Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Neuro-Ophthalmology_Wright, Spiegel, Thompson_2006

The incidence of MMC is approximately 1 in 1000 in the United States, and is slightly greater in whites than in AfricanAmericans; the risk of recurrence for parents with one affected child is approximately 4%.118 The etiology is unclear, and polygenetic inheritance, environmental conditions, and maternal malnutrition have all been implicated.88,98 Prenatal diagnosis during the 14th to 20th week of gestation is accomplished through amniocentesis by the detection of alpha-fetoprotein and acetylcholinesterase liberated via a leaky neural tube defect.3,56

Ocular Findings

Modern neurosurgical, urosurgical, and medical management have increased the quality and lifespan of patients with MMC; as a result, there is a greater opportunity for neuro-ophthalmic anomalies to manifest themselves and require proper care. Although the majority of neuro-ophthalmic findings associated with MMC are motility disorders,71 other findings, such as optic atrophy,112 are intimately linked with CSF dynamics and generally increase in proportion to the severity of hydrocephalus. Hydrocephalus has been reported in up to 90% of patients with MMC.13

Hydrocephalus results in ventricular dilation, chiasmal and optic nerve compression with secondary ischemia, midbrain distortion, and hindbrain herniation. Most MMCs, particularly in the lumbosacral region, are associated with some degree of ACM; in turn, more than 90% of children with MMC develop hydrocephalus secondary to aqueductal stenosis and the associated ACM.114,120 Nearly all patients with ACM type II or worse have an associated MMC12,115; conversely, almost all patients with MMC have an ACM.58

Papilledema (and eventual optic atrophy), cortical blindness, sixth nerve palsy, vertical gaze palsy, and downbeat nystagmus may all develop when intracranial pressure (ICP) rises acutely.12,71 In younger patients, before fusion of the fontanelles, the head often expands rather than progress to clinical papilledema. If a sixth nerve palsy from hydrocephalus does occur, the eventual level of binocular development and realignment is often poor.

Early comprehensive ophthalmic examination for optic nerve anomalies, nystagmus, and motility disorders, combined with appropriate amblyopia therapy and surgical intervention, have led to excellent visual outcomes (90%–95% have greater

than or equal to 6/12 visual acuity)12 in patients with MMC. Fortunately, there is a low rate of decreased visual function from amblyopia in these patients26,44,77,107 because early occlusion treatment yields good results.12 However, causes responsible for decreased visual acuity include amblyopia, latent nystagmus, optic atrophy, and cortical blindness, as well as apparently unassociated or “independent” reasons such as refractive errors, cataracts, and keratoconus.12 Generally, in patients with MMC and ACM, visual difficulty seems to be more often related to refractive errors and strabismus than to optic atrophy.71

Approximately 60% of patients with MMC are found to have some form of strabismus.23 Even without hydrocephalus, 50% of patients with MMC also develop strabismus.26 Of the many types of strabismus found, “A” patterns, with overacting superior oblique and superior recti muscles23,39 and esotropia,113 are the most frequent, although “V” patterns and exodeviations can occur.12,23,38,71,77 A patterns have also been more commonly seen with exodeviations than with esodeviations (Fig. 11-3).13

The A-pattern strabismus associated with spina bifida is most likely acquired and related to hydrocephalus.13 Skew deviation, with supranuclear origins, is generally considered a sign of posterior fossa disease, and can clinically mimic superior oblique over-action. In turn, some cases of superior oblique over-

FIGURE 11-3. “A” pattern exotropia in a patient with Klippel–Feil syndrome (KFS).

action can represent a form of skew deviation.58 This relationship is consistent with the finding that children with superior oblique overaction have a significantly higher incidence of concurrent neurological diseases than control subjects.58 A classic “setting-sun” sign, with the eyes deviated downward and outward in the presence of increased ICP, may actually represent a severe A pattern superimposed on a hydrocephalusinduced vertical gaze palsy. Lid retraction commonly seen in hydrocephalic infants may accentuate this sign. Theories surrounding A-pattern esotropias implicate inherently weak lateral recti, partial sixth nerve palsies, primary midbrain pathology (a supranuclear defect in the brainstem),77 and fluctuating ICPs.12,71,77 Both the deficiency of upward gaze and the lid retraction may be secondary to dilation of the posterior portion of the third ventricle and possibly the cerebral aqueduct.127 MRI studies demonstrate that beaking of the rostral area of the brainstem is associated with an A-pattern strabismus, which correlates with defects in the vertical gaze pathways.13,22

Treatment

Bilateral superior oblique tenotomies have had greater success for A-pattern deviations in patients with MMC than infraplacement of the lateral recti (A-pattern exotropia) or supraplacement of the medial recti (A-pattern esotropia).13 A tenotomy of the superior oblique muscle, nasal to the medial border of the superior rectus muscle, has been recommended for larger A-pattern deviations; a tenotomy lateral to the superior rectus muscle has been recommended for smaller A-patterns.13

ABNORMALITIES OF THE CRANIAL–VERTEBRAL BORDER: PLATYBASIA AND BASILAR IMPRESSION

Malformation of the base of the occipital bone (forming the base of the skull) and of the cervical spine may be manifested as platybasia and/or basilar impression. Other malformations of this region include defects of the atlas and axis, cervical fusional syndromes, such as the Klippel–Feil syndrome and the Arnold–Chiari malformation. Each of these entities may occur singly or within the scope of a larger syndrome, and may exist as a primary maldevelopment or as a secondary change from a systemic/bony disease.

Although the terms platybasia and basilar impression are often used interchangeably and are conditions that frequently occur together, they can each be distinctly defined. The condition of a flat skull base, or platybasia (from the Greek; platys, flat, and basia, base),41 exists when the angle formed by a line connecting the nasion, tuberculum sella, and anterior margin of the foramen magnum is greater than 143°.81 Normally this angle ranges from 110 to 140°.103 Essentially, the basal angle of the skull, made by the intersection of the plane of the sphenoid with that of the clivus, is flattened. The effect is that the anteroposterior (AP) diameter of the foramen magnum is decreased and the depth of the posterior fossa becomes shallow, thereby allowing less space for the cerebellum and brainstem to properly align themselves along the axis of the spinal column.

Basilar impression, or invagination, occurs when the margins of the foramen magnum are variably indented into the base of the skull. The foramen magnum contours become distorted and narrowed, more so in the AP dimension, by upward displacement of the odontoid process above Chamberlain’s line (hard palate to the posterior border of the foramen magnum) and into the foramen magnum.18,30,81 Localized thickening of the dura at the cranial vertebral junction is frequently associated, which further constricts the brainstem.62

As primary developmental anomalies, platybasia and basilar impression are relatively rare, and are more commonly associated with a systemic condition. When they are secondary to a systemic/bony disease, the cranial bones are usually softened so that the weight of the skull causes the cranial vertex to approach the occiput as the occiput and cervical spine press into the posterior fossa. This configuration may result from mucopolysaccharidoses (Hurler’s), Paget’s disease of the bone, rickets (osteomalacia), osteogenesis imperfecta, osteitis deformans, fibrous dysplasia, hyperand hypoparathyroidism, hypoand achondroplasia, cleidocranial dysostosis, histiocytosis X, rheumatoid arthritis, Conradi’s disease, and Klippel–Feil

syndrome.32,61,125,134

The clinical findings may be attributed to mechanical forces, such as direct compression or hydrocephalus, or to associated CNS defects. Clinical manifestations are proportional to the anatomic extent of the defect, and frequently to the degree of resultant hydrocephalus. Both platybasia and basilar impression lead to a diminished capacity of the posterior fossa. The pons, medulla, and cervical cord may be compressed, and the

posterior cranial nerves may become stretched as the occipital bone and cervical spine rise and the foramen magnum narrows. The cerebellum may also be compromised from above as it is pressed against the resistant tentorium cerebri.103 Hydrocephalus, often dependent upon head position about the atlantooccipital junction, is a frequent complication due to obstruction of CSF at the level of the crowded posterior fossa.106 A coincident vertebral basilar vascular insufficiency, also positionally dependent, has been implicated in producing clinical findings such as transient dizziness, hemiparesis, and bilateral blurring of vision, which may occur with platybasia and basilar impression.64,125

Atlantoaxial dislocation is a significant structural anomaly of the spine that often accompanies platybasia and basilar impression; this may lead to compression of the upper cervical cord (between the odontoid process and posterior arch of the atlas), and compromise of the lower medulla, which becomes distorted at its junction with the posteriorly displaced spinal cord.30 A variant of this dislocation, which requires surgical correction, is “odontoideum” in which the odontoid is separated from the body of the axis. The effect of this can range from mild transient neck discomfort to the extreme presentation of quadriplegia.1

Clinical and Ocular Findings

Platybasia and basilar impression often remain relatively asymptomatic until adult life, when they are aggravated by minor trauma. However, they may cause more severe clinical problems as a primary structural anomaly or as part of a progressive systemic disease (Table 11-2). Symptoms may be grouped in rela-

TABLE 11-2. Systemic Conditions Leading to Platybasia.

Mucopolysaccharidoses (Hurler’s)

Paget’s disease

Osteomalacia (rickets)

Osteogenesis imperfecta

Osteitis deformans

Fibrous dysplasia

Hyperhypoparathyroidism

Achondroplasia

Histiocytosis X

Rheumatoid arthritis

Conradi’s disease

Klippel–Feil syndrome

tion to the areas affected (such as cerebellum, medulla and pons, posterior and lower cranial nerves, upper cervical nerves, musculoskeletal system), and according to those that are associated with hydrocephalus.127

Cerebellar changes may cause atonia, ataxia, and poor coordination. Impingement of the pons and medulla by the clivus can induce pyramidal tract symptoms such as abnormal reflexes, spasticity, lateral spinothalamic tract impairment with dissociated sensory pain loss, weakness, and general paresthesias.127 Such a patient may also have loss of position sense, bladder dysfunction, and neuromuscular atrophy.30 Lower cranial nerves are typically affected, which causes descending trigeminal tract (facial) sensory loss and palatal and vocal cord weakness.30 Cervical nerve involvement may be detected by the dermatomal distribution of sensory loss, whereas musculoskeletal symptoms reflect the extent of malformation and include pain, shortening, and rigidity of the neck, in addition to torticollis and a sensation of heavy pressure.46

A syringomyelic syndrome may develop with hindbrain herniation and subsequent cavitation of the lower medulla and cervical cord. These patients typically exhibit spinal features such as upper limb neuromuscular atrophy early in their clinical course and often require a surgical laminectomy to decompress the expanding cord.30,33

Ocular findings reflect direct posterior fossa compression in addition to hydrocephalic changes. Nystagmus is often present and is usually horizontal, but, as seen with posterior fossa lesions, it also becomes vertical with upgaze and occasionally with downgaze.27,127 Hydrocephalus may lead to papilledema, secondary optic atrophy and decreased vision, ptosis and anisocoria, paralysis of convergence, and corneal anesthesia.54

Diagnosis

Clinical diagnosis requires recognition of early symptoms such as transient dizzy episodes seen with vertebral-basilar insufficiency,125 spastic weakness, ataxia, and focal neurological complaints. In addition, symptoms consistent with an elevated intracranial pressure such as nausea, vomiting, and headache may occur. Downward nystagmus, which is dependent on head position, and other neurological changes are followed by eventual signs of a bullneck appearance with decreased head movement and loss of the normal cervical lordosis.1 Syringomyelia,

demyelinating disease, and posterior fossa tumors must be ruled out in patients exhibiting the progressive and diverse findings of cerebellar, brainstem, and cervical cord syndromes.81

Confirmatory diagnosis is imperative and has traditionally been demonstrated with lateral X-ray studies; however, MRI is superior. With sagittal sections, one can see Chamberlain’s line (described earlier) intersect Bull’s line (an extended line drawn through the spine and body of the first cervical vertebrae), instead of parallel each other, as in the normal configuration.1 Some authors believe that posterior inclination of the odontoid, with respect to the foramen magnum, is the best radiographic indicator to determine if the patient is at risk for basilar impression.50

Treatment

Cases with minor cervical vertebral anomalies may simply require neck immobilization; however, worsening posterior fossa and cord compression or CSF obstruction require surgical decompression and possible shunting. Radiologic studies demonstrating the direction of compressive force on the cervicomedullary junction show the need for anterior or posterior decompression. If atlantoaxial dislocation is a concurrent condition, then skeletal traction is required for performing surgery and stabilization of the spine by bony fusion is recommended.30,63

KLIPPEL–FEIL SYNDROME

Klippel–Feil syndrome (KFS), also known as congenital brevicollis, was described in 1912. It is typified by a no-neck or head- on-shoulders appearance resulting from a congenital synostosis of the upper cervical vertebrae, limitation of neck movement, and a low posterior hairline68 (Fig. 11-4). This appearance is exaggerated by “pterygium colli” or webbing of the neck similar to the Turner’s syndrome patient. Although the primary skeletal defect is fusion of the atlas and axis, three types of KF dysostosis have been described according to their anatomic extent.10,31,52 Type I involves fusion of cervical and upper thoracic vertebrae, type II is solely a cervical fusion, and type III includes an associated fusion of lower thoracic and lumber vertebrae (Fig. 11-5). In addition to the variable degree of cervical fusion, there are

AR, autosomal recessive; AD, autosomal dominant.

associated deformities of the base of the skull, frequent cranial nerve anomalies, facial asymmetry, torticollis, and significant limitation of head movement.

A new classification comprised of four types has recently been proposed, and is based on the genetic heterogeneity and variable expression of Klippel–Feil syndrome (Table 11-3). KF I, with autosomal recessive inheritance, is typically most severe, the only class involving C1 fusion, and associated with a very short neck. KF II is autosomal dominant and typically includes craniofacial anomalies and C2–C3 fusion. KF III is either recessive, or autosomal dominant with reduced penetrance or variable expression, presenting with isolated cervical fusion. KF IV, with fusion of cervical vertebrae, is predominantly in females, possibly X-linked, and includes the cluster of hearing and ocular findings (Duane’s syndrome) known as Wildervanck’s syndrome.24

KFS may be considered a subset of an embryologic defect in neural tube development or “dysraphism” as in iniencephaly, cervical meningomyelocele, or spina bifida.43 This collection of neural tube anomalies may result in paraplegia, hemiplegia, and cranial, or more often cervical, nerve palsies45,96 (Fig. 11-6). Several other abnormalities associated with KFS have been reported, including syringomyelia,118 recurrent meningitis,51 varying degrees of deafness, palatal and bifid tongue deformities,132 various gastrointestinal anomalies (associated with congenital vertebral or spinal cord defects in general),109 congenital cardiac,53 genitourinary, and mental deficiencies, as well as musculoskeletal changes including abnormal ribs, scoliosis, and Sprengel’s deformity (a failure of the scapula to descend normally)31,92,105,118,132 (Fig. 11-7). Although sensorineural hearing loss may occur, deafness is usually conductive in nature. Unilateral absence of the auditory canal and microtia have even been described.59,78 A particular neurological characteristic of