11.7.3 Neuro-opthalmology And Optic Nerve Flashcards
Afferent visual pathways consists of
- Retina
- Optic nerve
- Optic chiasm
- Optic tract
- Lateral geniculate nucleus
- Optic radiation
- Visual Cortex in the occipital lobe
Detecting pathology in afferent visual pathways
- 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
Examination of pt with reduced visual acuity: RAPD
General
Examination
Interpretation
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.
Examination of pt with ⬇️ VA: Optic nerve
- 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).
Swollen optic disc
General types
- 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.
Papilloedema
- Bilateral disc oedema, may be asymmetric.
- Hyperaemia of disc.
- Splinter haemorrhages, exudates, cotton wool spots may be present.
Swollen optic disk
Ischaemic types
- 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!!
Ischeamic optic neuropathy
- If the optic neuropathy is ischaemic, the disc is usually pale and swollen, especially with Giant Cell Arteritis.
Pale optic disc: optic atrophy
- 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.
Pale optic disc: Advanced glaucomatous cupping
- 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.
Examination of pt with ⬇️ VA: Visual field
- 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.
Scotoma
Def
Types
- 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.
Hemianopia
Def
Types
- 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.
Altitudinal defect def
Altitudinal defect: the defect involves only the upper or lower half of the field.
Exam pt ⬇️ VA: Adjunctive test
- 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.
Ocular motility
General
2 Types
- 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.
Third nerve palsy (Oculomotor)
- 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!
Complete cranial nerve III palsy
- 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.
4th nerve palsy (Trochlear)
- 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.
Superior oblique plasy
Head tilt to opposite side
6th nerve palsy (Abducens)
- 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
Pupillary abnormalities
- 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
Horner’s syndrome
- 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.
Eyelid & facial abnormalities: Ptosis
- Myogenic ptosis (CPEO)
- Aponeurotic ptosis (involutional)
- Neurogenic ptosis (Horner’s syndrome, CN III)
- Neuromuscular ptosis (Myasthenia Gravis)
- Mechanical ptosis (Dermatochalasis)
- Traumatic ptosis
- Pseudoptosis ( Enophthalmos)
Important visual emergencies
- 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!
Markus gunn pupil: relative afferent pupil defect
- 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.
Adie’s pupil (Tonic pupil)
- 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.
