/nəˈstaɡməs/ (noun): rhythmic, involuntary movement of the eyes. Nystagmus is a key diagnostic feature in diagnosing the cause of a patient’s dizziness. There are many different types of nystagmus and each is associated with its own pathology.
Nystagmus is defined as any rhythmic, involuntary oscillation of the eyes. It can either be physiological or pathological in nature. Generally, the eyes are set in a forward gaze position due to the balanced input from both the left and right vestibular systems. This forward gaze position is maintained even when a person is not focusing on a visual target. The right and left vestibular systems are modified to quickly respond to any adjustment or movement of the head in relation to the body. For example, if a person turns his/her head to the side without fixing their eyes on a target, their eyes will remain centers in the primary gaze position. This is a result of the vestibulo-ocular reflex (VOR). The VOR is a reflex mediated by the vestibular system that functions to keeps the eyes centered during head positioning. For example, if a person turns his head to the left, the VOR will cause the eyes to move to the right; therefore keeping them centered in the socket.
Think of a ballerina spinning around during a pirouette. Theoretically, her eyes should follow her head at the same speed she is spinning. If she were to spin too fast, the ballerina would then start to experience a sensation of vertigo. This is because her eyes would not be able to keep up with the rotational speed of her head or body. The vestibulo-ocular reflex may have a slow latency which would cause a discrepancy in the speed of eye movements relative to head movement. Luckily, ballerinas have found a solution to this problem. Many ballerinas use a technique known as “spotting”, which actually allows them to induce physiological nystagmus. By fixating on a visual target during their rotational spin, they are able to interfere with the smooth movement of the eyes. When she reaches the point in her rotational spin where her eyes are in an extreme lateral position, she will quickly re-fixate her eyes on another visual target. This rapid, voluntary eye movement is representative of the “quick phase” of nystagmus and is a deliberate attempt of the conscious brain to fixate on a visual target. During a pirouette, the ballerina will continue to induce physiological nystagmus by alternating between quick and slow phases of eye movements as she fixates on new visual targets. This rhythmic oscillation of eye movements is referred to as nystagmus.
Pathological nystagmus is referring to any type of nystagmus caused by a disease or illness. A typical presentation of pathological nystagmus will cause the eyes to slowly drift from their central position. This is due to an abnormality of one of the “gaze-holding” regulatory systems which generally controls eccentric gaze such as looking side to side, up or down, or obliquely. As the eyes slowly drift from a central position, they will be suddenly be corrected by a rapid eye movement. This rapid eye movement back to their central position is a phenomenon of the conscious brain that brings the eye back to center.
When dealing with pathological nystagmus in a clinical setting, patients often present with many other symptoms that will vary depending on the underlying disease (e.g. BPPV, viral neronitis, peripheral vestibulopathy, acute cerebellar syndrome, etc.). Because of the wide variety of nystagmus patterns that may presents themselves, it is highly recommended that any medical professionals become knowledgeable about the different types of nystagmus and how to distinguish between them. The following are different patterns of nystagmus that may present in a clinical situation:
Jerk Nystagmus: Jerk nystagmus is defined as asymmetric rhythmic oscillation of the eyes with two phases – one slow (pathological) phase and one rapid (corrective) phase. For example, if the eyes drift slowly away from center to the right and then jerk back rapidly to the left, this would be called a left horizontal jerk nystagmus. If the nystagmus is occurring spontaneously while the patient is in forward neutral gaze, then the jerk nystagmus would be classified according to the direction of the drift (e.g. horizontal, vertical, torsional, oblique, or mixed). If a jerk nystagmus only occurs when a patient gazes their eyes to an eccentric positon (such as the far right or far left), this is called a gaze-evoked nystagmus. Jerk nystagmus may also be dependent on where the patient’s head is in space, rather than the direction of the patient’s gaze. If jerk nystagmus is defined by which position the patient’s head is in, then the nystagmus is classified as positional nystagmus.
Nystagmus patterns seen in BPPV: In BPPV, otoconia that are usually located in the vestibule are pathologically present in one or more of the semicircular canals of the vestibular organ. Gravity dependent movements of the head cause a shift of the otoconia which cause disruption of the endolymphatic fluid and ultimately stimulate the cupula. This stimulation causes vertiginous sensations in the patient, as well as nystagmus patterns consistent with the vestibulo-occular reflex. Nystagmus can be evoked during Dix-Hallpike testing, which involves vectoring the patient’s head into different positions so that they are in the same plane as each semicircular canal. Nystagmus patterns should be a direction reflection of which semicircular canal the otoconia are moving through during each Dix- Hallpike testing position. Nystagmus patterns may be horizontal, vertical, torsional, oblique, or mixed depending on where the otoconia or located.
Peripheral Vestibular Nystagmus: Our brain is constantly receiving input from the right and left vestibular organs about the movements our head makes in space. If one of these vestibular organs is affected by a disease process, the input signals to the brain would no longer be equal from both sides. Pathological nystagmus is the result of an imbalance in the different sides of the vestibular system. Let’s say a virus has damaged the right vestibular organ causing a loss of input signal on the right. The only remaining vestibular input to the vestibulo-ocular system is now from the other vestibular organ on the left. The left vestibular organ, being the stronger side, will slowly start the drive the eye in the opposite direction (towards the affected ear on the right). Because the conscious brain cannot tolerate the eyes drifting from their neutral, central position, it will instruct the eyes to rapidly jerk back to center – causing jerk nystagmus. This fast jerk phase is in the direction of the “good” ear (the left in our example). Even though the pathological movement is the initial slow movement, the nystagmus is defined by the direction of fast corrective phase. In a peripheral vestibulopathy where one of the vestibular organs has been damaged, there will be a continuous pattern of the eyes drifting to the damaged side and being corrected back to center. This constant jerk nystagmus is referred to as sustained nystagmus. If damage were to occur to both vestibular organs equally, then the drift or pull would be equal on both sides and cancel each other out – therefore nystagmus would not occur.
Vertical Nystagmus: Also consists of a slow phase and fast phase. Vertical nystagmus can be classified into two sub-classes – upbeat nystagmus and downbeat nystagmus. The nystagmus class is defined by the direction of the fast or rapid phase.
Gaze-Evoked (eccentric position) Nystagmus: Refers to nystagmus that is provoked by moving the eyes to an extreme gaze position only and is not present when the eyes are in a neutral centered position. This is more commonly due to an abnormality of the central neural integrators responsible for gaze- holding and less commonly due to weakness or fatigue of the extraocular muscles (also known as gaze- paretic nystagmus). Below are some examples of gaze-evoked nystagmus:
Periodic Alternating Nystagmus (PAN): can be either congenital or acquired and is defined as a persistent horizontal jerk nystagmus that periodically changes direction. It is the only conjugate gaze nystagmus that occurs spontaneously in the primary gaze position. PAN has a waxing and waning characteristic as the nystagmus pattern switches direction while increasing and decreasing in amplitude. For example, a left beating nystagmus will start to develop a progressively larger amplitude and higher frequency up to a peak, and then it will wane until it reaches a short period of downbeat or no nystagmus. Then, the nystagmus will reverse direction to the right and enter the same crescendo-decrescendo pattern until it again leads to the same short period of downbeat or no nystagmus. The short period of no nystagmus or null period typically lasts about 10 seconds while the crescendo-decrescendo periods in the horizontal directions last about 90 seconds. Some of the causes of periodic alternating nystagmus include: degenerative spinocerebellar disease, multiple sclerosis, Creutzfeldt-Jacob disease, ataxia-telangiectasia, brainstem infarcts, cerebellar mass lesions, neurosyphilis, hepatic encephalopathy, Trauma, anticonvulsant drugs, or following visual loss.
Horizontal Rotatory Nystagmus: involuntary rotation of the eyes about the visual axis. This is a common nonspecific finding in brainstem disease.
Brun’s nystagmus: caused by lesions in the cerebellopontine angle (e.g. acoustic schwannoma). This lesion produces two types of nystagmus – a gaze-evoked nystagmus that results compression of the brainstem and a horizontal-rotatory nystagmus that results from damage to the vestibular nerve. If a patient had a right acoustic schwannoma, the compression of the pons will cause a large-amplitude, low-frequency coarse gaze-evoked nystagmus to the same side of the lesion. In the primary neutral position, a left beating nystagmus of medium amplitude will be observed due to the damage of the peripheral vestibular system. If the patient gazes to the left, a small- amplitude, high-frequency fine nystagmus is evoked.
Rebound Nystagmus: a type of horizontal, gaze-paretic nystagmus in which the jerk nystagmus will gradually decrease in amplitude as the eyes remain in an eccentric position for many seconds. In some cases, the direction of the nystagmus can reverse, this is called centripetal nystagmus. There are two types of rebound nystagmus — Type I and Type II. Both types of rebound nystagmus are caused by disturbances to the cerebellum. They are described below:
Type I rebound nystagmus: occurs most often in alcoholic patients who have developed vestibulo-cerebellar degeneration. When these patients move their eyes into an eccentric position, they will have gaze-evoked jerk nystagmus that slowly fades in amplitude. As this gaze-evoked jerk nystagmus starts to wane, it is suddenly followed by a fast rebound jerk nystagmus in the opposite direction towards the neutral center gaze position; however their eyes will still try to maintain their eccentric gaze. The quick rebound jerk towards the central neutral position is called centripetal nystagmus.
Type II rebound nystagmus: occurs most often in patients with cerebellar parenchymal disease. After an extended period of eccentric gaze, the eyes will return to the central neutral position via a rapid phase of rebound nystagmus.
Pendular Nystagmus: a type of nystagmus resulting in oscillation of equal amplitude with no fast or slow phases. The eye moves in a sinusoidal trajectory that is similar to that of a pendulum. The pendular movements of the eyes may be in any direction including vertical, horizontal, torsional, or a combination thereof. Most pendular nystagmus occurs at a frequency of 1-4 Hz. Pendular nystagmus is generally due to either the loss of inhibitory feedback control or increased delay on a feedback circuit – ultimately resulting in a spontaneous oscillation. Visual deprivation pendular nystagmus is a common form of pendular nystagmus acquired secondary to blindness. Three other common causes/types as described below:
Motor Nystagmus: due to ischemia or multiple sclerosis affecting the brainstem. Oblique nystagmus is when the symmetric pendular nystagmus occurs diagonally. If the directional vector continually changes, this is called windmill nystagmus.
Spasmus nutans: usually a benign form a nystagmus that typically presents between the ages of 4 months to 1 year. It usually resolves by itself in several months to years. It occurs with a unique triad of symptoms including pendular nystagmus, torticollis, and head nodding. The pendular nystagmus may be horizontal, vertical, torsional, or mixed.
Ocular Myoclonus: rapid, involuntary, multi-vectorial (both horizontal and vertical) fast eye movements that occur without intersaccadic intervals. Symptoms can presents as a continuous pendular nystagmus and often occur with tremor of the face and palate. The palatal tremors and pendular eye movements are often present during sleep as well.
Binocular Dysconjugate Nystagmus: in this type of dissociative nystagmus the eye movements remain symmetric, however they occur in opposite directions. The three main types of binocular disconjugate nystagmus are as follows:
Seesaw Nystagmus: a vertical disconjugate nystagmus characterized by the alternating elevation and depression of one eye accompanied by the opposite movements of the other eye. Similar to a see-saw where the two ends alternatively move up and down in opposite directions, the eyes also alternate along a vertical plane. The rising eye may also intort while the falling eye extorts. Seesaw nystagmus can be congenital or acquired. Acquired seesaw nystagmus is often due to parasellar tumors compressing the optic chiasm or more rarely due to brainstem infarct, septo-optic dysplasia, multiple sclerosis, Arnold-Chiari malformation, Syringobulbia, head trauma.
Convergence-retraction nystagmus: occurs secondary to Parinaud’s syndrome (aka dorsal midbrain syndrome). This type of nystagmus occurs during attempted upgaze and causes convergence and retraction of the eyeball into the socket. Voluntary convergence can also cause downbeat nystagmus or convert an upbeat nystagmus into a downbeat nystagmus.
Convergence nystagmus: is a pendular nystagmus induced by convergence. This type of nystagmus is usually convergent. It may be accompanied by eyelid nystagmus, and often accompanied by synchronous contraction of the muscles of mastication, or synchronous palate and mandibular muscle contractions (oculomasticatory myorhythmia). This oculomasticatory phenomenon is pathognomonic of Whipple’s disease if it is associated with a supranuclear vertical gaze palsy.