Variable bursts of sustained convergence and retraction of the eyes on attempted upgaze suggest a midbrain disorder. Co-contraction of all the extraocular muscles causes retraction with convergence because of the greater strength of the medial rectus muscles. In infants, one would suspect congenital aqueductal stenosis, in children a pinealoma or obstructive hydrocephalus, and in older adults a vascular accident in the tectal or pretectal area. Parinaud syndrome is convergence-retraction nystagmus in association with vertical eye movement palsies, pupillary abnormalities, lid retraction, and accommodative spasm (3).

Seesaw nystagmus is a unique vertical-torsional oscillation of both eyes in which one eye rises and intorts and its fellow eye falls and extorts. The disjunctive vertical movement alternates to provide the seesaw effect. This disorder may be congenital, but most patients have suprasellar tumors expanding within the third ventricle and compressing the brainstem. Acquired oscillopsia and bitemporal hemianopsia are associated findings (4). Other causes include a lesion in the lateral medulla or pons, syringobulbia, or severe vision loss (5).

PAN is a horizontal jerk nystagmus in which the direction of the fast phase changes spontaneously and cyclically with an intervening neutral period. A typical cycle lasts 1 to 6 minutes. A sequence of jerk waveforms in one direction converts to a neutral period of pendular waveforms, followed by jerk waveforms in the opposite direction. Alternating head turns may accompany the jerk nystagmus periods. PAN appears to result from a spatial and temporal shift in the null zone (6). PAN usually remains horizontal in vertical gaze and is hypothesized to arise from an instability of the optokinetic-vestibular system. It may coexist with downbeat nystagmus, both of which suggest an abnormality in the caudal medulla (7). PAN was found on eye movement recordings of patients with idiopathic infantile nystagmus who had mutations on the FRMD7 gene on Chromosome Xq26 (NYS 1 locus) (8). Up to one-third of patients with albinism also demonstrate PAN (6). Acquired PAM occurs most commonly with disease involving the midline cerebellum. Inhibitory pathways that use gamma-aminobutyric acid (GABA) in the nodulus and uvula control the time course of rotationally induced nystagmus and therefore Baclofen, a GABA agonist, can be effective in its treatment (9).

Downbeat nystagmus is recognized by a deficit in downward pursuit whereby the eyes drift upward, and a corrective sac-cade returns the eyes to the primary position. It is maximal in downgaze, lateral gaze and when the head is erect or hyperextended. A rare congenital hereditary form has been identified that is usually self limited, and associated with good vision and a normal neurologic examination. A compensatory chin-down head posture is sometimes noted (10). More commonly, downbeat nystagmus is acquired and signifies an abnormality of the cerebellum (flocculus or paraflocculus). The commonest causes are cerebellar degenerations and structural lesions at the craniocervical junction such as Chiari malformation (Fig. 23.1). Alcohol, lithium, anticonvulsants, and thiamine, magnesium, and B12 deficiency may also give rise to this condition (11).

A vertical vestibular or smooth pursuit deficit similar in type but opposite in direction to downbeat nystagmus is thought to cause upbeat nystagmus, which may increase or convert to downbeat nystagmus with convergence. It may occur congenitally as a variant of congenital nystagmus (CN) with anterior visual pathway disease (12). Acquired forms occur with lesions of the brainstem (mainly the pontomesencephalic junction, rostral medulla, or caudal pons), cerebellar vermis, or after meningitis (13).

Lesions of the labyrinth or eighth nerve cause a horizontal and rotary jerk nystagmus on lateral gaze opposite the side of the lesion. Vertigo may be marked, and tinnitus and deafness occur in concert. Visual fixation decreases
the intensity of the nystagmus and vertigo, which lasts minutes, days, or weeks. Central pathways eventually compensate even if the underlying cause remains. Common causes are Meniere disease, infectious or vascular disorders, and trauma (14).

This autosomal-recessive disorder is characterized by diminished vision starting at or shortly after birth and can be divided into eleven subtypes depending on gene locus and defect. LCA2 results from RPE65 (retinal pigment epithelium-specific 65 KDa) deficiency that disrupts the retinoid cycle. Recent studies of subretinal gene therapy using recombinant adeno-associated virus to carry the RPE65 gene have shown promising results (16).

Acuity is less than 20/200 in up to 95% of affected individuals, and a searching nystagmus is present in 75%. The pupils are poorly reactive, and many children exhibit the oculodigital sign (habitual eye rubbing). The fundus is usually normal in infancy. Pigmentary disturbances of the peripheral retina develop during childhood in most patients, along with optic disk pallor and arteriolar attenuation. A markedly reduced or absent response to the ERG is noted in virtually all patients (Fig. 23.2). A variety of ocular conditions, including keratoconus, keratoglobus, macular coloboma, disk edema, cataract, and strabismus, have been associated with LCA. As many as 15% of patients may have mental retardation. Other systemic associations include medullary cystic kidney disease, cardiomyopathy, and skeletal abnormalities.

Optic nerve hypoplasia (ONH) is a congenital, nonprogressive condition characterized by a paucity of axons within the optic nerve and a diminished ganglion cell layer of the retina. Ophthalmoscopically, ONH is recognized by a small, pale nerve head. Classically, it is surrounded by a ring of white sclera and a second pigmented or nonpigmented ring outlining the scleral rim, giving rise to the term double-ring sign (Fig. 23.3). The retinal vessels usually appear relatively normal, but the retina may be deep red in color because of the thinness of the nerve fiber layer.

Severe bilateral ONH results in a searching nystagmus, whereas mild, unilateral ONH may not have visual symptoms. In one study, 78% of those with bilateral involvement, poor vision, and nystagmus had additional ocular abnormalities, compared with 21% of patients with unilateral ONH (17). Delayed development is the most frequent nonocular disorder, followed by hypopituitarism, cerebral palsy, and epilepsy. Other associations include midline facial defects and abnormalities of the cerebral cortex, brainstem, and cerebellum including absence of the septum pellucidum and agenesis of the corpus callosum (18

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