The optic nerve is the structure at the back of the eye that carries visual information from the eye to the central nervous system (CNS). Approximately 120 million rods and cones sense light in the outer retina. This information is transmitted to approximately 1 million ganglion cells in the inner retina. These ganglion cells converge to form the optic nerve. The electrical impulses generated in the optic nerve are transmitted to the occipital lobe via the lateral geniculate nucleus. Posterior to the eye, the optic nerve is covered by a dural sheath and arachnoid membrane. This structure surrounds the nerve until it enters the brain, where it is contiguous with the subdural space (Figure 33–1).
FIGURE 33–1
Normal microscopic appearance of optic nerve as it exits the eyeball. The neurons of the inner ganglion cell layer of the retina combine to form the nerve. The sclera surrounds the nerve. A portion of the central retinal artery is visible in cross-section. The retina is artificially detached due to histological processing. (Photograph contributed by Morton Smith, MD.)
The only portion of the optic nerve that is visible on examination is the site where the nerve attaches to the eye at the posterior retina (Figure 33–2). The normal nerve has a central area through which the central retinal artery travels, branching into vessels lining the inner layer of the retina. The central portion is called the cup, and the entire area is called the disc. The cup:disc ratio describes the relationship between these 2 structures. The normal ratio ranges from 0.1 to 0.4, although larger cups may be normal in certain ethnic populations, particularly African Americans. Increased intraocular pressure may cause enlargement of the cup, and increased intracranial pressure (ICP) may cause edema and obscuration of the cup.
Optic nerve problems in children occur infrequently. However, bilateral optic nerve hypoplasia (ONH) is one of the common causes of profound visual loss in infancy.
The optic nerves form from the optic stalk, the structure that connects the embryonic forebrain and optic vesicle. It contains an outer layer of cells from the neural crest, which differentiates into the structures of the sheath that surround the optic nerve (pia, arachnoid, and dura). An inner neuroectodermal layer regresses and is replaced by ganglion cells that originate in the inner layer of the retina and travel through the optic nerve to the lateral geniculate body. At 4 months of gestation, almost 4 million ganglion cells are present within the nerve. This number decreases to 1 million by the time of birth. Myelinization of the optic nerve occurs late in gestation. It begins at the optic chiasm and travels forward, stopping at the juncture between the nerve and the eyeball.
Optic nerve problems may be caused by intrinsic abnormalities within the nerve itself, or may be secondary to problems within the eye or brain. Increased pressure within the eye (e.g., glaucoma) or inflammation (e.g., autoimmune, infectious) may damage the nerve. Increased ICP can be transmitted through the sheath to the optic nerve head, creating papilledema. Primary congenital malformations and inherited optic nerve disorders also occur.
Children with bilateral visually significant optic nerve abnormalities, such as ONH, present within the first few months of life with poor visual fixation, nystagmus, and decreased or absent pupillary responses.
Patients with increased ICP may experience systemic symptoms that include headache and vomiting. Papilledema itself initially causes minimal visual acuity changes. Formal visual field testing may show enlargement of the blind spot, but this is usually asymptomatic. Chronic increased pressure can lead to loss of vision. Symptoms of diplopia (due to sixth cranial nerve palsy) and transient visual obscurations (episodes of decreased vision lasting a few seconds) are common ocular complaints in patients with increased ICP.
Compression of the nerve may lead to decreased visual acuity. In children, however, if the compression causes unilateral visual loss and the vision is normal in the opposite eye, the decreased vision may go undetected for prolonged periods, because the children function well using only the vision in the normal eye. Sensory strabismus may develop if the decreased vision persists.
A very sensitive sign of unilateral optic nerve dysfunction is a relative afferent pupillary defect (RAPD). This can be detected by the swinging flashlight test. The pupillary reactions are connected in the brain, so that a light that is shined into one eye normally causes equal constriction of both pupils. When the light is removed, both pupils dilate. If a light is swung back and forth between the 2 eyes, there should be an initial pupil constriction each time the light is shined into an eye. If one optic nerve is damaged, the abnormal pupil will still constrict when a light is shined into the normal pupil, because the neural pathway for this reaction does not travel through the damaged optic nerve. When the light is moved to the abnormal eye, the signal for constriction will be diminished because of the optic nerve problem. At the same time, a dilating signal is being sent from the normal eye, because the light has been removed from that eye. In the presence of optic nerve damage this dilating signal will be stronger than the weak constricting signal from the light in the abnormal eye. The pupil in the abnormal eye will therefore initially dilate, rather than constrict, when the light is shined into it (see also, Chapter 1, Figure 1–26).
Diseases that compromise the function of the optic nerve may interfere with color vision discrimination. Various methods of testing color vision are available, ranging from color test plates that can be used easily in the office, to sophisticated tests that require computer analysis. The ability of children to perform these tests varies with age and the degree of visual impairment. The color vision deficits in acquired diseases of the optic nerve and retina tend to be primarily in the blue-yellow spectrum, whereas inherited color vision defects, which affect approximately 7% of males, cause red-green discrimination difficulties.
Bergmeister’s papillae are benign remnants of the hyaloid artery. This vessel extends from the optic nerve to the posterior lens during embryological development of the eye. It usually regresses completely by birth. Small remnants of the hyaloid artery may persist overlying the optic nerve, producing a fibrous, elevated structure (Figure 33–3). These do not affect vision.
True aplasia (absence) of the optic nerve is extremely rare. It usually occurs unilaterally in otherwise healthy children. It is associated with other malformations of the eye, including abnormal development of the retina. Examination reveals no visible optic nerve structures in the area where these normally should be (Figure 33–4).
Ocular colobomas occur due to incomplete folding of the embryonic fissure during ocular formation. They may include the iris, lens, retina, and optic nerve. The degree of visual impairment is primarily related to retinal involvement. Lesions that include the fovea generally cause significant loss. Surprisingly, even large optic nerve colobomas are often compatible with near normal vision if the fovea is unaffected (Figure 33–5).
This malformation is characterized by an increased number of retinal blood vessels crossing the disc in a radial pattern, with a central area of glial tissue at the site of the normal optic cup (Figure 33–6). The effects on vision range from minimal to marked. Systemic associations include basal encephalocele and Moyamoya disease.1
ONH results from underdevelopment of the nerve during embryogenesis. The number of axons in the hypoplastic nerve is small. Therefore, the optic disc is smaller than normal. It is often surrounded by a region of decreased pigment, creating the double-ring sign. This ring can be mistaken for the optic disc, and the hypoplastic nerve for the optic cup (Figure 33–7A and B). Visual acuity in ONH is variably affected, but often there is significant loss.
If ONH is bilateral, infants usually present within the first few months of life with decreased visual responsiveness and nystagmus. If it is unilateral, children frequently develop strabismus secondary to the vision loss. On examination, decreased vision and a RAPD are present.
The etiology of ONH cannot be identified in most patients, but in some cases it is associated with other abnormalities (Table 33–1). Infants whose mothers have type 1 diabetes mellitus may have segmental ONH. In segmental ONH the visual acuity is usually normal, but patients have visual field defects that correspond to localized areas of disc hypoplasia. Children with aniridia often have ONH as one of the associated ocular abnormalities. Children who are born prematurely and have periventricular leukomalacia may have a form of ONH characterized by a large optic cup, rather than a small disc. This results from transynaptic degeneration of axons caused by damage to the optic nerve radiations.
The most common abnormality associated with ONH is septo-optic dysplasia (DeMorsier syndrome). In this disorder, bilateral ONH is present, in addition to absence of the septum pellucidum and agenesis of the corpus callosum (Figure 33–8). Variable associations include ectopic pituitary glands (Figure 33–9A and B), schizencephaly, and cortical heterotopia (Figure 33–10). The visual prognosis in these patients is dependent on the degree of ONH. Developmental delay and seizures may occur due to the associated CNS defects.2 Patients with ectopic pituitary glands have endocrinological disorders, including growth hormone and adrenocorticotropic hormone deficiency. These disorders require treatment due to the risk of developmental delay and potentially serious medical problems related to the inability to mount a normal stress response.
