Acquired Pendular Nystagmus
Acquired pendular nystagmus is a visually distressing form of nystagmus, in which oscillopsia and impaired vision are common. Acquired pendular nystagmus is a quasi-sinusoidal oscillation that may have a predominantly horizontal, vertical, or mixed trajectory (i.e. circular, elliptical, or diagonal); it can be either predominantly monocular or predominantly binocular [6, 61, 62]. The frequency of this type of nystagmus is 2-7 Hz . It is often associated with head titubation (a kind of head tremor with small amplitude and not synchronized with the nystagmus), trunk and limb ataxia, or visual impairment. The amplitude is small and can often be only seen with an ophthalmoscope.
Acquired pendular nystagmus occurs with several myelin disorders (e.g. multiple sclerosis, toluene abuse, Pelizaeus-Merzbacher disease). It is also a component of the syndrome of oculopalatal tremor (myoclonus) and is observed in Whipple's disease [6, 62]. Common etiologies in adults are multiple sclerosis and brainstem stroke [62, 64]. On the basis of observations that the nystagmus is often dissociated and that eye movements other than optokinetic nystagmus and voluntary saccades are also disturbed, it has been suggested that a lesion in the brainstem near the oculomotor nuclei is the cause . Alternative candidates such as an inhibition of the inferior olive due to lesions of the 'Mollaret triangle' or an instability of the gazeholding network (neural integrator) have also been proposed .
The first reported treatment option was anticholinergic treatment with trihexyphenidyl (20-40 mg p.o. daily) [65, 66]; however, Leigh et al.  reported in a double-blind study that only 1 of 6 patients improved during this oral treatment. Starck et al.  reported that nystagmus improved with memantine, a glutamate antagonist, in all 9 tested patients (15-60 mg p.o. daily). Gabapentin, an alpha-2-delta calcium channel antagonist, substantially improved the nystagmus (and visual acuity) in 10 of 15 patients (3 X 300-400mg daily) . Gabapentin was superior to vigabatrin in a small series of patients ; others have also reported an improvement due to gabapentin [70, 71]. Cannabis, which acts as a retrograde presynaptic inhibitory transmitter and in this way is similar to gabapentin, which also acts presynaptically, was recently reported to be equally effective [72, 73]. A bilateral retrobulbar botulinum toxin injection was successfully used in some patients to induce a complete external ophthalmoplegia, thereby diminishing the acquired pendular nystagmus [74, 75]; however, it proved unsatisfactory in other patients .
Opsoclonus and Ocular Flutter
Opsoclonus consists of repetitive bursts of conjugate saccadic oscillations, which have horizontal, vertical, and torsional components. During each burst of these high-frequency oscillations, the movement is continuous, without any intersaccadic interval. These oscillations are often triggered by eye closure, convergence, pursuit, and saccades; amplitudes range up to 2-15° . The same pattern is restricted in ocular flutter to the horizontal plane. The ocular symptoms are often accompanied by cerebellar signs, such as gait and limb myoclonus (the 'dancing feet, dancing eyes syndrome'). Most of the patients complain of very disturbing oscillopsias during these saccadic oscillations [6, 77].
A functional disturbance of active saccadic suppression by the pontine omnipause neurons is the most probable pathophysiological mechanism. Since histological abnormalities of these neurons have not been shown , a functional lesion of the glutaminergic cerebellar projections from the fastigial nuclei to the omnipause cells is the likely cause of their disinhibition. Opsoclonus can be observed in benign cerebellar encephalitis (postviral, e.g. Coxsackie B37; postvaccinal) or as a paraneoplastic symptom (infants, neuro-blastoma; adults, carcinoma of the lung, breast, ovary, or uterus) .
In addition to therapy for any underlying process such as tumor or encephalitis, treatment with immunoglobulins or prednisolone may occasionally be effective . Four of 5 patients with square-wave oscillations, probably a related fixation disturbance, showed an improvement on therapy with valproic acid . In single cases, an improvement has been observed during treatment with propranolol (40-80mg p.o. three times daily), nitrazepam (15-30mg p.o. daily), and clonazepam (0.5-2.0mg p.o. three times daily) [1, 77, 81].
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