ENG Principles and Practice

1. ENG Principles and Practice Dr.Mohammad Shafique Asghar Au.D, (USA). American Board of Audiology, M.Sc Audiological Medicine,(UK). MCPS (ENT), M,B.B,S (Pb).

2. Definitions • Electro-oculography (EOG): A method of monitoring eye movements which provides a permanent record, either with eyes open or closed, in light or darkness. • Electronystagmography (ENG):A method of using EOG for clinical study of dizziness and balance disturbance. The term is used because it is primarily used to measure the nystagmus.

3. Purpose of ENG • Most commonly used test • Detection of organic pathology within at least a portion of the vestibular system. • Monitoring changes in the vestibular function • Identification of general site of lesion • Central • Peripheral

4. ENG (limitations) • Deals primarily with only one of the major vestibular tracts- VOR pathway. • Ignores the vestibulospinal tract • Limited to horizontal SCC and superior branch of the vestibular nerve. • Does not diagnose or identify specific pathologies • Only tests very low frequency response of the vestibular system.

5. Different eye movement recording techniques • Visual inspection • Electronystagmography • Infrared • Magnetic search coil • Photographic • Image analyses.

6. 1. Visual inspection • Characteristics: • Full range; • Resolution to 10 minutes of arc with ophthalmoscope; • Vertical and torsional movements noted

7. Visual inspection (cont).. • Advantages: • No cost; • Brief exam allows easy comprehension of complex waveform; • may be able to correlate symptoms with eye movements.

8. Visual inspection (cont).. • Disadvantages: • No permanent record produced; • Poor estimation of velocity or fine details of waveform

9. 2. Electro-oculography • Characteristic • Range of ~ 40 degrees with resolution of 1 degree; • Vertical movements recorded but not accurately; • Torsional movements not sensed; • Bandwidth of 35 Hz.

10. 2. Electro-oculography (cont).. • Advantages: • Inexpensive ; • Noninvasive, well tolerated by children and non cooperative patients; • Vestibular stimulation in all plane possible; • Best general methods for clinical setting

11. 2. Electro-oculography (cont).. • Disadvantages: • Low resolution and low bandwidth obscure fine details, particularly of saccades; • Baseline drift and artifacts may be a problem; • No torsional and poor vertical records.

12. 3. Infrared • Characteristics • Range limited to 20 degree with resolution of 0.1degree, • Vertical recordings inaccurate, • Torsional movements not sensed • Bandwidth > 35 Hz

13. Infrared (cont) • Advantages: • High resolution • Non contact.

14. Infrared (cont)… • Disadvantages: • Moderately expensive • Horizontal record limited and poor vertical recording • Difficult to use with vestibular stimulation • Some limitations of field of view, • Eyes must be open.

15. 4. Magnetic search coil • Characteristics • Range of 40degree, • Full with special equipment, • Resolution of .02 degrees, • Records torsional and vertical movements accurately, • Bandwidth up to 1000 Hz.

16. 4. Magnetic search coil (cont).. • Advantages: • Best method for high resolution of all types of eye movements • Primarily for research.

17. 4. Magnetic search coil (cont).. • Disadvantages: • High cost • Eye contact needed to place the electrodes • Limited use in cooperative patients of ~ 30 minutes.

18. 5. Photographic • Characteristics: • Range limited to ~20 degree with accuracy of 1 degree, • Depends upon sharpness of images, • Horizontal and torsional movements visible, • Bandwidth limited by frame rate with 16 ms between images common.

19. 5. Photographic (cont)… • Advantages: • Non contact, • May be used in usual environments such as space • Disadvantages: • Very labor intensive, • Poor time resolution

20. Image Analyses. • Characteristics: • Technology is developing • High resolution of horizontal, vertical, and torsional moments possible.

21. Image Analyses. (cont).. • Advantages: • Promise of resolution to match Magnetic recordings without being invasive. • Disadvantages: • Investigational. • Very expensive.

22. Physical and physiological principles of ENG. Corneo-retinal potential (CRP): • The eyes act as a battery with the cornea as the positive pole and retina as a negative pole. • The potential difference between the poles is called the corneo-retinal potential and creates in the front of the head (face) an electrical field that changes its orientation as the eye balls rotates.

23. Physical and physiological principles of ENG (cont).. • These polarity changes are detected by electrodes placed on the skin, amplified, and transmitted to a computer or to a strip chart. • The tracing shows a displacement that corresponds to the degree and direction of eye displacement.

24. Preparation for ENG Assessment

25. The Patient Does the patient have: • An intracardiac catheter or pacemaker with exposed leads? • Alcohol “on board” (I.e., in the blood)? • Drugs in the system that could affect the test results? • A hearing loss? • A severe visual problem (e.g., blind)? • Seizure disorder? • A history of back or neck problems?

26. Otoscopic Examination • Check for excessive cerumen which must be removed prior to caloric testing. • With a tympanic membrane perforation, must use air or closed loop calorics. • Orientation of EAC. For accurate irrigation, it’s important for the tester to be familiar with bands in the EAC.

27. Electrodes • Five electrodes are typically used with a two-channel recording technique (one channel is for horizontal movement and one for vertical). • Apply the electrodes as soon as possible before actual ENG recording begins. Over the course of minutes after electrodes are applied, inter-electrode impedance tends to decrease.

28. Location of Electrodes in ENG • Picture

29. Electrodes (cont)… • Prepare the skin with an appropriate abrasive substance. • Check impedance. Inter-electrode should be lower than 10,000 ohms. • Darken the test room. A change in darkness will affect the CRP.

30. Calibration • Calibration verifies the relationship between recorder or pen deflection and the amount of eye movement (in degrees). • The patient alternately looks at two fixation points separated by a known horizontal distance, usually at 20º visual angle. • As the patient performs the task, the gain is adjusted so that 1 mm of pen movement equals 1 mm of eye displacement. The vertical channel is calibration similarly with the light points arranged vertically.

31. Calibration (continued) • When horizontal eye movements are recorded, • An upward deflection represents eye movement to the Rt • And downward deflection represents eye movement to lt. • With vertical eye movements, a upward deflection represents upward movement and a downward deflection represents downward movement.

32. ENG Test Battery • Tests comprising the basic ENG battery: 1. Saccade test (calibration) 2. Smooth pursuit tracking 3. Optokinetic test 4. Gaze tests 5. Dix-Hallpike maneuver 6. Positional tests 7. Caloric test with and without fixation.

33. Patient ready for ENG

34. Infrared ENG

35. PROTOCOLS

36. 1. Saccade test • Saccades are the rapid eye movements made to bring a point of regard onto the fovea. • Latency is about 200msec. • Effective stimulus for saccade generation is the object which is not in line with sight. • Saccades originate in the frontal eye field of frontal lobe • Parapontine reticular formation is the final common pathway.

37. 1. Saccade Test • General points. • Done during calibration • Patient looks back and forth between two dots at the wall. • If the saccades are normal , the patient’s eyes move rapidly and usually stop precisely on each target. • Some patients with normal findings overshoot or undershoot and then make corrective saccades.

38. Saccade Parameters. • Peak velocity • Latency • Accuracy

39. Saccades Peak velocity • Peak velocity varies with the method f measurement. With ENG it is 410 +-100 • It may be , • Too slow • Too fast • Asymmetric saccades.

40. Causes of abnormal saccadic velocity

41. Disorders of saccade latency • Prolongation of latency >400msec is considered abnormal.

42. Disorders of saccadic accuracy & causes

43. Normal variations on Saccade test. • Superimposed gaze nystagmus. • Inattentive patient • Eye blink • Head movements during calibration.

44. 2. Smooth pursuit • These movements are required when the object of interest is moving in space relative to the head. • Moving objects are the effective stimuli to generate the tracking movements. • Latency: 100msec

45. 2. Smooth pursuit • Supposed to be generated in the Cerebellar region at the junction of temporal, parietal and the occipital lobes • Final common pathway is mediated through the dorsolateral pontine nucleus on the same side.

46. 2. Smooth pursuit tracking test • General: • Patients eye movements are recorded while he or she follows a visual target moving in the horizontal plane. • The total excursion should be approximately 30 degree visual angle and the maximum target speed should not exceed 40-50 degrees/sec, because normal persons begin to have difficulty following targets at higher speed.

47. 2. Smooth pursuit tracking test Response parameters • Gain • Phase • Acceleration

48. 2. Smooth pursuit tracking test Disorders Causes of smooth pursuit Causes of asymmetric pursuit

49. Abnormalities • Saccadic Pursuit: When brainstem diseases involves the pursuit system, a patient may substitute saccadic movements for the smooth tracking capacity. This is called “cogwheeling”. • Disorganized and disconjugate pursuit: This is reduced horizontal gaze capacity and disconjugate eye movement, and may indicate a brainstem lesion.

50. Reasons for variation in patients with normal function • Inattentive patient • Head movement • Superimposed gaze or congenital nystagmus.