Spasmus Nutans
Definition
Spasmus nutans (SN) is a rare, self-limiting paroxysmal disorder of infancy classically defined by a clinical triad:
- Nystagmus — asymmetric, disconjugate, horizontal, pendular, small-amplitude and high-frequency (up to 15 Hz); may be monocular or, occasionally, bilateral or vertical (Bonnet et al., Movement Disorders, 2010).
- Head nodding — slow head tremor, usually “no-no” type (also “yes-yes” or rotatory), 2–3 Hz, variable amplitude, absent during sleep and thought to be compensatory for the nystagmus (Bonnet et al., Movement Disorders, 2010; Bellini et al., Neurology, 2022).
- Torticollis / anomalous head position — head tilt present in ~40% of cases (Bonnet et al., Movement Disorders, 2010).
Onset is typically between 4 and 18 months (occasionally up to 3 years). It usually resolves within 1–2 years of onset, though subclinical fine nystagmus may persist to 5–12 years of age (Bonnet et al., Movement Disorders, 2010; Gottlob et al., IOVS, 1995). Prevalence among children with asymmetric congenital nystagmus is ~2.5%, with no gender difference (Bonnet et al., Movement Disorders, 2010).
Etiology / Pathogenesis
- The etiology is unclear. Ocular motor control is thought to be transiently unstable and “plastic” in the early postnatal period; temporary disruption of afferent visual input in the first months of life may transiently affect motor control mechanisms (Bonnet et al., Movement Disorders, 2010).
- Head nodding and head tilt appear compensatory for the nystagmus on electronystagmographic recordings (Bonnet et al., Movement Disorders, 2010).
- Familial forms have been reported, suggesting a genetic contribution; SN may be more frequent in African-American and Hispanic families and in those of low socioeconomic status. Associations with low gestational age and low birth weight have also been described (Bonnet et al., Movement Disorders, 2010).
Evaluation
SN is a diagnosis of exclusion made only after normal clinical, ophthalmologic, electrophysiologic, and neuroimaging studies (Bonnet et al., Movement Disorders, 2010).
Recommended workup:
- Complete neurologic examination — should be normal.
- Ophthalmologic examination including fundoscopy — to exclude macular/optic nerve hypoplasia and retinal disease; assess for strabismus, amblyopia, and refractive error (Bellini et al., Neurology, 2022; Young et al., Ophthalmology, 1997).
- Brain (and orbital) MRI — to exclude SN-like structural lesions. Eye/head movement recordings cannot distinguish benign SN from SN-like disease with CNS lesions; this differentiation rests on neuroimaging (Gottlob et al., Ophthalmology, 1990).
- Visual-evoked potentials and electroretinography — normal in benign SN; useful to exclude retinal disorders (Bonnet et al., Movement Disorders, 2010).
Note on neuroimaging yield: Historically SN was associated with optic pathway gliomas (OPG), but the largest MRI series (n=40) found no OPG and no intracranial or orbital masses; in children with isolated SN and no other neurologic findings, neuroimaging may not always be required, though MRI remains standard to confirm the diagnosis (Bowen et al., Journal of AAPOS, 2017; Bellini et al., Neurology, 2022).
Differential diagnosis
- Congenital idiopathic nystagmus (persists throughout life)
- Opsoclonus-myoclonus syndrome
- Bobble-head doll syndrome
- Secondary “SN-like” syndromes — refractive errors, optic nerve/chiasmal gliomas, retinal disorders, cerebellar vermian hypoplasia, brain tumors, Pelizaeus-Merzbacher disease, Leigh encephalomyelopathy (Bonnet et al., Movement Disorders, 2010; Gottlob et al., Ophthalmology, 1990).
Management
- No pharmacologic treatment is necessary — SN is benign and self-limiting (Bonnet et al., Movement Disorders, 2010).
- Reassurance of the family regarding the benign natural history once serious mimics are excluded (Aung et al., Ann Acad Med Singapore, 1996).
- Ongoing monitoring to confirm that SN remains isolated and gradually resolves; watch for emergence of neurologic or ophthalmologic abnormalities that would suggest a secondary cause (Bonnet et al., Movement Disorders, 2010).
- Detect and treat associated visual sequelae — strabismus, amblyopia, and refractive errors are common (strabismus in 10/18 and amblyopia in 8/18 in one series); early spectacle correction and amblyopia therapy optimize visual outcomes (Young et al., Ophthalmology, 1997).
- Prognosis — good visual acuity is the rule and psychomotor development is usually normal, though rare cases with developmental delay have been reported; subclinical nystagmus may persist for years (Bonnet et al., Movement Disorders, 2010; Gottlob et al., IOVS, 1995).