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 Academic Information on the Following Topics

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Lynn S. Alvord PhD

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           "Old" Interpretation of VNG


-Gaze- Emphasizes eyes open as a CNS sign


-Saccade- Over Interpreted as CNS 


-Pendular- Over interpreted as CNS 


-OPK- Over Interpreted as CNS


-Spontaneous- Ignore minor nystagmus (>6deg/sec)


-Head Shake- Is only a minor abnormaility 


-Hallpike- Negative means not BPPV


-Positional- If direction changing, probably CNS



Calorics- 

-Low total (4 irrigations) usually means bilateral weakness

-Unilateral weakness >23% is significant 

                                  Additional Information Tests for Dizziness 

 



                              VNG Testing and Interpretation

                              
Test Methods and Suggestions






Calibration: 
    
     In calibration, the patient watches a target dot moving side to side, then, up and down, which adjusts the gain of the machine to set to a certain amount of eye movement equal to a certain amount of curser movement on the screen.    

     The display screen of the VNG shows two lines, one red and one blue.  The blue measures horizontal eye movement of both eyes, with up meaning rightward eye movement, and down meaning leftward eye movement of both eyes.  The red line measures vertical eye movement, with up meaning upward eye movement and down meaning downward eye movement. 

     A good idea is to recalibrate if the saccade or pendular tracings are too low or high.  Sometimes there is a lot of artefact in the tracing, which if you can’t solve by machine adjustments, can be helped by having the patient raise or lower the head slightly.  

 
Gaze Test: 

     The patient gazes steadily at a target 30 or 40 degrees to the right, left, up, down and center (the latter is called “spontaneous” nystagmus).  If normal patients gaze beyond 40 degrees (and in some, beyond 30 degrees) there will be “physiologic” nystagmus, meaning “normal” nystagmus at an extreme gaze.  Testing is done with eyes open as well as with vision denied (covered on VNG, closed on ENG). 

     The observer is looking for nystagmus, which is a rhythmic jerking of the eyes in a fast-slow pattern.  The nystagmus is named “right-beating” if the fast phase is to the right, and left-beating if the fast phase is to the left.  Vertical (up-beating or down-beating nystagmus) can indicate central nervous system disorder; however, a small amount of vertical nystagmus is seen in some normals, especially older individuals.  One type of vertical nystagmus is found in a common peripheral vestibular disorder, benign positional vertigo.   Nystagmus which is horizontal, is more often caused by a peripheral vestibular disorder, but can signify a central nervous system problem if it occurs in the eyes open condition.  The exception is, if the patient is being tested in the first few days after becoming dizzy with peripheral disorder (in the acute stage), there may be nystagmus even with eyes open, but greater with eyes covered.  If the nystagmus is peripheral vestibular, gazing in the direction of the fast phase will increase the degree of nystagmus compared to center gaze (Alexander’s Law). 

 
Saccade Test:

    Saccades are voluntary, rapid eye movements.  This test is classified as an “oculomotor” test, which is a function of the central nervous system.  The patient follows a quickly moving target (dot) in random directions.  The test is looking for CNS problems. Drugs affecting the CNS can affect the test, and the patient may need to be reminded periodically not to anticipate where the dot will be, but to “move the eyes as quickly as possible to the target, then stay on the target till it moves again.”



Smooth Pursuit Test (Pendular Tracking):

    This is probably the most sensitive of the “oculomotor tests”.  There is an old axiom in neurology which states, “normal pendular, normal brain,” because every lobe of the brain is involved in the smooth pursuit task.  This statement is somewhat of an exaggeration.  Modern MRI’s, etc. can detect much smaller lesions than when this statement was popular, nevertheless, smooth pursuit is a very sensitive test.  Unfortunately, this often results in the test results being over interpreted as “central”.  This is because older patients, those with eye (or vision) problems or those on medications that affect the CNS, will have abnormal smooth pursuit.  A more common cause of the test being “abnormal”, however, is the patient’s misunderstanding of the task.  It is often necessary to coach the patient by saying, “don’t jump ahead”, or, “stay back patiently with the dot”, which are useful phrases that can often turn and otherwise abnormal test into a normal one. 

        On the smooth pursuit test the examiner is looking for a smooth waveform without stair stepping (cogwheeling), or other pattern with poor morphology, any of which would suggest a central nervous system disorder.   On the other hand, a severe peripheral nystagmus will render the waveform abnormal, making it look “central”.  


Optokinetic Response (OKN, OPK):

     This test is the least sensitive of the oculomotor tests, consisting of a series of stripes or dots moving across the patient’s visual field, causing a tracing that looks like nystagmus, but the eyes are responding to a natural reflex that causes the eyes to always seek the middle after being drawn to the side by the target. Being a natural response such as when looking at train cars passing by, the only true test is considered by some experts to be one in which the test stimuli are large enough to cover most of the entire visual field; therefore, the dots used on VNG machines are arguably not a true or pure test of the OPK response, because the patient may be using fast saccades instead.  Hopefully, the test using dots has a greater chance of mimicking true optokinetic response if the examiner, instead of using too much coaching, lets the reaction happen naturally.  Many times, however, the patient’s eyes seem to just lazily look through the dots in a blurry fashion.  It may help to coach the patient, saying, “try to focus on the dots just in the middle section of the bar”, and “don’t let your eyes blur”.  Also, sometimes the patient tends to follow an individual dot all the way to the end of the bar, resulting to a slower than normal waveform.  It may be unwise to tell the patient to “look at every dot” or “count the dots” because that would more likely cause the attempted use of fast saccades.  The test is normal when the waveform is similar looking (or has similar values) in clockwise versus counterclockwise directions.  

Spontaneous Nystagmus:

     This test looks for nystagmus with eyes open fixating on a target as well as with the eyes covered.  Usually, a mental task will help bring out any nystagmus.  The patient is typically tested during the gaze test with the patient sitting still. 

     While a little spontaneous nystagmus is quite common in many normals, a small degree of  nystagmus can be very significant.  Small amounts of nystagmus can help diagnose a peripheral vestibular disorder or in rare cases, a CNS disorder if the characteristics of the nystagmus are quite consistent with either a CNS or peripheral vestibular disorder.  Characteristics of “peripheral vestibular nystagmus”, are: usually horizontal (unless BPPV, which is usually vertical rotatory), suppresses with visual fixation, follows Alexander’s Law, enhanced with head movement.  “Central nystagmus” is quite bizarre, namely, has characteristics such as:  worse with eyes open, does not suppress with visual fixation, direction may be up-beating or down-beating, or consists of any strange morphology (pendular, etc.).  Very often, neither peripheral nor central nystagmus possess all their expected characteristics.   

 
Head Shake Test:

     With vision denied and the patient sitting, the patient shakes the head laterally at 2 Hz (twice per second), for 30 seconds.  Nystagmus is observed after the shaking stops for 30 seconds at least.  The “abnormal” result is when there are approximately three beats of 3 deg/sec nystagmus.  There may be a slight reversal of direction after the initial nystagmus dies away.  Supposedly, 20% of “normals” have an abnormal headshake test; however, this needs to be studied further. 

     Abnormal headshake nystagmus usually means peripheral vestibular disorder. Headshake nystagmus could also be a CNS sign, but this is rare.  Richard Gans, Ph.D. recommends also doing the test in the side lying positions, since nystagmus might more readily appear when lying on the “bad” ear. 


Positional Nystagmus:

     This test looks for nystagmus with the patient lying with the head either to the right or left.  This can be accomplished in supine position with just the head turned either to the right or left, or by lying with the whole body to the right or left side.  Of prime importance is that the patient’s head is completely all the way to the right or left, therefore just “head left” and “head right” if normal, suffices.  However, if there is nystagmus in either of these positions, then to rule out the neck being the cause, the patient needs to turn the whole body to the side (thereby allowing the neck not to be strained and possibly causing the nystagmus).  If nystagmus is present, it is good to see whether visual fixation will suppress the nystagmus (peripheral).   

     Technically, “positional nystagmus” can indicate either a CNS or peripheral vestibular disorder.  This however, is somewhat misleading in that most cases of positional nystagmus (at least in our office) are peripheral.  With the new understanding of horizontal canal BPPV, it is now realized that most cases of positional nystagmus are related to that disorder rather than a CNS disorder. 

 Positional nystagmus would likely be of central origin if:  nystagmus changes direction while in the same position (there are exceptions), or, if nystagmus changes direction between right and left positions (this, however, is also usually horizontal canal BPPV).  At this point in the discussion, it would be good to refer to the BPPV section.  A lot of normals have a small degree of positional nystagmus.  The choice whether to make an issue of this is somewhat complex.  For example, in many elderly individuals, the feelings of vertigo often are not present even though a significant vestibular disorder is present, therefore, such a patient complaining of only “imbalance” may benefit from some vestibular rehabilitation in case there is horizontal canal BPPV.  To recommend this, however, is not without its perils, as the patient, believing that they have been diagnosed with an “inner ear” cause of their dizziness, might not seek out further etiologies.  Proper counseling with the patient and their physician, as well as proper documentation (that there is no current “proof” of vestibular disorder) are options which might allow such a course for some patients.      

 Caloric irrigations:  This test looks for nystagmus (vision denied) after irrigating the ear with warm then cool air or water for one minute.  For example, warm air at 44 degrees Centigrade is instilled into the right ear for 60 seconds.  Then this is performed in the left ear.  Similarly, cool air at 30 degrees Centigrade is utilized.  Shorter time periods can be used, if equivalent for all four irrigations.  We sometimes stop at 45 seconds if there is becoming a robust response by that time, in order to reduce the patient’s vertigo.  Again, the same amount of time must be used for all 4 irrigations. 
 
    The examiner hopes for robust and equal responses on the right and left ears.  Peripheral disorders have either a unilateral weakness (23% or more) or bilateral weakness (total caloric values for all four are less than 30 degrees per second).  Certain normal could have either hyperactive or hypoactive caloric response of any irrigation.  A hyperactive response on a caloric can be either due to anxiety, a CNS disorder (rare) or certain inner ear (vestibular) causes such as Meniere’s syndrome.  A hypoactive response is most often due to a small or occluded ear canal (the bane of caloric testing).  To solve the latter problem, rotational chair is a handy test.  There should be a suppression of the nystagmus with visual fixation if the disorder is peripheral.  A “directional preponderance” meaning that, when comparing all four irrigations, there is a preponderance of beating in either the right or left direction irrespective of the ear or temperature.  That is, with a right directional preponderance, there will still be nystagmus beating in the proper direction for the particular ear and temperature (see above), but the total intensity of the nystagmus will be greater on the right side than on the left.  A directional preponderance is most often seen in a severe peripheral vestibular disorder, because, if there is a strong right beating spontaneous nystagmus, this will often affect caloric results, causing a right directional preponderance.  However, in the absence of a strong spontaneous nystagmus, directional preponderance can be a CNS sign. 

Some suggestions about calorics:  Don’t “psych the patient out” about the dizziness they may have.  Underplay this and the patient won’t feel as dizzy.  Say instead, “this might make you just a little dizzy for a minute.  That would be normal.”           

     Clinical experience by our lab as well as other labs has shed new light on the manner in which VNG should be interpreted.  The new philosophy centers around recognizing as abnormal, smaller amounts of nystagmus if it is of the “vestibular” type (follows Alexander’s Law and/or enhanced after head shake).  With the foregoing in mind, gaze nystagmus becomes more of a peripheral vestibular test when used with vision denied to determine Alexander’s Law.  With greater emphasis on this issue, small amounts of nystagmus becomes more significant if the nystagmus is of the peripheral type.  Also, with the recent greater understanding of horizontal canal BPPV, positional nystagmus is less often considered possibly a central sign, especially if it follows the patterns seen in BPPV.  Finally, the so called “CNS” signs on the oculomotor portions of the VNG should be regarded less often as central, the feeling being that their interpretation as CNS is overused.  This is likely because of the underuse of peripheral nystagmus issues just mentioned.  In other words, because peripheral nystagmus is under recognized, abnormalities on the oculomotor tests (which are very common) are being over interpreted as the major abnormality on VNG.  

 

                             The following table summarizes “old” versus the “new” ways of interpreting VNG results.          

 
                                                                   “New” Interpretation Guidelines for VNG

 





 




















 



Example Case 1: 

Patient had sudden onset vertigo 4 weeks ago, now just slightly dizzy with head turns. 

Results- “Traditional” VNG: Patient’s only finding was mild spontaneous nystagmus, 3 deg/sec.  Head shake and gaze (vision denied) were not tested.  Patient considered to have “Normal” VNG.

Results - “New” style VNG: In addition to the mild spontaneous nustagmus, 3 deg/sec., Gaze with vision denied was also tested.  Gaze toward the fast phase was found to enhance the nystagmus (Alexander’s Law).  Head shake nystagmus was 3 deg/sec.  Overall, these findings are strongly consistent with uncompensated peripheral vestibular disorder.

 
Example Case 2:

Patient in her 80’s complains of very gradual onset dizziness when walking. 

Results:  “Traditional” VNG:  Patient had normal VNG except for low total score on calorics.  Total of all 4 calorics was 12 deg/sec ( Rt warm = 4, Lt warm = 2, Rt cool = 4, Lt cool = 2).   Patient was considered to have bilateral vestibular weakness and 50% left vestibular weakness.

 Results:  “New” style VNG: Same VNG results, but – patient’s narrow ear canals were considered as the possible cause of the low caloric values.  Also, the difference between ears for warms and cools was only 2 deg/sec for each irrigation.  While this resulted in a 50% left vestibular weakness, the low absolute values of individual irrigations makes this calculation less valid. 

 The patient was retested using rotational chair, which showed normal symmetry between Rt and Lt rotational, as well as normal gain and phase values.  Therefore, the “unilateral weakness” and bilateral weakness found on the VNG were considered to be spurious, and due to the narrow ear canals.

 
Example Case 3:

 Patient complains of momentary dizziness when rolling to the right side in bed, also when bending down or coming up. 


Results: “Traditional” VNG: Normal VNG.  Normal Hallpikes. Patient was considered normal.  It was suggested she might have another cause of her symptoms such as postural hypotension, or just aging.   

 Results: “New” style VNG: Normal VNG.  Normal Hallpikes.  Patient was given a right Epley maneuver according to recent AAOO guidelines due to her symptoms consistent with BPPV.  

 
 

                     "New" Interpretation of VNG 


-Gaze- Alexanders Law with eyes covered


-Saccade- Probably non- significant if minor and isolated findings


-Pendular- Probably non- significant if minor and isolated findings


-OPK- Asymetry can be peripheral vestibular if greater value for targets toward bad ear 


-Spontaneous- even minor nystagmus is significant if follows Alexanders law


-Head Shake- is often the sole defining sign of uncompensated vestibular disorder (3 deg/sec) 


-Hallpike- Negative may still mean BPPV if patient symptoms consistent with BPPV (AAOO Position Statement)


-Positional- If direction changing, usually horizontal canal BPPV (unless in a single position)


Calorics- 

-Low total could mean narrow canals or poor irrigations. Verify with rotational chair. 

-UW not significant if all individual values are low