage to retinal ganglion cells and their axons.
- presence of an RAPD does not result in anisocoria.

Question 4

Examination of pt with ⬇️ VA: Optic nerve

Answer 4

• If the visual acuity is reduced and the patient has an RAPD, the next step is to locate the pathology.
• In the vast majority of cases when an RAPD is present, there will be retinal or optic nerve pathology.
• Fundoscopy will assist in diagnosis.
• If the retina appears normal, the optic nerve head should be examined carefully with attention to vasculature, colour of the neuro- retinal rim, disc margin and cup/disc ratio
• findings can be broadly divided into the swollen optic disc and the pale optic disc (optic atrophy).

Question 5

Swollen optic disc
General types

Answer 5

• Papilloedema from CNS tumours etc: bilateral disc oedema, normal vision (if no direct compression of visual pathway), no RAPD (in the acute phase)
• Idiopathic Intracranial Hypertension causing papilloedema: bilateral disc oedema, normal vision, no RAPD( in the acute phase and non- fulminant)
• Optic neuritis(papillitis): usually unilateral, can be bilateral, acute impairment of vision, RAPD present.

Question 6

Papilloedema

Answer 6

• Bilateral disc oedema, may be asymmetric.
• Hyperaemia of disc.
• Splinter haemorrhages, exudates, cotton wool spots may be present.

Question 7

Swollen optic disk
Ischaemic types

Answer 7

• Nonarteritic ischaemic optic neuropathy: usually unilateral, acute visual loss, RAPD, age 60-70yrs, vascular risk factors
• Arteritic ischaemic optic neuropathy (Giant cell arteritis): acute, RAPD initially if unilateral, pale disc, scalp tenderness, jaw claudication, new onset headache, age 70-80yrs. OPHTHALMIC EMERGENCY!!

Question 8

Ischeamic optic neuropathy

Answer 8

• If the optic neuropathy is ischaemic, the disc is usually pale and swollen, especially with Giant Cell Arteritis.

Question 9

Pale optic disc: optic atrophy

Answer 9

• Optic atrophy is the hallmark of damage to the retinal ganglion cells.
• Atrophy does not occur immediately but takes 4-6 weeks from the time of axonal damage.
• During examination compare the colour of the two discs, the vasculature and the disc margin.
• All the causes of a swollen optic disc can result in optic atrophy including papilloedema if chronic.

Question 10

Pale optic disc: Advanced glaucomatous cupping

Answer 10

• can be difficult to distinguish from optic atrophy.
• Raised intra-ocular pressure more indicative of glaucoma.
• VA can be helpful, central VA often spared until end-stage glaucoma.
• In glaucoma the cup is pale, but the neuro-retinal rim is pink.

Question 11

Examination of pt with ⬇️ VA: Visual field

Answer 11

• Visual field defects can assist in localising neurological lesions causing visual loss.
• Central, centrocecal, paracentral and arcuate scotomas suggest optic nerve pathology.
• Bitemporal hemianopia is due to interruption of decussating nasal fibers in the chiasm.
• All retrochiasmatic lesions result in a contralateral homonymous hemianopia.

Question 12

Scotoma
Def
Types

Answer 12

• Scotoma: an area of loss of visual function in the visual field.
• Absolute scotoma: an area of total loss of visual function.
• Relative scotoma: an area of partial loss of visual function.

Question 13

Hemianopia
Def
Types

Answer 13

• Hemianopia: scotoma involving half of the visual field.
• Homonomous hemianopia: the same side of each visual field is involved.
• Heteronymous hemianopia: opposite sides of the visual fields are involved e.g. bitemporal hemianopia.

Question 14

Altitudinal defect def

Answer 14

Altitudinal defect: the defect involves only the upper or lower half of the field.

Question 15

Exam pt ⬇️ VA: Adjunctive test

Answer 15

• Severe reduction in colour vision is indicative of optic neuritis*
• In suspected optic nerve disease colour vision testing is essential.
• In optic nerve disease, the degree of dyschromatopsia may be proportionately greater than the degree of Snellen visual acuity loss.
• Colour vision testing should be performed separately for each eye to detect unilateral disease.

Colour vision testing
- Ishihara colour plates can be useful.
- Beware congenital colour blind patients.

Question 16

Ocular motility
General
2 Types

Answer 16

• The most important symptom in ocular motility pathology is diplopia.
• The following questions are important:
  1: Is the diplopia monocular or binocular?
  2: Is the double vision the same in all fields of gaze (comitant) or does it vary (incomitant)?
  3: Is the double vision horizontal, vertical or oblique?

MONOCULAR DIPLOPIA
Usually abnormalities in refractive media:
- High astigmatism
- Corneal irregularity
- Lens opacities
- Retinal pathology is less common cause.

BINOCULAR DIPLOPIA
- Causes of diplopia can be divided anatomically into supranuclear, nuclear, internuclear and infranuclear causes.
- Infranuclear causes include cranial nerves III, IV and VI.

Question 17

Third nerve palsy (Oculomotor)

Answer 17

• Complete third nerve palsy presents with:
  1. complete ptosis
  2. eye down and out
  3. dilated pupil with poor response to light
• A new onset isolated CN III palsy with a dilated pupil is due to posterior communicating artery aneurysm until proven otherwise! EMERGENCY!

Question 18

Complete cranial nerve III palsy

Answer 18

• can be difficult to see if superior oblique (SO) function is intact in a patient with a CN III palsy because SO works as a depressor in adduction.
• eye cannot adduct because medial rectus is affected.
• Look for intorsion on attempted downgaze.

Question 19

4th nerve palsy (Trochlear)

Answer 19

• Vertical diplopia (double vision)
• Diplopia worse in downgaze
• Ipsilateral head tilting increases vertical strabismus
• Patients tilt their head to the opposite side to reduce diplopia.

Question 20

Superior oblique plasy

Answer 20

Head tilt to opposite side

Question 21

6th nerve palsy (Abducens)

Answer 21

• Most frequent cause of an isolated ocular motor palsy.
• Horizontal diplopia worsening on ipsilateral gaze.
• Typically associated with an esodeviation
• Ischaemic mononeuropathy is the most common cause.

Right CN6 palsy
- right eye cannot abduct

Question 22

Pupillary abnormalities

Answer 22

• Relative afferent pupillary defects as described earlier.
• Horner’s syndrome (ptosis, miosis, anhydrosis)
• Adie’s tonic pupil (dilated, poor reaction to light, slow constriction for near)
• Other causes for light near dissociation.
• Pharmacologically dilated pupils

Question 23

Horner’s syndrome

Answer 23

• Ptosis is mild, usually < 3mm.
• Upside down ptosis of the lower lid.

All signs are on the same side of the face
- Partial ptosis of the upper lid due to paralysis of Müller’s muscle. This may mimic enophthalmos (pseudoenophthalmos).
- Miosis which is more obvious in the dark.
- Anhydrosis of the side of the face.
- Heterochromia of the iris only if the syndrome is congenital.
- Pharmacological tests can be done to confirm the diagnosis.

Question 24

Eyelid & facial abnormalities: Ptosis

Answer 24

• Myogenic ptosis (CPEO)
• Aponeurotic ptosis (involutional)
• Neurogenic ptosis (Horner’s syndrome, CN III)
• Neuromuscular ptosis (Myasthenia Gravis)
• Mechanical ptosis (Dermatochalasis)
• Traumatic ptosis
• Pseudoptosis ( Enophthalmos)

Question 25

Important visual emergencies

Answer 25

• Unilateral optic neuropathy in an elderly patient with jaw claudication and scalp tenderness in an emergency. Giant cell arteritis must be promptly treated to prevent bilateral blindness.
• Complete Third nerve palsy with a dilated pupil is an emergency!

Question 26

Markus gunn pupil: relative afferent pupil defect

Answer 26

• afferent pathway of one eye is appreciably weaker than that of the other eye.
• detected by means of the swinging flashlight test.
• lesion is prechiasmal: it is caused only by a unilateral or very asymmetrical optic neuropathy or by extensive unilateral or very asymmetrical retinal damage.
• Light-near dissociation occurs as accommodation becomes a stronger stimulus than light for pupillary constriction.
• Note that:
  (a) Medial opacities do not produce an afferent pupil defect.
  (b) A unilateral enlarged pupil cannot be caused by an optic neuropathy.

Question 27

Adie’s pupil (Tonic pupil)

Answer 27

• Light-near dissociation with slow constriction in response to accommodation.
• Usually unilateral.
• Usually in young (20-40 years) women.
• Caused by a lesion of the ciliary ganglion of unknown aetiology.
• reason for the light-near dissociation is that many more efferent fibres subserve accommodation than light, so that nonselective nerve cell damage affects the light pathway much more than the accommodation pathway.
• In isolation a tonic pupil is of no neurological importance and requires no further investigation.
• Pharmacological tests can be done to confirm the diagnosis.