A factor common to disorders that have been previously reported in association with pendular nystagmus (Table 3) and the peroxisomal disorder that afflicted our patients is the involvement of central myelin. Furthermore, the visual system is commonly involved in disorders of central myelin, including the peroxisomal disorders. For example, Zellweger syndrome is characterized by pigmentary retinopathy and NALD by optic atrophy, whereas visual involvement in X-linked ALD is less severe, with most patients having normal visual acuity.6- 8 Unlike with X-linked ALD, disorders of peroxisome assembly may show neuronal lipidosis.9 Nystagmus has been previously noted in NALD, but it was neither characterized nor recorded.10,11 Our 3 patients' nystagmus was pendular in waveform, with predominant frequencies of 3.0, 5.5, and up to 6.5 Hz; these characteristics are similar to the forms of pendular nystagmus reported with other disorders of myelin, such as multiple sclerosis.1,12 A correlation was noted between the severity of visual acuity loss and the amplitude of nystagmus in patients with multiple sclerosis, which led to the hypothesis that delays in visual transmission may be responsible for these ocular oscillations.13 This hypothesis, however, was tested, and findings indicated that delays in the control of eye movements by vision are not the root cause of such nystagmus; it was suggested that these oscillations occur when feedback loops between the brainstem and cerebellum go awry.12 Furthermore, disorders that do not affect vision, such as oculopalatal tremor following brainstem stroke, may lead to pendular nystagmus.1 In such disorders, the frequency of oscillation tends to be lower (1-2 Hz) than in our patients or in patients with other demyelinating diseases. These disorders may reflect the interruption of visual inputs that reach the cerebellum via the inferior olive and are important for optimal eye movements. Magnetic resonance imaging of the brain of patient 2 showed an area of demyelination in the paramedian dorsal pons and medulla. This is similar to the location of demyelinative lesions in patients with acquired pendular nystagmus,14 and it has been suggested that cell groups of the paramedian tracts,15 which project to the cerebellar flocculus, may be involved. Lesions within these brainstem-cerebellar feedback circuits might lead to instability and produce pendular nystagmus.12 Autopsy findings have shown that cerebellar degeneration is particularly severe in patients with NALD,11 with marked gliosis and demyelination of the cerebellar white matter and atrophy of the granular layer, along with Purkinje cell loss and Bergmann gliosis.