2. What is evoked potential?
Electrical potentials that occur in the cortex
after stimulation of a sense organ which
can be recorded by surface electrodes is
known as Evoked Potential.
eg. SEP, ABR and VEP
3. Introduction
O The VEP tests the function of the visual
pathway from the retina to the occipital
cortex.
O It assesses the integrity of the visual
pathways from the optic nerve, optic
chiasm, and optic radiations to the
occipital cortex.
4. Cont..
O The VEP is very useful in detecting an anterior visual
conduction disturbance.
O However, it is not specific with regard to etiology.
For example a tumor compressing the optic nerve, an
ischemic disturbance, or a demyelinating disease may
cause delay in the P100.
5.
6. Cont…
O VEPs are most useful in testing optic
nerve function and less useful in
postchiasmatic disorders.
O In retrochiasmatic lesions, the MRI is a
more useful test.
7. Comparison of VEP with
MRI
VEP
O The VEP explains the
functionality of visual pathway.
O VEP gives us information about
the physiology of a anatomical
pathway with much less spatial
or localizing information
O VEP is useful primarily in
assessing optic nerve function
in the anterior (prechiasmatic)
portion.
O It is lateralizing but not
localizing to the lesion.
MRI
O The MRI largely remains an
imaging, structural, or
anatomical test.
O The MRI scan gives more
accurate information about
structural problems
O MRI is a highly accurate
localizing modality
Under given circumstances they may be complementary
to each other.
.
8. Waveforms
(The NPN complex)
O The initial negative peak (N1 or N75)
O ِA large positive peak (P1 or P100)
O Negative peak (N2 or N145)
N75
P100
N145
9.
10. Maximum Value for P100
O P100 is 110 milliseconds (ms) in patients
younger than 60 years
(it rises to 120 ms thereafter in females and
125 ms in males. )
(Even though published norms are available in the
medical literature, each individual laboratory
should have its own norms to control for lab-to-lab
variability in technique. )
O Interocular P100 latency difference is upto 5 – 6
ms. > 10ms is gross abnprmality.
11. VEP generator site
O Visual Cortex (occipital lobe)
The generator site is believed to be the
peristriate and striate occipital cortex .
12. Procedure
O The room should be dark.
O Test mono-ocularly with other eye covered.
O Stimulus:
Checkerboard pattern (or less often, flash) is
used as stimulation two reversal/sec.
O Stimulus rates of 1-2 Hz are recommended
O The recommended recording time window (ie, sweep length)
is 250 ms.
O Seating distance:
70-100 cm from the monitor screen
O Fix the gaze at a colored dot in the center of the screen.
13. Cont…
OApply three scalp electrodes at;
Oz : 2cms above the inion.
Cz : at vertex
Fz : on frontal bone.
OCheck the impedance of the electrodes.
OIn the menu enter patient’s info (name, age ,
sex,ID no. ref. Dr.
OStart averaging process.
OContinue averaging till 1000 stimulus repetition
complete. It will stop automatically.
20. Cont…
O After the stimulus are over you will get NPN complex.
O Identify the waves & apply the wave markers. the values will
appear in the table.
O Repeat the procedure & get another record.
O Display both the recordings and superimpose them to show
the reproducibility of the test results.
O Repeat the procedure for other eye.
21. Analysis
O Identify the waves (NPN complex)
O Determine the absolute peak latencies.
O Determine the amplitude of the waves.
O Determine the interocular latency
difference.
23. Factors influencing VEP
OThe size of the checks
OPupillary size
OGender (women have slightly shorter P100 latencies ), and
OAge: below 1 yr of age P100 may be 160ms, &
above 60 yrs. also it get delayed.
OSedation and anesthesia abolish the VEP.
OVisual acuity deterioration up to 20/200 does not
alter the response significantly .
ODrugs.(eg. carbamazepine and sodium valproate prolong P100
latency)
24. O Delayed P100 is due to,
1. Demyelination of optic nerve.
2. Axonal degeneration.
O Low voltage of P100 is due to,
Problems of refrective medias of eye.
eg. Corneal opacity, cataract , vitreous
hemorrhage.
O Voltage should not be less than 5mv.
25. Differential diagnosis with
abnormal (prolongP100 latency)
VEP
O Multiple sclerosis
O Optic neuropathy
O Optic neuritis
O Toxic amblyopia eg. Tobacco smoking,
alcohol.
O Glaucoma
O Ischemic optic neuropathy
O Tumors compressing the optic nerve - Optic
nerve gliomas, meningiomas,
craniopharyngiomas, giant aneurysms, and
pituitary tumors
O Normal VEP virtually excludes an optic
nerve or anterior chiasmatic lesion.
26. Clinical usefulness of VEPs
O More sensitive than MRI or physical
examination in prechiasmatic lesions
O Objective and reproducible test for
optic nerve function
O Abnormality persists over long periods
of time
O Inexpensive as compared with to MRI
O Under certain circumstances, may be
helpful to positively establish optic
nerve function in patients with
subjective complaint of visual loss;
normal VEP excludes significant optic
nerve disorder
27. Multiple Sclerosis (MS)
O Its a chronic demyelinating disease of the
central nervous system, which
predominantly affects young adults during
their most productive years. Viral and
autoimmune etiologies are postulated.
Genetic and environmental factors are
known to contribute to MS, but a specific
cause for this disease is not identified.
28. O Pathologically, MS is characterized by the presence of
areas of demyelination and T-cell predominant
perivascular inflammation in the brain white matter. Some
axons may be spared from these pathological processes
O Differential diagnosis for MS includes other demyelinating
diseases of the nervous system, often of a viral or
postinfectious origin
