Brain tumors can affect the optic nerve itself and the cranial nerves that control eye movement. Tumors in different parts of the brain affect different nerves and often affect several nerves at once. According to "Adams and Victor's Principles of Neurology," common causes for disordered eye movements include tumors at the base of the brain in adults and brainstem tumors in children.
Tumors Affecting the Optic Nerve
The pituitary gland sits just beneath the optic chiasm, where the two optic nerves from either side of the brain meet and exchange fibers. A pituitary tumor typically causes bi-temporal hemianopsia, which means loss of lateral vision in both eyes. If the tumor isn't treated, it can lead to blindness. A lesion in the temporal lobe can cause loss of vision in one quarter of the visual field. Tumors in the occipital lobe can cause loss of vision on one side of the visual field in both eyes. If the left occipital lobe is affected, the patient will not be able to see the right side of the visual field.
Eye Movements
Eye movements are controlled by three different cranial nerves. They originate from the brain stem and travel a long way to the eye muscles. Tumors located at their source or in their pathway can affect their function. Symptoms include inability to move the eyes side to side and inability to converge the eyes downward, as needed when going downstairs. Eyes may move in a jerky way, which is known as nystagmus. Certain lesions block the ability to look upward; others impair the ability to look toward the midline. Depending on the impairment, the examining physician can determine where the tumor is located. Disruption of eye movements can cause double vision, nausea and headaches, symptoms that often lead the patient to seek medical attention.
Cortical Blindness
Occasionally the patient can't see, despite intact optic pathways. Tumors of both occipital lobes—for example bilateral gliomas—can cause dysfunction of the areas responsible for interpretation of visual signals, resulting in blindness. In Anton's syndrome, a blind patient denies that he can't see. He'll attempt to function as though he can, will walk into obstacles, and stumble. The cause is a lesion affecting higher association areas, leading to a lack of awareness of the neurological deficit.
References
- "Adams and Victor's Principles of Neurology"; Maurice Victor and Allan Ropper; 2001
- "Harrison's Principles of Internal Medicine"; Anthony Fauci et al.; 2008
