Head shaking nystagmus

By Marcello Cherchi, MD PhD

For clinicians

Head-shaking nystagmus (HSN) was first described in 1907 by Robert Bárány (Bárány 1907). Subsequent studies (Borries 1923; Klestadt 1936; Moritz 1951; Vogel 1929) described different techniques of eliciting the nystagmus and proposed various hypotheses regarding its pathophysiology (Fetter et al. 1990; Hain and Spindler 1993; Halmagyi and Curthoys 1988; Katsarkas, Smith, Galiana 2000; Minagar, Sheremata, Tusa 2001; Perez et al. 2004).

Head-shaking nystagmus (HSN) may follow a prolonged sinusoidal head oscillation. A common technique for eliciting HSN is as follows: The patient is seated in an upright position and instrumented so that fixation is removed but horizontal and vertical eye movements can be observed; this is best performed using ENG, VNG or VFO. Optical Frenzel goggles can be used, but are probably less sensitive because they allow some vision. Performing the test without eliminating fixation is not diagnostic.

Eye movements are observed in darkness for 10 seconds to obtain a baseline. Next, the examiner grasps the patient’s head firmly and moves it briskly back and forth in the yaw plane (around the vertical axis), aiming for a frequency of about 2 Hz and a displacement of the head of approximately 30 degrees to either side. If possible, the head should be tilted approximately 20 degrees anteriorly (with respect to vertical) so that the axis of rotation is close to being parallel to the axes of the lateral semicircular canals, though small deviations from this are probably not clinically significant. The head-shaking is continued for 20 cycles and then abruptly stopped.

In normal subjects, or patients with symmetrical vestibular loss (such as bilateral vestibular loss), no nystagmus is expected; one study (Li 1991) found that not a single individual out of 60 healthy subjects exhibited HSN; in contrast, other studies report HSN in healthy controls ranging from 9.6% (7 out of 73) (Vicini, Casani, Ghilardi 1989) of individuals, to 14.3% (4 out of 28)(Choi et al. 2007) of individuals. In persons with a dynamic imbalance between the ears (such as due to unilateral vestibular neuritis or an acoustic neuroma), a nystagmus is often seen (usually with the fast phase beating towards the “better” ear (Hain, Fetter, Zee 1987; Katsarkas, Smith, Galiana 2000)) which decays over about 30 seconds. This is often referred to as the first phase of nystagmus, because in some cases it is followed by a second phase of nystagmus that is weaker, decays more slowly, and is in the opposite direction (i.e., with the fast phase directed towards the “bad” ear). Rarely, horizontal head-shaking produces a non-horizontal nystagmus, such as vertical nystagmus (Wu et al. 2005) or torsional nystagmus (Califano et al. 2001); this is called a “perverted” head-shaking nystagmus.

Variations in technique of the head-shaking test mainly involve use of a different axis of rotation. These include oscillation of the head vertically, about the naso-occipital axis (by alternating lateral flexion of the neck), or using a combination of horizontal and vertical movement so that the nose traces out a circular trajectory (Haslwanter and Minor 1999). Some studies have also explored head-shaking with the body in other positions than upright (Kamei, Iizuka, Matsuzaki 1997; Palla, Marti, Straumann 2005), but these techniques are not in wide clinical use. Generally, head-shaking using these other axes of rotation or other body positions does not have a well established clinical indication.

Hain and Spindler (Hain and Spindler 1993) review several possible mechanisms of HSN, and point out that it likely involves more than one mechanism since HSN can have central or peripheral etiologies.

The first hypothesis postulates the existence of a tone asymmetry that predisposes to an underlying potential for nystagmus that is usually absent due to compensatory mechanisms, but becomes manifest when the compensatory mechanisms are disrupted (such as by head-shaking). (We avoid here the term “latent nystagmus,” which refers to a specific pathological process.)

The second hypothesis postulates asymmetries in vestibular gain due to a variety of factors, such as loss of vestibular hair cells, lesions of vestibular nerve or its root entry zone, asymmetrical central gain or asymmetrical cervical afferent input. Asymmetry in the peripheral vestibular input leads to nystagmus by virtue of Ewald’s second law, which in its general form states that excitation is a relatively better vestibular stimulus than is inhibition (Leigh and Zee 2006), and which in its specific form states that ampullopetal endolymph flow in the horizontal canal causes a greater response than ampullofugal endolymph flow (Baloh and Honrubia 2001; Ewald 1892). Ewald’s second law is thought to be due to the inability of inhibitory stimuli to decrease vestibular nerve firing rates to less than zero (Baloh, Honrubia, Konrad 1977; Hain and Spindler 1993).

The third hypothesis postulates asymmetries in timing. Hain and Spindler (Hain and Spindler 1993) conceptualized this class of mechanisms as deriving from errors in central head movement response storage. Storage mechanisms (often termed “leaky integrators”) play a role in many central nervous system circuits by providing low-pass filtering and averaging. Responses are stored in three places in the vestibular system (the semicircular canals, the central velocity storage mechanism, and adaptation circuitry); consequently there are three corresponding potential sources of asymmetry in timing. First, response storage asymmetry in the semicircular canals could result, for example, from asymmetrical endolymph viscosity between the ears, leading to a timing mismatch and filtering across the two ears. Second, there can be asymmetry in the central velocity storage mechanism. The central velocity storage mechanism prolongs the raw vestibular signal (Leigh and Zee 2006); its neuroanatomical substrate is thought to include the medial vestibular nuclei of Schwalbe and their commissural connections (Baloh and Honrubia 2001; Leigh and Zee 2006). Third, there can be asymmetry in the compensatory changes involved in central and/or peripheral adaptation (Leigh and Zee 2006).

The fourth hypothesis postulates simple mechanical causes, such as improper technique of eliciting HSN. When head movement is not confined to a single plane, such as in a “circular trajectory” (Haslwanter and Minor 1999), there is an effective rotation about an axis of the head that induces a per-rotatory or post-rotatory nystagmus. Thus, one must be careful to maintain a constant and consistent axis when doing head-shaking to avoid extraneous nystagmus.

Since there are several possible mechanisms for HSN, it probably has a relatively high sensitivity and low specificity.

There are few studies on the normative values of incidence of HSN in healthy individuals, and the available studies describe a rather broad range. Li (Li 1991) states that of 60 healthy individuals, none had head shaking nystagmus. Vicini et al.(Vicini, Casani, Ghilardi 1989) studied 73 normal subjects and observed HSN in 7 (9.6%). Levo and colleagues (Levo, Aalto, Petteri Hirvonen 2004) studied 20 healthy subjects using videonystagmography; they reported that head-shaking nystagmus was observed in 35% of individuals.

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