2What 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
5Introduction:The VEP tests the function of the visual pathway from the retina to the occipital cortex.It assesses the integrity of the visual pathways from the optic nerve, optic chiasm, and optic radiations to the occipital cortex.
6Cont..The VEP is very useful in detecting an anterior visual conduction disturbance.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.
7Cont…VEPs are most useful in testing optic nerve function and less useful in postchiasmatic disorders.In retrochiasmatic lesions, the MRI is a more useful test.
8Comparison of VEP with MRI The VEP explains the functionality of visual pathway.VEP gives us information about the physiology of a anatomical pathway with much less spatial or localizing informationVEP is useful primarily in assessing optic nerve function in the anterior (prechiasmatic) portion.It is lateralizing but not localizing to the lesion.MRIThe MRI largely remains an imaging, structural, or anatomical test.The MRI scan gives more accurate information about structural problemsMRI is a highly accurate localizing modalityUnder given circumstances they may be complementaryto each other..
9VEP generator site Visual Cortex (occipital lobe) The generator site is believed to be the peristriate and striate occipital cortex .
10Types of VEP Flash VEP Pattern VEP- full fieldhemi fieldcentral fieldpartial fieldChromatic patterned stimuli-best method of separating red, green , and blue coloured channels. Helpful in detecting colour blindness.
11How to do the test ? SKIN PREPARATION APPLY ELECTRODE USING JELLY RECORDING – OzREFERENCE – FpzGROUND – CzELECTRODE IMPEDANCE BELOW 5KΏAMPLIFICATION , FILTER , SWEEP SPEED, AVERAGING TO BE CHECKED
12Procedure The room should be dark. Test mono-ocularly with other eye covered.Stimulus:Checkerboard pattern (or less often, flash) isused as stimulation two reversal/sec.Stimulus rates of 1-2 Hz are recommendedThe recommended recording time window (ie, sweep length) is 250 ms.Seating distance:cm from the monitor screenFix the gaze at a colored dot in the center of the screen.
13Cont… Apply three scalp electrodes at; Oz : 2cms above the inion. Cz : at vertexFz : on frontal bone.Check the impedance of the electrodes.In the menu enter patient’s info (name, age , sex,ID no. ref. Dr.Start averaging process.Continue averaging till 1000 stimulus repetition complete. It will stop automatically.
14Cont… After the stimulus are over you will get NPN complex. Identify the waves & apply the wave markers. the values will appear in the table.Repeat the procedure & get another record.Display both the recordings and superimpose them to show the reproducibility of the test results.Repeat the procedure for other eye.
15Stimulus factors & VEP PATTERN FIELD SIZE DISTANCE CONTRAST LUMINANCE COLOURSRATE OF REVERSAL
17Factors influencing VEP The size of the checksPupillary sizeGender (women have slightly shorter P100 latencies ), andAge: below 1 yr of age P100 may be 160ms, & above 60 yrs. also it get delayed.Sedation and anesthesia abolish the VEP.Visual acuity deterioration up to 20/200 does not alter the response significantly .Drugs.(eg. carbamazepine and sodium valproate prolong P100 latency)
25Analysis Identify the waves (NPN complex) Determine the absolute peak latencies.Determine the amplitude of the waves.Determine the interocular latency difference.
26InterpretationNegative components of NPN complex may be absent even in normal subject. The only persistent wave is P100.
27Waveforms (The NPN complex) The initial negative peak (N1 or N75)ِA large positive peak (P1 or P100)Negative peak (N2 or N145)N75P100N145
28Delayed P100 is due to,1. Demyelination of optic nerve.2. Axonal degeneration.Low voltage of P100 is due to,Problems of refrective medias of eye.eg. Corneal opacity, cataract , vitreous hemorrhage.Voltage should not be less than 5mv.
30Full field PVEP- criteria for abnormality Latency criteriaProlongation > 3 sdInterocular latency of p100>10 msec,longer latency abnormalAmplitude criteriaInterocular amplitude ratio>2Abnomally low or high amplitudeAbsence of identifiable VEP from midline and lateral occipital sites.
31Maximum Value for P100P100 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. )Interocular P100 latency difference is upto 5 – 6 ms. > 10ms is gross abnprmality.
32Flash VEPCortical response to flash – widespread , complex and variableAbsence is abnormalUsed in situations when PVEP not possible –ocular scarring, ocular haemorrhagesDone through closed eyesUnaffected by refractive errorUseful in children and uncooperative patients
33Chromatic patterned VEP Best method to separate blue, red and green coloured channelsCan help in detecting color blindness
34Hemifield VEP Indications Optic chiasmal or retro chiasmal lesion Marked interhemispheric asymmetry of VEP on monocular stimulation
35Bilateral abnormal full field PVEP Bilateral lesions in optic nerve, optic chiasm, behind chiasm, retinal degeneration, butterfly gliomas of corpus callosum.
36Monocular abnormal full field VEP Conduction defect anterior to optic chiasmDemyelination commonretinal disease , glaucoma , compression of optic nerve cannot be excluded.
37Full field PVEP interpretation VEP normal in unilateral lesions of optic chiasm with monocular stimulation because each eye projects to both occipital lobes.
38Full field PVEP interpretation If P100 abnormality occurs in stimulation of one eye the lesion is anterior to optic chiasm i.e optic nerve, retina or other intra ocular structures.
39Differential diagnosis with abnormal (prolongP100 latency) VEP Multiple sclerosisOptic neuropathyOptic neuritisToxic amblyopia eg. Tobacco smoking, alcohol.GlaucomaIschemic optic neuropathyTumors compressing the optic nerve - Optic nerve gliomas, meningiomas, craniopharyngiomas, giant aneurysms, and pituitary tumorsNormal VEP virtually excludes an optic nerve or anterior chiasmatic lesion.
40Clinical usefulness of VEPs More sensitive than MRI or physical examination in prechiasmatic lesionsObjective and reproducible test for optic nerve functionAbnormality persists over long periods of timeInexpensive as compared with to MRIUnder 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
41Clinical applications of PVEP Detects subclinical lesions in MSDetects subclinical involvement of optic pathways in neurofibroma,AntibioticsEvaluation of visual loss in hysteria, malingeringMonitoring visual functions during operations around pituitaryConfirmation of lesion of optic nerve tumour
42Multiple Sclerosis (MS) 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.
43Pathologically, 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 processesDifferential diagnosis for MS includes other demyelinating diseases of the nervous system, often of a viral or postinfectious origin
45PVEP in demyelinationProlonged latency of P100 or absence of VEP during acute attackAbnormal PVEP in 90% patients with optic neuritisP100 prolongation persists many years – unilateral marked increase in latency with normal wave form and amplitude.
47PVEP in disorders of optic nerve Ischemic optic neuritisInvariably abnormalAmplitude affected or absent VEPMild latency increaseToxins/ drugsPVEP absent / delayedCan revert following treatmentUsed to monitor drug toxicity
48PVEP in disorders of optic nerve Compression of anterior visual pathway –decreased amplitude, distorted wave, mild latency prolongationLeber’s atrophy, spino cerebellar degenerations –increased latency
49PVEP in retinopathiesIncreased latency with preservation of waveform similar to demyelinationHence opthalmological examination mandatory in symptomatic visual loss.
50PVEP in glaucoma PVEP abnormal in 50% of patients with field defects Increased latency with decreased amplitude of P100
51PVEP in hysteria and malingering Normal wave and latency makes severe visual loss unlikelyNormal PVEPs occasionally reported in cortical blindness when part of striate cortex is preserved.