The Visual System and Childhood Brain Tumor

Michael X. Repka, M.D., neuro-ophthalmologist, Johns Hopkins University School of Medicine

Michael X. Repka, M.D. Professor of Ophthalmology
Associate Professor of Pediatrics
Johns Hopkins University School of Medicine

Childhood brain tumors present with visual symptoms about 50% of the time. Additional children will develop visual symptoms and or signs during and after treatment. Such signs may become permanent or may resolve. The input of an ophthalmologist in the overall treatment plan may be important in monitoring the oncologic therapy, as well as suggesting simple interventions to prevent unnecessary loss of vision or ocular motor function.

The visual system is comprised of two different subsystems. These are the visual sensory system which is designed to focus, detect, transmit, and interpret an image and the ocular motor system which is designed to keep the two eyes aligned with each other and aimed at the target of interest.

The visual sensory system begins with the refractive or focusing elements, the cornea and lens of the eye. The role is to focus the light reflected by or transmitted from the object onto the retina. Only defects in this portion of the sensory system can in some cases be improved with glasses. Abnormalities after this step cannot be improved with regular glasses.

The retina is the innermost layer of the eyeball and is the tissue responsible for detecting the image. A layer of the retina gives rise to the optic nerve which exits the eye at the optic disc. Each optic nerve is comprised of one million optic nerve fibers. When some of these fibers are damaged and lost, the child's optic disc develops pallor, which is often described as optic nerve atrophy. Substantial numbers of these fibers can be lost with only minimal effect on visual acuity, though peripheral vision may be harmed.

The optic nerves enter the skull and fuse forming the optic chiasm. The fibers once again divide into the two optic tracts. It is at the chiasm that fibers from the nasal retinas of each eye cross to the other side. This allows the left brain to receive fibers from each eye that are concerned with the right visual field. There is a similar crossing of fibers from the left eye destined to reach the right brain. This crossing merges fibers from both eyes for the left visual field.

The optic tracts exit the chiasm and proceed a short distance to the thalamus. From there the optic radiations move around and then behind the lateral ventricles to reach the occipital lobe, which is the visual cortex. This part of the brain is responsible for the perception of vision.

The testing of vision in a child involves determination of the visual acuity in each eye independently. Reduction of acuity in one eye establishes a problem somewhere between the eye and the chiasm. Abnormalities affecting the visual sensory system from the chiasm all the way to the occipital lobe will generally preserve normal central vision, as long as there is one side functioning normally. Color vision may be tested in slightly older children. Loss of color vision is a marker for optic nerve and optic chiasm disease. Visual fields are also tested. In younger children this may include simply placing some toys in varying parts of the field of vision and watching the child turn toward the toy. With maturity, more rigorous computerized visual field tests are possible. These are used to carefully watch for changes in the function of the visual pathways, helping to monitor the tumors growth/shrinkage. Abnormalities in front of the chiasm affect only the visual field of that eye, while those behind the chiasm affect the visual field of both eyes in a similar manner.

The two tumors of childhood which most commonly affect the vision of children are the optic glioma and the craniopharyngioma. The optic glioma is a well-differentiated tumor arising in the optic nerve, optic chiasm, or optic tract that compresses and destroys the nerve from which it originated. The craniopharyngioma is a tumor that arises in the region of the chiasm. As it gets larger it may compress the chiasm, the pituitary gland, hypothalamus and the third ventricle. Each of these tumors in children are fairly silent, with the key symptoms being slow visual loss. Other symptoms of endocrine dysfunction and headache, common in adults, are uncommon in affected children, often leading to difficulty in diagnosis. Warning signs in the visual system would be unexplained visual loss, jiggling or nystagmus of the eyes, exotropia (an outward deviation of one eye), and amblyopia that does not improve with conventional therapy.

The prognosis for vision with optic pathway gliomas is variable. Some of these remain stationary both in terms of their deleterious effect on vision and of their size on MRI. Others may worsen, while a few seem to completely disappear without any treatment. For the most part therapy is used when there is evidence of an increase in size and worsening of vision. Surgery cannot cure these lesions and retain vision, because the tumor replaces the nerve. However, surgery may be performed to reduce a large mass arising from one nerve when it is compressing the fellow nerve, the chiasm, or the hypothalamus. Chemotherapy and radiation therapy are the most common treatments prescribed.

Craniopharyngiomas present similarly to optic nerve tumors, usually as unexplained visual loss. The treatment is surgical removal and on occasion adjuvant radiation therapy. The visual prognosis for these children appears to be governed by their vision at the time of surgery. Most retain that level of vision, but they do not typically recover any of the lost vision. Most have significant optic nerve atrophy at the time of tumor diagnosis, which is likely why there is no recovery.

The ocular motor system is a complex interconnection between the brainstem (pons and midbrain) which is responsible for the final control commands for eye position and nearly every other part of the brain which is responsible for producing or modulating those commands to the eye movement centers. There are three ocular motor nerves. The Oculomotor Nerve (CN III) controls 4 eye muscles, the medial rectus, the inferior rectus, the superior rectus, and the inferior oblique, as well as the ciliary muscle which is concerned with focusing of the lens to allow near vision. This nerve also controls the muscle which elevates the eyelid. The key fact about the Oculomotor nerve is that it operates 4 of the 6 muscles, so that if its injury is great, it is very difficult for the ophthalmologist to restore any reasonable cooperative movement with the other eye. The Trochlear Nerve (CN IV) controls the superior oblique. This is the finest nerve and is often injured, leading to vertical and tilted double vision. The Abducens Nerve (CN VI) controls the lateral rectus muscle and has the longest course in the intracranial space of these nerves. This nerve is frequently impaired in children with brain tumors. Impaired outward movement of theaffected eye can be improved for many patients with a series of injections of Botox and or eye muscle surgery. The most commonly performed surgery in this situation is called a transposition. In such a surgery, other functioning muscles are moved to the outside of the eye to help in creating the necessary outward pull.

Ocular motor testing includes having the patient move the eyes in all directions of gaze. Deviations may be measured by the physician or technician with prism to allow sequential monitoring. Binocular vision is measured with polaroid glasses and 3-D books. A change in alignment or in depth perception signals a similar change in the function of the ocular motor system.

Ocular motor nerve problems may occur from direct pressure by a tumor. A brainstem glioma would be a common cause. Other posterior fossa tumors, like a medulloblastoma, cerebellar astrocytoma, or ependymoma, may cause weakness through the remote effect of raising the pressure in the brain, causing the abducens nerve to work more poorly and the eyes to deviate inwards. On occasion the surgery to remove these tumors may required damage to these nerves and their consequent dysfunction. Radiation therapy to the brainstem in children does not damage these structures.

The visual system of children under the age of 8 years is still developing and abnormal visual input when the eyes are not aligned properly may lead to a permanent impairment, known as amblyopia. The young brain is able to eliminate the double image seen when the eyes are not properly aligned, by turning off the image from the deviating eye in the visual cortex. While this adaptation is good because it allows easier function, it does lead to permanent impairment of vision from the deviated eye. This could be important if the eyes are realigned or the better eye for some reason is damaged in the years that follow. The treatment is simple in concept, make the child use the less favored eye, either with a patch, blurring lens, or blurring eye drop. The actual performance of this therapy is impacted by the oncologic treatments, the child's current health, and prognosis. The parents and the physicians need to have a thorough discussion about this issue before deciding to institute therapy.

Some ophthalmological exams are performed to monitor the state of the optic nerve. Usually the referring doctors are interested in whether there is swelling of the optic disc, often called papilledema. The presence of papilledema means that the pressure within the skull is too high, or in other words, Increased Cerebrospinal Fluid Pressure. Such a sign may frequently be present at the time of the diagnosis of a brain tumor and alerts the neurosurgeon to the need to do a shunting procedure. If the swelling of the optic nerve is allowed to persist, the child may have persistent headaches and vomiting, but most importantly may suffer irreversible optic nerve atrophy and consequent loss of vision. Other eye exams are performed to monitor the quality of the optic nerve fibers, looking for the presence of atrophy or a change in the quality of the atrophy. In older children with minimal optic nerve damage, optic nerve photographs may be taken to be used to monitor the nerve. If there is substantial damage, both photos and clinical exams are much less sensitive than the measurement of acuity, color vision, and visual field.

Brain tumors may also impair sensation of the eye and blinking of the eyelids. Sensation is part of the Trigeminal Nerve (CN V) and blinking is controlled by the Facial Nerve (CN VII). Damage to these nerves can lead to poor wetting and erosion of the cornea. If this is unchecked there can be permanent scarring and opacification of the cornea which will damage vision. In some cases the only correction is a corneal transplant, which is always a difficult procedure in young patients.

This article was written for the Childhood BrainTumor Foundation, Germantown, Maryland, www.childhoodbraintumor.org.
Reviewed 2010

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