Nystagmus
What is nystagmus
Nystagmus is a involuntary, rapid, rhythmic, oscillatory eye movement with atleast 1 slow phase. JERK nystagmus has a slow phase and fast phase. Pendular nystagmus has only slow phases. pathologic forms of nystagmus generallly result from disease affecting the peripheral vestibular apparatus, brain stem, or cerebellum; less commonly, they affect the anterior visual pathways or cerebral hemispheres.
Nystagmus differs from saccadic intrusions and oscillations, such as square wave jerks, ocular flutter, and opsoclonus, in which inappropriate saccades take the eye away from a target during intended fixation. Nystagmus is characterised by numerous phenomenological attributes, many of which are linked to specific underlying pathologic mechanism. specific forms of nystagmus are sometimes given special names based on combinations of these attributes.
NYSTAGMUS TRAJECTORY: Referance Frames, Axes, Planes and Directions
when the eye is pointing straight ahead with the line of sight perpendicular to the frontal plane, it is in the "straight-ahead position"or "center gaze position". the eye's motion path taken from starting to ending positionis known as the trajectory.
Describing trajectring in 3 dimensions requires specefying a referance frame depending on a clinical or scientific circumstances, one may choose an eye-referanced cordinate system, a head referanced cordinate system or even earth-referanced cordinate system. For spontaneous nystagmus in the straight ahead position the farther the eye is moved into an eccentric position orthogonal to the nystagmus direction the greater will be the differance between the eye reference and head reference frames description of the nystagmus.
EYE MOVEMENT AXES and PLANES OF ROTATION IN 3 DIMENSIONS
| Axis | Anatomy and radiology of plane | Aeronotics and vestibular plane | Ocular movements direction with eyes in straight ahead position |
|---|---|---|---|
| Craniocaudal, superioinferior or Z Interaural or Y | Horizontal or axial sagittal | Yaw | Horizontal right vs left |
| Nasooccipital Anteroposterior or X | Coronal and frontal | Roll | Torsional- Top pole toward right ear versus left ear |
| RALP | 45 degree from midsagittal, perpendicular from a mean plane of RA and LP SCCs | RALP: Right Anterior and Left Posterior | Mixed vertical-Torsional: Up with torsion moving the top of the globe toward left ear- down with torsion moving the top of the globe toward right ear |
| LARP | 45 degree from midsagittal, perpendicular to the mean plane of LA and RP SCCs | LARP: Left Anterior Right Posterior | Mixed Vertical-Torsional: Up with torsion moving the top of the globe toward right ear--- down with torsion moving the top of the globe toward left ear |
ADDITIONAL NYSTAGMUS ATTRIBUTES AND INFLUENCING FACTORS
They are follows as-
Binocularity
Monocular and Binocular
Conjugacy
Conjugate (both eyes rotate together in same direction by the same amount)Disconjugate(they do not)
Velocity
Quantitave measurement of slow phase velocity in degree per second.
Waveform
the oscillatory appearance of nystagmus on an oculographic trace. jerk (Constant, decreasing and increasing) Pandular(maybe horizontal, vertical, diagonal, eliptical)
Freaquency
measuring beats per second is most useful for low freaquency nystagmus forms.
Temporal profile
Continious, intermittent, or changing overtime.
Age at first appearance
congenital or infentile if present since birth or early life and acquired if it developed at later age.
CLASSIFICATION OF NYSTAGMUS
They are divided into 2 terms-
Physiological
Pathological
PHYSIOLOGICAL NYSTAGMUS
Physiologic end-point nystagmus
gaze evoked nystagmus in the absence of pathology, attributed to normal variation in gaze-holding ability.
per-rotational nystagmus
Post-rotational nystagmus
Optokinetic nystagmus
Optokinetic after nystagmus
Caloric nystagmus
Magnetic vestibular stimilation-induced nystagmus
PATHOLOGICAL NYSTAGMUS
Spontaneous nystagmus
Nystagmus present while looking in a straight-ahead gaze position with the head stationary in the upright and neutral position.
Spontaneous peripheral vestibular nystagmus
spontaneous jerk nystagmus due to an imbalance in vestibular tone between the labyrinths or vestibular nerves
spontaneous peripheral vestibular nystagmus consists of the following characterstics-
binocular and conjugate in head referanced coordinates, regradless of gaze postion
obeys Alexender's law
suppressed by visual fixation
constant velocity slow phases
Spntaneous peripheral vestibular nystagmus excitatory type
uncommon but may be seen during attacks of maniere disease or vestibular poraxysmia.
Spontaneous peripheral vestibular nystagmus inhibitory type
Spontaneous peripheral vestibular nystagmus whose vestibular tone assymetry is due to a unilateral reduction or loss of vestibular function.
Recovery nystagmus
spontaneous peripheral nystagmus that has reversed direction after a period of time and is attributed to recovery from an underlying vestibular disorder causing an initial inhibitory nystagmus.
spontanous central vestibular nystagmus
it is due to dysfunction of central nervous system circuits that contribute to the vestibular ocular reflex or adaptive control of these reflaxes.
Direction fixed horizontal central vestiular nystagmus
it is a predominantly horizontal and remains direction fixed in the straight-ahead gaze position. this can be easily mistaken for spontaneous peripheral vestibular nystagmus normally seen with vestibular neuritis or other condition.
periodic alternating nystagmus
it is binocular horizontal jerk nystagmus that spontaniously reverse direction usually every 90-120 seconds, seen with dysfunction of the cerebeller nodules or uvula.
Latent nystagmus
conjugate horizontal jerk nystagmus absent during binocular viewing that appears with monocular occlusion. fast phases of both eyes beat toward the side opposite the covered eye.
Downbeat nystagmus
it is due to vestibulocerebeller dysfunction. it commonly incresed in lateral and downward gaze or might become evident only in lateral gaze
Upbeat nystagmus
less common and less well localizing than downbeat nystagmus but most often occurs with lesions of the peramedian medulla.
Torsional nystagmus
most often from medullary or midbrain lesion
infentile nystagmus
conjugate horizontal nystagmus present at birth or developing during infancy it remains horizontal in eye referanced cordinate in upward and downward gaze
Acquired pendular nystagmus
Pendular nystagmus developing after infancy that may have horizontal, vertical and torsional components, acquired pendular nystagmus is multiple sclerosis typically has a higher frquency and lower amplitude than that associated with oculoplatal tremor.
oculoplatal tremor
A form of acquired pendular nystagmus characterized most commonly by large amplitude, low frequency and often disconjucate vertical, torstional and horizontal oscillations that may be enhanced by eye closure.
NYSTAGMUS LIKE MOVEMENTS
Saccadic intrusions and oscillations
Inappropriate saccades that distrupt foveal vision by taking by taking the eye away from the target during intended fixation .
square- wave jerks
pairs of small horizontal conjugate saccades that takes the eyes away from the fixation position and then return them after an interval of about 200 to 400ms.
Macrosaccadic ossilations
oscilation around a fixation point due to saccadic hypermetria
saccadic pulses
brief instruction upon steedy fixation caused by an unintended saccade away from the fixation position sometimes followed by an immidiate drift back.
OTHER NYSTAGMUS -LIKE MOVEMENTS
Convergence-retraction nystagmus
A condition in which upward saccades or fast phases are replced by rapid convergent or retractory eye movements, or both, as can be seen with dors
Superior oblique myokymia
bursts of small, irregular, high frequency, predominently horizontal pendular oscilations that vary in their amplitude and phase relationship between the 2 eyes attributed to spasmus nutans.
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