Visual evoked potential

Visual evoked potential

1. VISUAL EVOKED POTENTIAL

2. Record of gross electrical signal generated at visual (occipital) cortex in response to visual stimulation The amplitude of VEP-3 to 25 microvolt is considerably smaller than that of EEG, which can be as large as 100 microvolt

3. Averaged and amplified record of action potentials in visual cortex VEP is the only objective technique available to assess clinically the functional state of the visual system beyond the retinal ganglion cells.

4. VEP abnormality Visual pathway abnormality Non specific

5. Since foveal projection is magnified at cortex , it is an objective indication of macular function

6. At rapid rates of stimulation the waveform becomes sinusoidal – STEADY STATE VEP Not used routinely due to inferior information on latency Low rates of stimulation – discrete deflections Known as TRANSIENT VEP This is commonly employed

7. STIMULI FLASH Occipital cortex is relatively insensitive to flash PATTERN Cortex is sensitive to edges of contrast

8. TYPES OF VEP

9. FLASH VEP Response to diffusely flashing light stimulus that subtends a visual field of 20 degrees Cruder response than pattern VEP Merely indicates that light has been perceived by cortex Indications – media haze, infants,poor patient cooperation

10. PATTERN REVERSAL VEP Response to a patterned stimulus - checkerboard or square and sine wave gratings Frequency of gratings is decribed in CPD- cycles per degree For check pattern visual angle subtended by a single check is used Preferred technique for most clinical purposes, gives an estimate of form sense and thus visual acuity

11. PATTERN ONSET/OFFSET VEP A pattern is abruptly exchanged with an equilluminant diffuse background More intersubject variability than pattern reversal VEP Useful in detection of pts with malingering , pts with nystagmus

12. PREREQUISITES There should be no distracting sound or light waves Pattern and flash must both be done in all patients as pattern cannot be detected in pts with media opacities Pattern VEP followed by flash VEP Significantly affected by eccentric fixation, excessive blinking of eyes and partial closure of eyes

13. EQUIPMENT REQUIRED Visual stimulus producing device Scalp electrodes Amplifier Computer and read out systems

14. TECHNIQUE OF RECORDING Undilated pupil Monocular recording Refractive correction Relaxed position 1m distance from monitor

15. COMPONENTS OF VEP AMPLITUDE LATENCY

16. NORMAL DATA P 100 LATENCY ( m sec ) = 102  5 R-L difference ( msec) = 1.3  2.0 Amplitude ( μ V) =10  4.2 Duration = 63  8.7

17. If acuity of the patient is in question , the amplitude is more important If detection of a lesion in visual pathway is in question , latency is more important Latency is more reliable than amplitude (Variabilty – 5% as compared to 25%)

18. Presence of reduced amplitude is non specific, gains importance only on serial testing B/L symmetry is seen both with flash and pattern VEP , thus an asymmetrical response is more indicative of an abnormality

19. FACTORS INFLUENCING VEP SIZE OF STIMULUS – Decrease in size of stimulus increases amplitude of VEP Position of electrodes on scalp AGE - amplitude decreases with age ATTENTION OF PATIENT – If subject looks to side of stimulus , there is rapid fall in size of response

20. CLINICAL APPLICATIONS DELAYED LATENCY Demyelinating optic neuritis Neurotransmitter disorders Glaucoma Uncorrected refractive error Ageing REDUCED AMPLITUDE Optic atrophy Toxic Compressive Uncorrected refractive error Ambylopia

21. OPTIC NEURITIS / MULTIPLE SCLEROSIS Increased latency of P100 Even when no defect in visual acuity , colour vision or field of vision. About 96% of pts with MS have delayed latency

22. COMPRESSIVE OPTIC NERVE LESIONS – decreased amplitude without much change in latency DURING ORBITAL OR NEUROSURGICAL PROCEDURES - continuous record of optic nerve function in form of VEP to prevent inadvertent damage to the nerve during surgical manipulation

23. DEGENERATIVE DISEASES that affect the spinal cord, cerebellar pathways, or both, such as Friedreich's ataxia, Huntington's disease, neurosyphilis, and AIDS, also can affect the optic nerves and cause visual defects, including a delay of the pattern VEP

24. The earliest VEP abnormalities in LEBER'S optic neuropathy appear to be increases in P100 latency or changes in the waveform morphology (i.e., the development a double positive peak). As the condition progresses, the VEP amplitude decreases to a point where responses become immeasurable.

25. Patients with THYROID OPHTHALMOPATHY may have a prolonged latency of the pattern VEP before a clinically apparent optic neuropathy. 55 SUBACUTE COMBINED DEGENERATION secondary to vitamin B 12 deficiency causes demyelination and prolongs the pattern VEP latency, 56 , 57 even with an unremarkable neuroophthalmologic examination. 58

26. AION Low amplitude but normal latency TO ASSESS MISPROJECTION OF OPTIC NERVE FIBRES IN ALBINISM Nerve fibers that originate in the temporal retina are misrouted at the optic chiasm. This misrouting results in an anomalous temporal nerve fiber decussation and an abnormal projection to the occipital cortex.-definite VEP asymmetry

27. TO ASSESS VISUAL POTENTIAL IN PATIENTS WITH OPAQUE MEDIA Flash VEPs may be useful for detecting maculopathy or optic neuropathy in patients with dense media opacities. An amplitude reduction of more than 50% or a latency delay of more than 15 ms is highly suggestive of dysfunction in the central visual field. important in patients with opacities who are at high risk for neuronal dysfunction, such as patients with diabetes, ocular hypertension, or ocular trauma.

28. TO ASSESS VISUAL ACUITY IN NON VERBAL CHILDREN , MENTALLY CHALLENGED AND APHASIC PATIENTS Useful in assessing the integrity of macula and visual pathway Pattern VEP gives a rough estimate of visual acuity objectively

29. EVALUATION OF OPTIC NERVE FUNCTION IN PATIENTS WITH HEAD INJURY Pupillary reflexes are often inaccessible because of periocular edema and/or motoric pupillary involvement, and patients are often comatose or sedated. Under these circumstances, the flash VEP can provide valuable information regarding optic nerve integrity. AMBYLOPIA – decrease in amplitude with relative sparing of latency

30. GLAUCOMA Decreases in pattern VEP amplitude and prolonged VEP latencies are found in many patients with glaucoma and some glaucoma suspects Steady-state VEPs appear to be more sensitive for detecting glaucomatous damage thantransient responses

31. MALINGERING AND HYSTERICAL BLINDNESS R/O malingering by confirming the fact that visual pathway is intact even in patients claiming no PL Hysterical response – shows large variations from moment to moment , eg first half may produce an absent VEP and second half may produce normal VEP

32. VISUAL FIELD DEFECTS Asymmetry of amplitudes of VEP recorded over each hemisphere implies a hemianopic visual pattern Decreased amplitude of VEP recorded over contralateral hemisphere when each eye is stimulated separately indicates bitemporal field defect

33. MULTIFOCAL VEP Generated simultanously from 60 regions of central 20 to 25 degrees of visual field Local defects are easily missed in conventional VEPS Can detect local demylination - f/u of cases of optic neuritis To confirm unreliable visual fields

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