Just like strabismus (or tropia), intermittent (sometimes referred to as transient) strabismus is a condition in which the eyes are not properly aligned with each other. However, unlike constant strabismus, the condition is only evident intermittently (not all the time). Sometimes intermittent strabismus is missed in an eye exam if the patient doesn't report symptoms or the eyes are aligned during the visit and the clinician cannot induce the eye turn. This is why it is important to tell your eye care provider about all of your symptoms—they will have a better overall picture of what may be occurring with your vision and be able to refer you to a developmental optometrist.
Intermittent strabismus is often traceable to a problem with binocular vision. A patient may have a natural inward or outward eye posture (called a phoria) that a patient's binocular system is unable to compensate for. Often these patients complain of intermittent double vision (diplopia) when fatigued for when performing a task, such as reading, for an extended amount of time. Occasionally difficulties with the focusing ability of the individual eye can also lead to intermittent strabismus.
Patients with intermittent strabismus have a slight advantage over constant strabismus in that they use both eyes at least SOME of the time. Because of this, a patient's visual system has likely developed critical skills, such as stereopsis. However, a patient that suffers from intermittent strabismus does have a risk of progressing to constant strabismus and developing compensatory mechanisms, such as suppression, to alleviate double vision. Patients who have intermittent strabismus can often times tell you which eye they are using if they are able to control each eye independently. Alternatively, the patient is sometimes able to use both eyes together to achieve binocularity. With these patients, their eye or eyes are not always in the wondering position and therefore can be easier to treat with vision therapy.
Most cases of intermittent strabismus are horizontal - the eye turns out (exo) or in (eso). In many cases, the patient has a somewhat fragile binocular vision system, and additional stressors (such as extended near work, computer use, or homework) cause the eye to deviate. Some cases can also be vertical - the eye turns up (hyper) or down (hypo). These are more rare and extreme cases though, they can also be treated in a similar if not the exact same way as a patient with intermittent strabismus. Vision therapy can also reveal inadequacies of the patients ability to maintain fixation with their strabismic eye, even if it is only intermittent. Often, patients will have more than one type of intermittent turn of one or both eyes.
The eye occasionally turns inwards. Many times intermittent esotropia is associated with excessive focusing (called accommodation). These patients are more likely to have a moderate or high plus (hyperopic) glasses correction. Excessive convergence and esophoria may also cause one of the eyes to turn inwards. Excessive convergence may occur after working on a computer or other near work for many hours of the day. This type of prolonged near work is often first interpreted as eye strain or fatigue, as the patient may not know that their visual system is working overtime just to be able to hold both eyes in that position for an extended period of time. Often, patients will have more than one type of intermittent turn of one or both eyes.
The eye occasionally turns outwards. Intermittent exotropia is more common, in fact, more patients have intermittent exotropia than constant exotropia. Inability to maintain convergence and exophoria cause one of the eyes to turn outwards. Patients can find the intermittent exotropia to be the most frustrating. Often, patients will have more than one type of intermittent turn of one or both eyes. This is a relatively common feature of the disorder convergence insufficiency, where extended near work leads to similar symptoms of fatigue and eye strain, but is due to the inability of the visual system to keep the eyes aligned. Most patients have a moderate degree of exophoria (natural outward deviation of the eyes) at near, but relatively normal eye position when looking at something far away.
The eye occasionally turns upwards (hypertropia) or downwards (hypotropia). Often the deviating eye with the hyper (upwards) deviation is reported. These types of intermittent vertical tropia are less common than the horizontal intermittent tropias, but still exist within the strabismus category. Often, patients will have a moderate vertical phoria (resting eye position) that becomes more difficult to compensate for with age. Vertical intermittent strabismus often responds well to prism lenses.
Many times patients that suffer from intermittent strabismus avoid tasks like reading. Symptoms may include double vision, headache, and eyestrain. Oftentimes these symptoms are overlooked by regular physicians who won't necessarily be looking for binocular conditions such as intermittent strabismus. A developmental optometrist will be able to associate these symptoms with the correct binocular disorder and then assign a full and comprehensive vision therapy program to the patient. Some patients who have intermittent strabismus often don't seek treatment because it only happens during stress and sleeplessness. Other patients don't feel their condition is worse enough to warrant seeking treatment in the first place.
Since intermittent strabismus is associated with at least occasional binocularity, vision training is often helpful for the patient to improve their ability to keep the eye straight. The patient needs to be motivated and patient with the overall vision therapy process. The vision therapy process takes time as it is rewiring the brain to use both eyes together instead of one at a time some of the time. When the eyes are used together, visual changes start to occur that are new to the patient. These new visual changes will help the patient stay motivated throughout the vision therapy process. This vision therapy process can take anywhere from 3 months to a year or even longer. Progress will depend on the type of intermittent strabismus as well as the patient's ability to continue treatment with home therapy tools.
A baby's visual system is rapidly developing—you may notice that a newborn has very poor eye teaming/coordination. This is normal! Transient misalignment of a baby’s eyes is very common up to the age of four months. The eyes may be intermittently esodeviated or exodeviated, but by three months of age, the eyes should be straight.
If the baby's eyes persist in misalignment, it will be important to have the baby seen for an eye exam either by an optometrist or a developmental optometrist. If strabismus is caught early, the patient is able to enter vision therapy before their reading age and therefore may be more likely to have an easier time with things like comprehension and reading ability over time. Programs such as InfantSee provides free eye examinations for young children, with the goal of discovering potentially concerning visual conditions at a young age.
TedXPioneerValley - Sue Barry