Neuro-ophthalmology 1

introductory video

History :

Ask about past medical history, drug history, family history, and occupation. Smoking, alcohol use and diet may be particularly important. The history is guided by the presenting complaint.

Examination:

Basic neuro-ophthalmic examination includes: near and distance VA, colour vision, confrontation field tests, pupil tests, cover tests, eye movements, cranial nerves, fundoscopy including optic nerve examination, and other tests as indicated.

Full neurological examination may be required.

■ Cover test and eye movements:

■ Pupils:

1. Observe: look for anisocoria, ptosis, and iris abnormality (heterochromia, rubeosis, posterior synechiae, coloboma).
2. Check the near response using an accommodative target (some patients fi nd it easier to converge on their own finger).
3. Test pupillary constriction to light: dim the lights and ask the patient to look at a distance target. Check the direct response (pupil constriction in the same eye) and consensual response (fellow eye constriction). Use a very bright light such as an indirect
ophthalmoscope.
4. Look for a relative afferent pupillary defect (RAPD) using the swinging light test. Ask the patient to fix on a distant object. Shine light in one eye for 3 seconds, then move to the other for the same interval. Repeat as required but do not spend longer on one eye than the other as this may bleach the retina and create an artificial RAPD. If the left
optic nerve is damaged, shining the light in the right eye will produce a normal efferent response in the right and left pupil. Moving the light to the left eye then causes a paradoxical dilatation of both pupils as the stimulus intensity is effectively diminished (left RAPD). Retinal disease, if extensive enough, will also impair the pupil light reflex. Only one functioning pupil is required to test for an RAPD; for example, in a patient with a 3rd nerve palsy with a fixed dilated pupil the test can be carried out as above but only the active pupil is observed. Remember that in bilateral symmetrical optic nerve disease the pupil deficit will be balanced and no RAPD will be elicited.

■ Confrontation visual field testing:

1. Establish the VA and explain that you are testing peripheral vision.
2. Ask the patient to cover one eye with the palm of their hand: ‘Look at my nose: are any parts of my face missing or blurred? Can you see both my eyes?’
3. Hold up both your palms, one in each hemifield and ask, ‘Is each hand equally clear, are there any differences or does each hand look the same?’
4. Now ask the patient to fix on your eye. Always begin by testing for a defect to hand movements. Advance your hand, with fingers moving, from the periphery towards fixation, asking the patient to report when they see movement. A patient will not be able to count fingers in visual field region in which he or she cannot detect hand movements.
5. Ask the patient to fixate on your eye and hold up a finger in one quadrant; ask how many fingers can be seen.
Repeat in each quadrant, presenting 1, 2, or 5 fingers (3 or 4 is too difficult). This will identify areas where vision is better than hand movements.
6. In visual field areas where finger-counting vision has been established look for red desaturation using a red target such as a 4 mm hat pin (a red bottle top may also be used but it is too large for the blind spot or small scotomas). Compare subjective colour intensity in four quadrants whilst fixating your eye. Simultaneously holding two red pins either side of the vertical midline (in each eye separately) may help detect bitemporal red desaturation from pituitary tumours or a relative homonymous hemianopia.
7. Test the midperipheral and central field with the red target. ‘Tell me when you first see the red colour, rather than the pin, and tell me if it then disappears or the colour changes at any point.’ Move obliquely in each quadrant from periphery to fixation, equidistant between you and patient. Compare to your own monocular visual field. Look for scotomas.
8. Consider comparing the blind spot size with your own using the red target.
9. It is possible to ascertain the isopter to a small white target such as a 4 mm white hat pin. Failure to detect the white target indicates greater severity of loss than is revealed by the ‘red desaturation’ technique. Similarly, the white target can also be used to plot scotomas and the blind spot.
10. Test the fellow eye.
11. If any defect is detected, draw fields and arrange formal testing. Document the left eye visual field on the left side of the page; the right visual field on the right side of the
page (opposite convention to VA).

■ Colour vision.

Optic nerve damage can result in loss of color saturation and poor color discrimination before significant acuity loss is present. This is very helpful in determining the likely cause of visual loss (i.e. refractive, lenticular, retinal, optic nerve, postchiasmal) and is usually tested with Ishihara plates. The procedure is as follows:
1. Patients should wear reading correction if required.
2. Cover one eye and ask patient to read through the book. Allow 2 or 3 seconds per plate only.
3. Patients with VA better than 6/60 can usually complete the test, but do not proceed if they cannot see the first test plate. Test patients before examining them with bright lights
(e.g. fundal examination or RAPD check) or allow them several minutes to recover after such examination. Patients who have had dilating drops can still perform the test but  may need to use a +1.00 or +2.00 lens. Consider testing red desaturation instead if VA is very poor.
4. Score the number of plates read out of 13, 17, or 21 depending on which book is used (1/17 or ‘control only’ if only the test plate was read). Record the reading distance and any refractive correction.
5. There are some plates which only some patients with congenital anomalous color vision (Daltonism) will be able to read. Do not include these when testing for acquired visual loss unless the patient has Daltonsim.
6. Patients who cannot identify numerals (e.g. small children) can be asked to trace the outline of the numbers with their finger or trace the wiggly line across the page in the color plates at the end of the book.
7. The Ishihara book comes with instructions on how to interpret performance on the missed plates in cases of Daltonism. The missed and misread numbers should be the same using each eye.

■ Red desaturation.

Ask the patient to report any difference in the appearance of a red target viewed with each eye in turn.

■ Cranial nerves.

II. Test VA, pupil reactions, visual field and color vision. III, IV, VI. V. Warn the patient then test corneal sensation by lightly touching a wisp of cotton-wool on the peripheral cornea. Sensation may be decreased in contact lens wearers, recurrent herpetic keratitis, or if anaesthetic drops have been instilled to check IOP. Check skin sensation to light touch and pin prick on the face. Ask the patent to clench the teeth together and feel for contraction of the masseter muscles. Ask the patient to open the mouth against the resistance of your hand and look for jaw deviation towards the weaker side. VII. Ask the patient to show their teeth (smile), purse lips, blow out cheeks against the resistance of your fingers, raise eyebrows (forehead wrinkling is spared in central/upper motor neurone lesions) and screw up eyes (assess Bell’s phenomenon). Look for loss or asymmetry of nasolabial folds. Assess for lagophthalmos and corneal exposure by asking the patient to close the eyes lightly. VIII. Ask about hearing loss and vertigo. Cover one ear and whisper a number in the other for the patient to repeat. To test more formally, check that a 256 Hz or 512 Hz tuning fork held on the midline forehead (Weber’s test) is heard equally in both ears, and that air conduction persists after audible bone conduction (with the tuning fork held on mastoid) ceases (Rinné’s test). IX, X. Ask the patient to open their mouth and say ‘Ahhh!’. Look for symmetrical elevation of the uvula, or deviation away from the affected side. Ask about any choking or problems swallowing. Test the gag reflex with an orange dressed stick if concerned. XI. Ask the patient to lift both shoulders and press down to check power. Ask the patient to turn the
chin towards one shoulder and press against this action with your palm. With the other hand, feel the muscle bulk of sternocleidomastoid. XII. Observe the patient’s tongue when it is in their mouth for fasciculations or wasting. Ask the patient to stick out their tongue (deviates to the weaker side).

Investigations

Computed tomography (CT)

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Principles of CT scans


1. How is the X-ray beam used in CT scanning?

Answer

In CT scanning a beam of X-rays is used to image sections of the
body.

The X-rays fall onto detectors rather than onto film, as
is

the case with radiographs. The X-ray tube rotates around the
patient.

This allows multiple data to be collected concerning all of
the

tissues in the section being imaged. A computer records the
data

and two- and three-dimensional images can be generated.

 

2. How is the computer used to form CT images?

Answer

The X-rays that penetrate the patient strike iodide crystals within

the
detectors. These crystals emit photons of light, which

are detected
by a photomultiplier. This converts the light into

electrical pulses.

The number of pulses is directly

controlled by how much of the
X-ray hits the crystals, and

thus how radio opaque (dense) the
subject is. The electrical pulses

are recorded as digital
information and converted into images

by a computer.

3. What are the advantages of CT scans?

Answer

Useful in showing bony lesions. More useful than MRI for
investigating

cortical bone fractures and calcification of organs.

Good contrast between different tissues. Good contrast is seen
between

tissues which are mainly bone, fat, water, and air.
Use of

a narrow X-ray beam and windowing can produce
detailed images.

However CT cannot differentiate well
between different parts

of the same organ.

Useful in imaging metallic foreign body. MRI is unsuitable in metallic
foreign

body as it can move the object causing dislocation or
tissue

damage.

Rapid imaging. Modern machines can produce images in a matter

of a few seconds, depending on the type of scan where as MRI
scan

takes much longer to complete.

4. What are the main disadvantages of CT scans?      

Answer

High

ionizing dose.

Bony artefacts. Brain scans may be distorted by
bony artefacts

Very small lesions may be missed. The wavelength of the X-ray
beam

means that lesions under 1cm may be missed

Limited contrast. CT cannot differentiate between tissues of very
similar

density or between areas of inconsistancy within an organ.

 

Magnetic Resonance Imaging (MRI)

Useful for imaging the anterior and posterior visual pathways (including optic nerve, chiasm, optic tracts, optic radiation and visual cortex), brain, soft tissue or vascular masses, and nonorganic, nonmetallic foreign bodies (FB). Does not show bone or calcium well. More expensive and less readily available than CT, and less useful for some orbital disease. Contraindications include a pacemaker and possible metal FB. Patients with clipped intracranial aneurysms need documentation to show that their clip is not ferromagnetic.

■ Commonly used sequences:
a. T1 image: best for structural defi nition of anatomy. Water (vitreous and CSF) appears black (hypointense), fat is hyperintense, which degrades orbital images.
b. T2 image: best for identifying diseased tissue. Water appears white, so pathological oedema shows as a high signal; fat is also hyperintense.
c. Fat suppression: Suppression of high signal from orbital fat allows clear defi nition of the optic nerve and extraocular muscles

d. Fluid-attenuated inversion recovery sequence (FLAIR): CSF appears dark, making it easier to detect small T2- hyperintense lesions (e.g. MS plaques) adjacent to the
lateral ventricles.
e. Short tau inversion recovery sequence (STIR): ideal for imaging the intraorbital optic nerve as orbital fat is supressed but water (infl ammatory oedema) can still be
seen as high signal.
f. Gadolinium contrast: useful when looking for lesions with disrupted vessel permeability. Consider if suspected meningioma, acoustic neuroma, lymphoma, metastatic disease, optic neuropathy (e.g. sarcoid), or active MS plaques. Contrast enhancement can be seen on T1 weighted images but not on T2 weighted images.

 

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Substance T1 weighted T2 weighted
Water/Vitreous/CSF black Light grey or white
Fat White Light grey
Muscle Grey Grey
Air Black Black
Fatty bone marrow White Light Grey
Brain: White matter Light Grey Grey
Brain: Grey matter Grey very light grey

 

 

1. How can magnetic resonance be used for imaging?

Answer

An imaging technique that uses the magnetic properties of the
hydrogen

atoms to produce images. The nucleus of the hydrogen
is a spinning

charged proton with magnetic properties.

Two magnetic fields are used in MRI. The first being a strong static
magnetic

field which cause the hydrogen atoms in the bodies to
align

in a direction parallel to the field. A second magnetic field
(radio-frequency

RP pulse) is then applied at right angle to the
first field

causing the hydrogen atoms to change their alignments.
When

the RP pulse is turned off the hydrogen atoms
return to their

alignment with the static magnetic field. In so doing,
they

release the extra energy as magnetic signal which can be
detected

by coil placed around the patient.

The magnetic signals depends on many factors amongst which are:
density

of the hydrogen, the environment of the hydrogen atoms
(in

fat or in water), flow (CSF or blood) and relaxation time (T1
and

T2). T1 (longitudinal relaxation) depends on the time the
hydrogen

atoms take to return tot he axis of the magnetic field
where

as T2 (transverse relaxation time) measure the interaction
of

hydrogens with other tissues.

2. What are the differences between T1 and T2 weighted images?

Answer

There are two main forms of MRI, known as T1 weighted and T2

weighted.  The two image types are also known as T1 contrast and
T2

contrast. As these names imply, the reason that more than one
image

type is required is that some tissues are contrasted better,
and

thus are easier to see, with one technique rather than the
other.

T1 contrast shows better anatomical detail and better
distinction

between solid and cystic structures. T2 contrast shows
pathological

changes better.

3. What are the advantages of MRI scans?

Answer

Non-ionizing radiation.  Because MRI does not ionize tissue it

is
considered amongst the safest of radiological techniques.

There are
no known physiological side effects of being exposed

to a
magnetic field.

High soft-tissue contrast MRI images provided very detailed
information

about soft tissues. They can differentiate between
normal and abnormal

tissues and may show damage
missed on CT

Visualization of areas deep within bony structures MRI is thus
invaluable

for the diagnosis and treatment of brainstem tumours.

Shows vasculature without contrast.

Good for angiography.  MRI is excellent for imaging blood flow.
Functional

MRI maps changes in blood flow in the brain
during specific

tasks. This provides valuable information
about how the brain

works.

4. What are the main disadvantages of MRI scans?

Answer

High cost of equipment.

Claustrophobia. Up to 10% of patients experience claustrophobia

during

an MRI scan and 1% of scans have to be aborted
because of

it.

Long imaging time. A complete image may take up to30 mins and

movement at the wrong time can cause artefacts in the image.
However,

new techniques have reduced imaging time.

Strong magnetic field. MRI imaging is unsuitable for many patients
with

metal implants (e.g. artificial joints) and is especially
dangerous

for patients with pacemakers, neural stimulators,

or cochleal implants. Loose metal objects must be removed before
coming

near to the scanner otherwise they may be attracted so
strongly

by the magnet that they fly through the air like tiny
missiles!

Unable to image calcium  Because MRI detects water rather than
molecular

density, calcium is not well visualized. This means
that tissue

calcification, a feature of a number of disease processes,
can

not be detected. Bone is also less obvious than on
a CT scan.

Acoustic noise  Switching on and off of the gradient coils causes
repeated

loud bangs. Noise levels may reach 95 dB for much
of the scan.

Ear plugs are advisable to reduce the risk of
temporary or

even permanent hearing loss.

■ Common examples:

a. Optic nerve disease: judge size on coronal sections. Acute optic neuritis shows high signal on T2 imaging and gadolinium enhancement. The acute phase, anterior ischaemic optic neuropathy shows no abnormality. Both will show high signal chronically due to Wallerian degeneration.
b. Vascular imaging: blood fl ow produces a ‘flow void’ on MRI, appearing as low signal in most of the above sequences. Lack of flow void implies complete or partial occlusion. Consider axial T2 cuts to look for dural sinus thrombosis or carotid artery dissection. MR angiography (MRA): can confirm blood fl ow without contrast injection, but may fail to detect very slow fl ow. MRA excludes clinically signifi cant intracranial arterial aneurysms of >3 mm but has not yet completely replaced intra-arterial angiography, e.g. in acute, painful 3rd nerve palsy with pupillary involvement. CT angiography and MR venography are also available.
c. Stroke: early (within 1–8 days) CT excludes clinically significant haemorrhage and allows early aspirin use. CT may be normal for the first 24 hours after a nonhaemorrhagic cerebral infarct. MRI may not detect acute intracerebral haemorrhage but is more sensitive for diagnosing small infarcts such as brainstem lesions.

Diffusion weighted images (DWI) on MRI and FLAIR imaging are the most sensitive way of detecting early ischaemic CVAs.

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