Physician Assistant Exam: Eye Movements and Muscle Conditions
Various conditions affect the ability of the eye to move as it should. For the Physician Assistant Exam, you will need to know about conditions such as strabismus and nystagmus.
Point the eye in the right direction
Strabismus is a disorder of the muscles of the eyes, preventing both eyes from focusing on the same object at the same time. The extraocular muscles don’t work in synchrony, so each eye essentially does its own thing. Here are two types of strabismus, based on the direction of deviation:
Esotropia: Esotropia (deviation of the eye toward the nose) is a form of strabismus in which the person is cross-eyed.
Exotropia: Exotropia is the opposite of esotropia. The eyes deviate outward, so the person is walleyed.
If strabismus is left untreated, the muscles of the weaker eye can become even weaker, leading to strabismic amblyopia (“lazy eye”). Amblyopia leads to vision loss in the affected eye because the two eyes aren’t working in synchrony. The child with amblyopia needs a full ophthalmic examination and full medical evaluation.
Studies show that approximately 3 percent of children suffer from amblyopia, and it needs to be detected by age 4. Although a vision screen for young children can be difficult, the Blackbird Vision Screening system is very effective, mainly because it’s a fun test and kids like to take it.
Strabismus has many causes, from cranial nerve lesions to medical conditions such as hyperthyroidism and myasthenia gravis. Treatment of strabismus involves treating the underlying cause. Sometimes interventions are done to strengthen the weaker eye. With amblyopia, this typically involves putting an eye patch over the stronger eye to force the muscles of the weaker eye to work harder, making them stronger.
Ectropion refers to an eyelid that’s turned outwards. You can see it in certain genetic disorders because it’s a weakness in collagen framework. You can also see it with Bell’s palsy. With ectropion, the affected person can have really dry eyes that can be painful and irritated. Conjunctival irritation is common, and treatment is directed at giving artificial tears to those with keratoconjunctivitis sicca as well as surgery if necessary.
Entropion is an eyelid that’s turned inward. When this happens, the eyelashes turn in as well and can rub against the eye. As with an ectropion, the eye can become infected and irritated over time. Treatment again involves the use of artificial tears to help keep the eye lubricated as much as possible. If the eyelashes are rubbing against the eye, surgery is usually indicated.
Nystagmus is involuntary eye movement. It may be physiologic (normal) or pathological (bad), with variations within each type. The key is the way the eyes normally move. The eyes can move horizontally (side to side), they can move vertically (up and down), and just to keep things interesting, they can also rotate. Pathological nystagmus involves three basic characteristics of abnormal eye movement: too slow, too fast, and too jerky.
Causes of nystagmus include congenital disorders, acquired or central nervous system disorders, toxicity, pharmaceutical drugs, and alcohol. Actually, there are 53 possible causes of nystagmus.
Although nystagmus was previously considered untreatable, several drugs have been identified for treatment in recent years. Think of memantine (typically for treating Alzheimer’s), levetiracetam (an anticonvulsant), amifampridine (for rare muscle diseases), dalfampridine (for multiple sclerosis), and acetazolamide (for epileptic seizures).