to recognize the significance of this observation has contributed to the conclusion that the VOR is inherently defective in infantile nystagmus.

As discussed in Chap. 8, this myth has now been largely dispelled.38,61,98 Unlike the head nodding in spasmus nutans, however, electro-oculographic recordings in infantile nystagmus have shown that the associated head oscillations do not contribute to an improvement in vision. On the contrary, it is now believed to be an involuntary cephalomotor tremor that presumably shares a common pathogenic origin with the nystagmus.60 Electro-oculographic recordings of infantile nystagmus have shown that, during foveation periods, the position of gaze remains stable during head rotation despite the superimposed oscillations of infantile nystagmus. With rare exceptions, the VOR is preserved, and electro-oculographic recordings show no flattening of foveation periods.

These findings are consistent with the fact that older children and adults are often aware of their intermittent head shaking and do not believe it helps them see.60

Carl et al38 documented an exception to this rule in a patient who showed true compensation of infantile nystagmus by the head movements. This patient suppressed his VOR gain, and improvement occurred only during foveation periods. Simultaneous eye and head movement recordings showed that the foveation periods were not flat when the head was still, but they became flat when the head movements occurred. Four other individuals who were studied showed no improvement in foveation periods during head shaking. Dell’Osso and Daroff60 have stressed that most individuals with congenital nystagmus have flat foveation periods and can therefore achieve no visual benefit from shaking their heads.60 From both a theoretical and an evidentiary standpoint, however, head nodding in infantile nystagmus is almost never visually adaptive.

Head nodding in infantile nystagmus can also be asso­ ciated with a fine-amplitude nystagmus in several congenital retinal dystrophies (achromatopsia, blue-cone monochromatism, congenital stationary night blindness), producing a clinical appearance that may mimic spasmus nutans.92,138

Neurodegenerative Disorders, Metabolic Defects,

and Genetic Syndromes

Pelizaeus-Merzbacher disease may be associated with intermittent shaking movements of the head and a rapid, irregular, often asymmetric pendular nystagmus.1 Children with 3-methyl- glutaconic aciduria and neurological signs of Behr syndrome may display head nodding, nystagmus, and optic atrophy.187 Dhir et al64 described two siblings with reduced visual acuity, nystagmus, hypopigmentation of the maculae, head nodding, dysarthria, and other neuromuscular coordination resulting from

histidinemia.ReportsfromtheJapaneseliteraturehavedescribed a newly recognized condition in boys characterized by ataxic diplegia, mental retardation, horizontal pendular nystagmus, head nodding, and abnormal auditory brainstem responses.2 Head nodding and nystagmus can occasionally be seen in multisystem genetic disorders.71,157

Head Nodding without Nystagmus

Bobble-Headed Doll Syndrome

Children with large third ventricular cysts or tumors that are associated with obstructive hydrocephalus occasionally develop a to-and-fro bobbing or nodding of the head and trunk. This 1- to 3-Hz anterior–posterior movement is named for its resemblance to the movement of a doll whose weighted head is mounted on a coiled spring.11 This disorder can be distinguished from benign familial tremor by its slower rate, its invariable association with hydrocephalus, and the child’s ability to volitionally inhibit it.63 With rare exceptions, it is unique to children, with an average age of 7 years at diagnosis.120

The head nodding of the bobble-headed doll syndrome is a slow (1–3 Hz) anteroposterior movement that disappears with sleep and in the lying position, that can be suppressed or decreased at will, or that decreases during voluntary head motion or activity.63,200 The head nodding may be accompanied by a synchronous gentle rocking of the trunk or movement of the hands.158

The cause of the head and trunk movements is unclear. Rapidly progressive hydrocephalus is not associated with the bobble-headed doll syndrome. Similarly, the classic setting sun sign of infantile hydrocephalus has not been reported in conjunction with the bobble-headed doll syndrome, although it is related to third ventricular dilatation and periaqueductal dysfunction. The bobble-headed doll syndrome seems to signify a slowly progressive hydrocephalus, while the setting sun sign is a sensitive sign of rapidly progressive hydrocephalus or shunt blockage in shuntdependent hydrocephalus.63

Most children with bobble-headed doll syndrome have been found to have a cyst or mass in or near the anterior part of the third ventricle.120 Most of the remaining cases have been found to have hydrocephalus secondary to aqueductal stenosis or, rarely, ventricular shunt obstruction,59 suggesting a slow dilation of the third ventricle as the possible common denominator. Some children with bobble-headed doll syndrome harbor, a subclinical nystagmus, and the head nodding may actually contribute to an improvement in vision.85 It has therefore been suggested that the head nodding in the bobbleheaded doll syndrome may share a common pathogenesis

(compression of the floor of the third ventricle) with the head nodding in spasmus nutans-associated suprasellar tumors.85

In some children, the head bobbing precedes other signs or symptoms of elevated intracranial pressure by as much as 6 months.200 Associated neurological and endocrinological abnormalities are common. Apart from a large head size, neurological findings may include abnormal pyramidal-tract findings, ataxia or intention tremor of the trunk, and optic disc pallor – which may reflect compression of the optic nerves, chiasm, or tract, depending on the underlying cause.200 Endocrinological abnormalities may also be present, including diabetes insipidus, precocious puberty, and advanced bone age.200 Resolution or improvement of the tremor following surgical removal of the cyst or tumor is noted in some patients.120

Cerebellar Disease

Midline cerebellar lesions may give rise to a slow, (3 to 4 cps) predominantly anteroposterior oscillation of the head (titubation) which may simulate the bobble-headed doll syndrome.44,200 However, cerebellar head tremors are accompanied by other cerebellar signs, such as truncal ataxia and motor incoordination, as well as cerebellar eye signs (ocular dysmetria, impaired pursuits, gaze-evoked nystagmus, impaired VOR suppression).47 Kalyanaraman et al123 described three related children with head nodding and nystagmus associated with a cerebrocerebellar degeneration of unclear etiology.

Idiopathic torsion dystonia is characterized by twisting and repetitive movements and postures that are not attributed to exogenous factors (e.g., trauma, neuroleptics) or other neurologic disorders (e.g., Wilson’s disease, Parkinson’s disease). Age at onset of idiopathic torsion dystonia ranges from early childhood to the eighth decade, and almost any skeletal muscle may be involved. When idiopathic torsion dystonia begins in childhood, it is likely to start in a limb and then spread to other body regions.19

Benign Essential Tremor

Benign essential tremor is a hereditary, monosymptomatic condition in which an intermittent, involuntary, high-frequency tremor affects the head and hands. Although it more commonly develops during later childhood or adolescence, it may appear at as early as 2 years of age. The head movements may precede the hand movements by several years. Benign essential tremor may initially manifest as shuddering or shivering attacks in which the head flexes and turns along with other body movements.203 Because these brief shuddering attacks

can occur several times a day then cease spontaneously for a week or two, affected children may be misdiagnosed as having epilepsy, psychogenic disturbances, tics, or paroxysmal choreoathetosis.158

The high frequency of benign essential tremor (5 to 1.5 cps) distinguishes it from the bobble-headed doll syndrome. Unlike the bobble-headed doll syndrome, in which the head movements are inhibited by activity, benign essential tremor persists or worsens with activity.200 Ingestion of small amounts of alcohol (one drink) abates the tremor, whereas ingestion of larger volumes exacerbates it. Treatment with propanolol effectively abolishes the tremor. Autopsy studies of the basal ganglia and other structures have failed to detect any pathological abnormality.176

Paroxysmal Dystonic Head Tremor

Paroxysmal dystonic head tremor is a rare, nonfamilial disorder characterized by attacks of horizontal head tremor (frequency 5–8 Hz).77,178 The attacks last 1–30 min and cannot be suppressed. They begin in adolescence, but some children may have an associated head tilt that predates the onset of the tremor by 5–10 years.77 Neuroimaging studies are negative. The condition is nonprogressive, and its cause is unknown. Daily clonazepam reduces the frequency and severity of attacks.77

Autism

Autistic children may display rocking of the head and trunk along with other motor stereotypes (hand flapping and spinning), sensory stereotypes, and impaired communication and socialization. Necropsy studies implicate severe Purkinje cell loss in the posterior cerebellar vermis and cerebellar hemispheres as a neuroanatomical correlate of autism.52 This abnormality is reflected on theMR imaging as either hypoplasia or hyperplasia of the cerebellar vermis, causing previous quantitative MR estimates of the mean posterior cerebellar size to fall within the normal range. However, the MR imaging shows increased total brain volume attributable to generalized enlargement of gray and white matter cerebral volumes, but not cerebellar volumes.51,103 Current theories focus on autism as a disorder of connectivity, particularly involving the association cortex.152

Infantile Spasms

Infantile spasms may manifest with nodding attacks that occur in isolation or together with mental retardation, myoclonic jerks, or various neurological deficits.76,154 Morimoto