“Sensory” and “Motor” Nystagmus

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Most patients with infantile nystagmus syndrome (INS) 1 (formerly known as congenital nystagmus) exhibit nystagmus with several of the waveforms first identified in 1975. 2 The past 45 years of recording and analyzing nystagmus waveforms have repeatedly demonstrated that most INS waveforms result from the same deficit in one of the several ocular motor subsystems, specifically smooth pursuit. 3-6 This applies to all patients with INS whether they have associated visual sensory deficits, are familially predisposed to have INS (ie, there is a true genetic anomaly), or exhibit INS without an associated ocular or central nervous system deficit (so-called idiopathic). Despite this eye-movement data, textbooks in ophthalmology, neuro-ophthalmology, and neurology as well as current peer-reviewed literature continue to use the terms sensory and motor nystagmus to describe the ocular oscillations of INS. Those descriptions imply not only that there are at least 2 different types of nystagmus but also that they have independent pathophysiology caused by different primary developmental deficits; both implications are unsupported by decades of ocular motor electrophysiological data. The adoption of this terminology and its presumed ability to clinically differentiate pendular from jerk waveforms are responsible for the misdiagnosis of many cases of INS. Ocular motor recordings demonstrate that owing to the complexity of INS waveforms, clinical differentiation is impossible in many patients. These recordings also show that another type of nystagmus of infancy, fusion maldevelopment nystagmus syndrome (formerly known as latent/manifest latent nystagmus), cannot be differentiated from INS with a latent component; Alfred Kestenbaum, MD, first noted the existence of these 2 clinically similar types of nystagmus in his 1946 textbook and its expanded 1961 second edition.