________________________________________________________________________ Dizziness and Vertigo
With vestibular disturbance, when the head is rapidly turned toward the affected side, the eyes move with the head and visual fixed gaze is broken, followed by a refixation (“catch-up”) sac- cade back to the original visual target [105]. This is nystagmus. In peripheral or central vestibular disorders, understanding the simple “rules” of nystagmus allows rapid, confident diagnoses. With peripheral causes of nystagmus, the fast-phase direction is away from the affected side (of canal/vestibular nerve), and this direction remains constant, regardless of gaze direction. The pattern is primarily horizontal with a torsional compo- nent—never purely torsional or vertical. In central nystagmus, the fast-phase varies with the direction with gaze (i.e., right- beating on rightward gaze, then left-beating on leftward gaze). The pattern is any direction, but primarily torsional or vertical. HINTS is an acronym for three ocular motor tests—the head impulse test (HIT), gaze testing for nystagmus, and alternate cover test for skew deviation—that are combined to differenti- ate central from peripheral causes of dizziness. Nystagmus is a key observed response. Normal visual fixation can suppress mild nystagmus, but Frenzel lenses worn by the patient block visual fixation and magnify examiner view of eye movements [8; 10; 19]. When conducting the HIT, have the patient fixate his/her gaze on a midline target (e.g., the examiner’s nose), then rapidly rotate his/her head 20 degrees to the right or left, bring head back to midline, then rotate to the other side. The presence of a corrective saccade is “positive” for abnormal vestibulo-ocular reflex, which generally indicates a peripheral vestibular process. Gaze that remains locked on the midline target is normal. A normal HIT in patients with AVS is highly suspicious for stroke, but HIT is only useful in patients with AVS and nystagmus. In patients with dizziness (with urosepsis or dehydration) without nystagmus, a normal HIT is a mislead- ing false-positive for stroke. Gaze testing for nystagmus consists of having the patient look straight ahead (“neutral” or “primary” gaze) and observing for eye movements. Patients whose eyes drift leftward and snap back horizontally to the right have right-beating horizontal nystagmus. Next, look for “gaze-evoked” nystagmus by having the patient look to the right and then to the left, each for several seconds; observe for nystagmus and direction of its fast-beating component. Two patterns suggest stroke or other central cause: • Dominantly vertical or torsional nystagmus in any gaze position • Dominantly horizontal nystagmus that changes direction in different gaze positions, termed bilateral or gaze-evoked nystagmus However, the most common pattern in patients with AVS and stroke is direction-fixed horizontal nystagmus, an acute vestibular neuritis mimic that requires further testing.
Explicitly assess for the following symptom-triggering circum- stances and note whether symptoms are triggered, exacerbated, or improved [15; 103]: • Rolling over in bed, looking up, changing head position relative to gravity • Turning the head to the side • Any head movement
• When standing up or walking • When supine or recumbent • Vibration or loud sounds • Seeing objects in motion
Assessment of triggers establishes the distinctions between positional (change of head orientation with respect to gravity), head motion-induced (head motion regardless of direction), and orthostatic position changes. Obligate triggers are specific triggers that consistently cause dizziness [8].
TARGETED EXAMS Eye Movement Assessments
At this point in the workup, the patient is placed into one of six vestibular syndromes after some possible etiologies were ruled out during the patient history. The differential diagnosis does not extend beyond possible etiologies within the vestibular syndrome, which substantially narrows the range. The next task is to differentiate dangerous central mimics from benign causes in acute and episodic vestibular syndromes and to diagnose BPPV by identifying the affected canal. This involves the use of targeted examinations, also called “bedside examinations,” which emphasize the assessment of eye move- ments to detect and localize the underlying pathology of dizzi- ness. The accuracy of eye movement assessment is superior to CT or MRI imaging in differentiating serious CNS from benign causation. Healthcare costs are also reduced by eliminating (in many but not all cases) expensive but unhelpful brain imaging. Nystagmus is a repetitive, involuntary movement of the eyes, and nystagmus assessment is a core component of the diagnostic workup. Whether spontaneous or triggered, the characteristics of nystagmus help to localize the pathology and suggest a diagnosis. The nystagmus is tested first, because some disorders are diagnosed solely by the nystagmus characteristics [10; 19; 47; 104]. The vestibulo-ocular reflex keeps vision focused on a target dur- ing head movement. When looking to one side, the labyrinth on that side signals the head-turn, and the eyes automatically move in the opposite direction to maintain fixed gaze. The vestibulo-ocular reflex maintains gaze for continued reading while turning the head alternately to each side [104; 105]. Vestibulo-ocular reflex assessment is used in the diagnosis of vestibular disorders, because vestibular dysfunction causes inappropriate activation of the vestibulo-ocular reflex that manifests in nystagmus [16].
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Book Code: ANCCTX2027
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