Author: Paul Chamberlain, 4th Year Medical Student, Baylor University
Strabismus is a misalignment of the two eyes. It has a broad differential that significant varies by age and varies in urgency. First it is important to understand how to correctly describe strabismus and its nomenclature. The deviated eye (in relationship to the other eye) can be elevated (hyper-), depressed (hypo-), pointed nasally (eso-) or pointed temporally (exo-). Next, the deviation may be occurring spontaneously (-tropia, called a “manifest” deviation) or only when fusion of the two eyes is broken during cover testing (-phoria). Putting these first two elements together you can properly describe most strabismus—for example, a left hypertropia (the left eye is deviated upwards relative to the right eye). In general, phorias are not problematic and do not require treatment but can decompensate overtime into tropias. In further describing strabismus, it is important to delineate if the misalignment is constant or intermittent as well as comitant or incomitant. An incomitant deviation is one that varies or changes based on the direction of gaze. For instance, a left 6th nerve palsy is incomitant because it is worse in left gaze but better in right gaze.
As in many medical diagnoses, a careful history is paramount in narrowing a differential and understanding the urgency and nature of the strabismus. Some critical questions include: When did you first notice this? Has the amount of deviation worsened, improved, or stayed the same during that time? Is it constant or intermittent? Are you seeing double vision? Are the images splayed horizontally, vertically, or diagonally? A medical history also provides numerous clues to the etiology, such as assessing vascular risk factors, birth and pregnancy history, thyroid disease, prior eye disease or surgery, etc.
In eliciting and quantifying the deviation, it is important to perform a complete ophthalmic exam where possible: specifically visual acuity, motility, cycloplegic refraction (checking the patient’s prescription after dilating their eyes) and dilated retinal exam. There are a few specific tests that help the examiner assess the eye alignment, such as the cover-uncover test. The examiner instructs the patient to look at a target, covers one eye for several seconds and then uncovers that eye, repeating this in the fellow eye. In a patient with a tropia, the misaligned eye will move to fixate on the target when the fixing eye is occluded. For instance, if a patient has a left esotropia (i.e. the left eye is deviated towards the nose) the examiner should see the left eye move laterally when the right eye is occluded. The second test is the alternate cover test which involves moving an occluder back and forth from eye to eye watching for eye movements in the uncovered eye. Prisms can be used to quantify the amount of deviation, which are mainly used for surgical decision making.
There are several types of strabismus of which all physicians should have a fundamental knowledge. When the strabismus presents in the first year of life, specifically by 6 months of age, it is deemed congenital and warrants prompt referral to a pediatric ophthalmologist as failure to correct misalignment in a timely fashion may result in a permanent loss of binocular vision such as depth perception (stereopsis). The optimal time to correct congenital misalignments is not universally agreed upon, but ranges from 6 months to 24 months.
Strabismus that presents in the toddler or young child is uniquely different that adult-onset strabismus in that children are still developing vision and have the potential to lose vision in a deviated eye due to amblyopia, or disuse of a deviated eye to avoid double vision (diplopia). This is why children tend not to complain of diplopia with strabismus, they suppress the misaligned eye as an adaptive mechanism which is clever in the short term but can cause long term damage to visual development.
Sudden onset strabismus in a middle-aged individual has a broad differential, including a cranial nerve palsy (III, IV or VI) from a compressive lesion such as a mass or vascular anomaly. Neuro-imaging of the brain and orbits is often warranted, with and without contrast. The key to identifying cranial nerve palsies that affect eye alignment is looking for incomitance, or a deviation that varies depending on direction of gaze. Patients who may not require neuro-imaging are those who are older than 50 with multiple vascular risk factors and have an isolated cranial nerve palsy. In these cases the etiology is more likely ischemic and they tend to recover in variable degrees. All of these cases warrant a consult or referral to ophthalmology, more specifically neuro-ophthalmology or pediatric ophthalmology.
If a patient or parent simply says they have a “lazy eye,” the interviewer should ask for an explanation of what this means to the patient as they may be referring to a number of things, most commonly ptosis, amblyopia or strabismus.
Treatment for strabismus includes watchful waiting, glasses with prism, eye patching or surgery. If there is an underlying medical problem causing the strabismus than this is typically addressed first. If left untreated in young children, strabismus may lead to amblyopia, loss of stereopsis, and negative psychosocial effects. Much of this is correctable if addressed promptly, but becomes irreversible within a matter of months to years, especially if not addressed by around age eight.