Keeping Your Focus: Eye Care

Eye care in neuromuscular disorders

by Margaret Wahl on December 1, 2000 - 5:00pm

When people think about neuromuscular disorders, eye problems usually aren't the first thing that comes to mind. That makes sense, because most eye problems in neuromuscular disease are, thankfully, not too severe, treatable with therapy for the underlying disorder, or correctable with special lenses or surgery. But in some disorders, problems can persist, and they range from nuisances to major impediments to quality living.

Not too surprisingly, weakness of the muscles that control the eyeballs or those that open and close the eyes is probably the most common eye problem seen in an MDA clinic.

This weakness can stem from deteriorating muscles themselves, as in muscular dystrophies and other muscle disorders; abnormalities in parts of the brain that have to do with eyes and eye muscles, as, for example, in some mitochondrial disorders; or a problem in the communication between nerve and muscle, as is the case in various types of myasthenia. Results in all three types of weakness can be similar, but depend, of course, on the particular muscles involved.

Drooping lids: Surgery is easy 'when somebody knows how to do it'

One muscle that's often affected has a long name with a simple meaning. It's called the levator palpebrae superioris, which literally means "lifter of the upper eyelid," and that's just what it normally does.

Weak levators are common in some neuromuscular diseases, particularly myotonic muscular dystrophy (MMD), myasthenia gravis (MG), congenital myasthenic syndromes, mitochondrial disorders and, most strikingly, in oculopharyngeal muscular dystrophy. In OPMD, as its name implies, weakness of ocular muscles is a defining characteristic.

When levators are weak, explains ophthalmologist Francois Codere, a condition known as ptosis, a drooping or dropping of the eye, results.

Codere, who practices ophthalmology and eye surgery at the Royal Victoria Hospital in Montreal, specializes in surgery to correct drooping eyes. He sees the problem frequently in Montreal, Quebec, in the heart of French Canada. Quebec has the highest OPMD incidence in the world.

Better understanding of the course of OPMD thanks to genetic and other discoveries in recent years has allowed surgeons like himself to better plan the nature and timing of a patient's surgery, Codere says.

One way to "strengthen" the levator muscle is by shortening it and pulling up on it. Known as a levator resection, this procedure is often done when levators are weak in stable, nonprogressive conditions. "If the levator is strong enough, we can just shorten it as you would tighten a loose elastic, by taking out a piece of it and reattaching it to the eyelid," Codere says. (The surgery doesn't really strengthen, but rather tightens, the muscle.)

But in most people with OPMD, particularly those whose course is predicted to progress rapidly (easier to do now, he says), the results are likely to be unsatisfactory.

"Since that muscle is sick in this disorder, and it's a progressive disorder, unfortunately that operation often only gives a temporary relief of the symptoms," Codere says. "It lifts the eyelid, but there's a tendency for it to droop again over the next five to 15 years."

levator palpebae superioris
A small muscle called the levator palpebrae superioris opens the eye, while the circular orbicularis oculi closes it. When the levator is weak, the frontalis can sometimes be used to hold the eye open.

For this reason, Codere prefers a different operation, called a frontalis suspension because it makes use of the large frontalis muscle in the forehead, which maintains its strength in OPMD.

Codere says, "The strategy is to use the movement of the brow and transmit it to the lid edge. The way we do that is by passing a kind of suspenders between the brow and the eyelid, so that when the patient lifts the brow, the lid edge will lift."

MDA grantee Guy Rouleau, a neurologist and neuroscientist at Montreal General Hospital and a colleague of Codere's, says such surgery can be "very, very effective" and usually lasts for many years. "In my experience, people do absolutely fine," he says, but notes that he's in a place where OPMD is well understood and easily recognized.

Problems sometimes occur with ptosis surgery when it's attempted by a plastic surgeon without much experience in the procedure, Rouleau says. He recommends finding someone who specializes in this type of surgery, such as an oculoplastic surgeon (Codere's specialty). "Up here, we see a lot of these patients, and the eye thing is a trivial problem, easily corrected. Well, it's easy when somebody knows how to do it."

Surgery, however, isn't for everyone. Sometimes a patient's condition isn't fit for surgery, and many times the weakness in the levators is rapidly progressing or fluctuating.

In some situations, such as myasthenic disorders (congenital myasthenic syndromes, which are genetic; or autoimmune myasthenia gravis, which results from an immune system attack on muscle cells), the solution is to treat the patient with medications and hope the eye problem clears up. It usually does.

In the meantime, temporary solutions, such as "ptosis crutch" eyeglasses, can be employed. These support the eyelids for reading, driving or watching television. They shouldn't be used for too long a time at a stretch, because they can lead to drying of the eye, but they work well for some situations and some patients.

Of course, some people bypass fancy solutions.

Ptosis Crutch
Ptosis crutch eyeglasses can help compensate for weak levator muscles as long as they're not used for prolonged periods.

"I had an older myasthenic patient, a man in his 70s who ended up using Scotch tape on his lids," says neurologist David Chad, who directs the MDA Clinic at University of Massachusetts Memorial Health Care in Worcester. "That's the crutch I'm most familiar with. The only worry is that if you keep the eye open, you can have a corneal abrasion [scratch on the cornea, the covering of the eyeball]. But a lot of ophthalmologists hesitate to do surgery in this disorder because of the fluctuating course."

In children, a severely droopy eye can affect development of the still immature visual system. The brain needs normal visual input from an open eye in order to develop all the right connections between the eye and the brain (see Lazy eyes). For this reason, it's crucial to treat a droopy eye in a child if at all possible.

When eyes won't close: Prevention better than treatment for dry cornea

The muscle that closes the eye is a circular one (see illustration above) called the orbicularis oculi, Latin for "little circle around the eye."

Weakness of this muscle doesn't occur as often in neuromuscular disease as does weakness of the levator palpebrae, but it can be a problem, particularly in facioscapulohumeral muscular dystrophy. In this form of MD, which involves weakness of several facial muscles, weakness of the orbicularis oculi muscles can keep the eyes from closing completely. That can be a serious problem during sleep, because the cornea — which covers the front of the eyeball like the glass on a watch — is left partially exposed all night, allowing it to become dry or scratched.

Eye surgery to lift the upper eyelid, if done incorrectly so that the lid is pulled up too high, is another common cause of difficulty with eye closure.

Symptoms of a dry cornea are a gritty or burning sensation in the eyes, as well as red or watery eyes.

Incomplete eye closure can be managed by wearing an eye mask during sleep and lubricating the eye with drops or ointments during the day and before sleep, Codere says.

A dry cornea can become infected — a "disastrous" situation, Codere notes, one that's better avoided than treated. In extreme cases, the patient may have to wear goggles to keep moisture from evaporating around the eyes, at least temporarily. Surgery to lift the lower lid can be done, but results aren't always satisfactory.

Six extraocular muscles — four straight and two diagonal — surround each eyeball and allow it to move in all directions. Weakness in any of these can lead to an inability to move the eyes or, if the two eyes are differently affected, to strabismus.

Lazy eyes: 'Don't make two problems if you only have one

Most neuromuscular conditions affected muscles that are considered voluntary — that is, those that can be moved at will. Such muscles share a similar structure and function, and look similar under the microscope. But not all voluntary muscles are exactly the same.

The extraocular muscles, six small muscles that move the eyeball in all directions, are voluntary muscles, but they have characteristics that make them slightly different from other such muscles. That may explain why they're particularly hard hit in some neuromuscular conditions, such as the myasthenias, mitochondrial disorders, myotubular myopathy, OPMD, myotonic dystrophy and thyroid-related muscle disorders. It also may indicate why they're almost never involved in Duchenne or Becker muscular dystrophy, limb-girdle muscular dystrophies, the merosin-deficient form of congenital muscular dystrophy, or the motor neuron disorders spinal muscular atrophy and amyotrophic lateral sclerosis.

There are six extraocular (EO) muscles for each eye — four straight, or rectus, muscles; and two slanted, or oblique, ones. Normally, the EO muscles work together with each other and with the muscles of the other eye to allow the gaze to be focused in any direction and to ensure that both eyes are sending the same image to the brain.

When one or more of the EO muscles weakens, the eyeball can't be moved in the direction in which that muscle would normally move it, such as upward (superiorly), downward (inferiorly), inward toward the nose (medially), or outward toward the ear (laterally).

If EO muscles in both eyes weaken at about the same rate and in the same way, there isn't much to worry about, says Andrew Lee, a neuro-ophthalmologist — eye doctor who specializes in eye-brain interactions — at the University of Iowa Hospitals and Clinics in Iowa City.

Fortunately, says Lee, that's usually the case in neuromuscular disorders. EO muscles may weaken, but they tend to weaken equally in both eyes. Such patients don't need to have eye muscle surgery, he says, because, although the eyes are unable to move, they're equally unable to move.

People with this condition "just turn their heads to see," Lee says. "They just pick their heads up or turn the head from side to side to see in different directions. If it's been like that for a long time, they just get used to it. Many patients don't even know that their eyes don't move."

Trouble comes when EO muscles in the two eyes don't weaken the same way or at the same rate. That causes a condition known as strabismus, meaning eyes that can't focus on the same object at the same time.

People with strabismus see one image with one eye and get a slightly different view with the other eye, sending two images to the brain. The result is double vision (sometimes of the same object, sometimes of different objects, depending on the type of weakness).

After childhood, double vision is a nuisance but not an emergency, because double vision that begins in adolescence or adulthood isn't likely to damage the visual process itself. In childhood, however, the situation requires immediate attention.

A simple treatment for double vision that starts after childhood is to block the vision from one eye, which can be done by wearing a patch or, more elegantly, a lens that looks transparent from the outside but blocks vision from the inside (called a Min lens, for its inventor).

In some cases, prism lenses can help align the eyes. These can be ground into the eyeglass lenses, if the problem is stable, or applied to the glasses as a thin, sticky sheet, if the problem is thought to be temporary or fluctuating. (These last are known as Fresnel prisms, after a company that distributes them.)

A new treatment for some kinds of strabismus is botulinum toxin, a bacterial poison that weakens muscles. It can be injected into a muscle that's too strong (pulling too tightly) to temporarily weaken it and thus straighten the eyeball. It's not a permanent solution, but it's useful in certain circumstances, particularly if surgery can't be done.

As in ptosis treatment, EO muscle weakness in myasthenias and thyroid-related myopathies can often be corrected by treating the underlying disease with medication.

If medications aren't effective and the eye muscles aren't expected to change much over the years, surgery to correct strabismus can be undertaken.

In children, strabismus is more serious, requiring immediate action. The reason for the urgency is that the child's brain is still developing, with connections between the eyes and the brain still forming up to about age 8 to 10.

When the brain receives two separate images, an uncomfortable situation for it, it simply shuts off the image from one eye. If that condition persists for a while, sometimes just a few weeks in young children, vision in the "rejected" eye is likely to be damaged. The resulting condition is known as amblyopia which literally means "dull eye" but is more often called "lazy eye."

Lazy eye isn't really eye damage, but a form of brain damage, in that the developing brain stops processing images from the eye it's decided to ignore and ultimately loses the ability to do so. Eye-brain connections in adolescents or adults are stable enough so that strabismus doesn't cause amblyopia. By the same token, however, amblyopia that starts in childhood isn't likely to be correctable with interventions after childhood.

Lazy eye is probably most often associated with strabismus, but it can also result from ptosis or from anything that interferes with the visual signals the brain normally receives from each eye during childhood. Keeping an eye covered can also cause amblyopia, which is why care has to be taken with therapies that involve patching of children's eyes.

"With children," cautions Lee, "it's urgent to prevent lazy eye if they have a droopy eyelid or a crossed eye. Don't make two problems if you only have one. If you wait until the child is older to correct the problem, you might have a lazy eye on top of, for example, congenital myasthenia. All these children should be examined and treated when they're young."

The treatment may ultimately be surgical, if the strabismus or ptosis is considered stable, but other therapies can also be used, in addition to or instead of surgery.

Most children with strabismus or ptosis in one eye will undergo treatment to patch or in some other way block the vision in one eye (for example, with Min lenses), forcing the brain to process signals from the eye it's been ignoring. To avoid amblyopia in the blocked eye, a careful schedule for the patching or blocking has to be followed.

Other vision problems: Not common, sometimes treatable

Neuromuscular disorders are more likely to affect the muscles that move the eye than they are the structures of the eye itself, but there are exceptions.

In myotonic dystrophy, a disorder that affects many organs, cataracts are found in nearly everyone. These are cloudy or scratchy spots on the eye's lens that can, as they increase in severity, interfere with vision. Fortunately, they can almost always be safely removed or dissolved and an artificial lens inserted.

The eye receives light through the pupil, a hole in the colored part of the eye, the iris. The cornea, which covers the pupil like the glass over a watch face, helps focus the light, as does the lens, just in back of the pupil. Drying and damage to the cornea can result when the eye doesn't close properly. A cataract can occur in myotonic dystrophy or with prolonged use of corticosteroids. Abnormalities in the retina, the "screen" onto which images are projected, occur in some mitochondrial disorders.

Precautions with respect to anesthesia are important when anyone with a neuromuscular disease, particularly myotonic dystrophy, has surgery. The neurologist and the surgical team must consult before any procedure is undertaken.

Cataracts can also occur as a side effect of prolonged treatment with corticosteroids, the prednisone family of drugs. These are often used in inflammatory myopathies, autoimmune MG and Duchenne MD.

The eye's retina isn't often a target of neuromuscular disease, but it can be in mitochondrial disorders. Websites and print materials unfortunately sometimes describe the problem as retinitis pigmentosa, which, if the reader looks it up, describes a group of genetic eye diseases, several of which can lead to blindness with time.

A better term is pigmentary degeneration of the retina, says Robert Daroff, a neuro-ophthalmologist at University Hospitals of Cleveland, and the problem usually doesn't lead to blindness, although it can lead to poor vision.

Some mitochondrial disorders, such as Leigh's syndrome, can affect the optic nerves, "cables" that carry information from the eyes to the brain. This, too, can interfere with vision.

Neither Daroff nor David Chad has seen many neuromuscular patients with vision-threatening problems in their many years of practice. Daroff notes that vision problems in neuromuscular disease in general tend to be minor and pose less of a threat to one's quality of life than do other issues in these disorders. "I don't think patients should be overly sensitized about this," he says. "But if they have trouble seeing, they should see an eye doctor."

Parents, he says, should be on the lookout for crossed eyes or droopy lids in their children, and for the child who continually bumps into things or holds his head in an odd position and resists having it straightened. "Amblyopia," he says, "is absolutely avoidable."

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