By: Edward J. Atkins, M.D.
What is Internuclear Ophthalmolplegia?
Internuclear ophthalmoplegia (INO) is the classic visual problem of the eye movement system in MS and is seen in 22 percent of individuals with the disease. INO can involve abnormal movements of one or both eyes. INO is the result of an inflammatory demyelinating lesion in the pathway joining the eye muscles that allows us to move both eyes simultaneously when looking to the side. This pathway is called the medial longitudinal fasciculus (MLF), and is located in the brainstem. With an MS lesion affecting the MLF on the right side of the brainstem, the right eye can not look towards the nose (to the left), but the left eye can move out towards the left. The right eye will, in a sense, try to ‘call the other eye back’ towards the right side if gaze to the left is maintained. This will result in involuntary, rapid, rhythmic horizontal movement of the left eyeball (nystagmus). It is this combination of failure of one eye to look towards the nose, and nystagmus of the other eye that identifies INO. INO results in double vision (diplopia), since the eyes are not able to focus on the same target. When looking to the one side, if either eye is covered, the double vision will go away. Another abnormality that may be seen is double vision on upward gaze, since the eye on the side of the affected MLF will often be able to look “higher” than the other eye. Some INOs can be quite subtle, and difficult to identify, while others are obvious. In addition to double vision, individuals can experience visual confusion, transient visual sensations of objects swaying back and forth (oscillopsia), reading fatigue, and loss of depth perception (stereopsis). Intravenous steroid therapy can be administered for acute INO.
What is Nystagmus?
Nystagmus is the most common abnormality, seen on examination in 35 percent of individuals with MS (excluding the nystagmus described above seen with INO). The most common form of this involuntary rhythmic back and forth motion of the eyeball is seen with either horizontal or vertical gaze. Nystagmus arises from imbalances in the complex networks required for holding gaze. More severe forms of nystagmus are present when the eyes are looking straight ahead (primary position), and can involve upward beating, downward beating, pendular, or see-saw oscillations. These types of nystagmus that are not gaze-evoked can be suggestive of the location in the brain of the MS lesion causing the problem. For individuals with nystagmus, the false perception of the rhythmic movement of the surrounding stationary world (oscillopsia) can be quite disabling. Several drugs including gabapentin, memantine, baclofen, and clonazepam can be helpful for visually disabling nystagmus.
What are Saccadic Intrusions?
Rapid eye movements are called “saccades.” Saccadic intrusions are small oscillating eye movements that produce impaired fixation on a target, and look like small fluttering movements of the eyeball. Due to an MS lesion, the nerve cells that control these eye movements in the brainstem or cerebellum are not functioning properly. People with saccadic intrusions may complain of blurred, jiggling, shimmering, or wavy vision. Baclofen and some anticonvulsant medications are sometimes effective treatment options.
What is the Vestibulo-Ocular System?
The Vestibulo-Ocular System coordinates eye position with head movement, allowing us to fix our vision on a target even if our head or body is moving or tilting. In MS, this system is often dysfunctional. For example, when the head or body moves, a mismatch can occur between the input from the inner ear and the cerebellum and the position of the head and eyes. If the eye position does not change in a coordinated way with head or body movement, then the ability to fixate on a target is lost. This results in blurred vision, oscillation of vision, or a sense of disorientation, particularly when in visually complex environments such as grocery store aisles, or in settings where there is a lot of head motion. Currently, there is no effective treatment.
What are Gaze Palsies?
Eye movements are controlled by specific nerves that are directly linked to brainstem centers. MS lesions that disrupt the ability to look with either eye in a given direction (i.e., gaze palsy) can affect these brainstem centers. Sometimes, a gaze palsy in one direction and an INO on attempted gaze to the other side can produce an unusual disorder called the one-and-a-half syndrome wherein the eye on the affected side remains fixed in the middle and the other eye can turn outward but not inward. While these are rare occurrences in MS, it is worth mentioning that prisms inserted into eyeglass frames can be an effective and simple treatment.
Choosing the Right Specialist
MS by definition involves multiple inflammatory demyelinating lesions that can occur at different times, and can affect different parts of the brain (dissemination in both time and space). This can lead to interruption and dysfunction of pathways involved in vision as described in this article. Because there is some healing that occurs after a demyelinating attack, the symptoms improve considerably in the majority of cases, but patients may be left with some ongoing visual disability, and relapses or progressive visual disability may occur. For help with eye movement abnormalities, speak to your neurologist. You may be referred to an ophthalmologist or optometrist for prisms to correct double vision or you may be prescribed lenses to optimize visual acuity. A neurologist who is an MS specialist, or a neurologist and ophthalmologist with additional training in neuro-opthalmology, will have the knowledge to offer you the most up-to-date recommendations on complex treatments.
Dr. Edward J. Atkins received his medical degree from McMaster University in Hamilton, Ontario, Canada, in 2003. He will begin a fellowship in Neuro-ophthalmology in 2008 with Dr. Nancy J. Newman and Dr. Valerie Biousse at the Emory Univerisity School of Medicine in Atlanta, GA.
(Last reviewed 7/2009)