Case Conference: Shaky Vision & Tired Left Eye

Arriving at a correct diagnosis for a complex case earns a clinician their patient’s appreciation and their colleagues’ praise. Avoiding diagnostic error, however, garners few—if any—accolades, although it may be no less challenging and important than establishing the correct diagnosis. In the US, millions of people come to serious harm, and an estimated $100 billion is wasted annually because of diagnostic errors.¹ Surely, reducing the rate of diagnostic error is a worthy, if underappreciated, goal.

Excessive Coherence & Diagnostic Errors

Causes of diagnostic errors are usually divided into organizational or system-based failures and cognitive errors. Our focus here is on the cognitive errors that contribute to the wrong diagnoses in over 90% cases.² A common cognitive error is to “shoehorn” the particulars of a case into a familiar diagnostic category by overemphasizing the parts that seem to fit and disregarding the parts that do not. Such wedging of particulars into categories stems from a psychologic phenomenon known as excessive coherence. The temptation of excessive coherence is especially strong when encountering an uncommon clinical scenario and can be avoided by using C. Miller Fisher’s 3-for-5 heuristic, which we described in our previous column.³ The 3-for-5 heuristic is, in essence, a quick screening tool that may be especially helpful for unfamiliar situations. With this heuristic, a clinician cross-checks features of their case against 5 of the most common clinical, laboratory, and imaging features of the proposed diagnosis and ascertains whether at least 3 of the 5 core features are present. We illustrate the use of the heuristic with an interesting example from our practice.

Differential Diagnosis

When this case was shared with our neurology residents, the 2 most popular diagnostic possibilities put forth were myasthenia gravis (MG) and functional neurologic disorder (FND), which are not unreasonable starting points of a diagnostic process. MG is a well-known cause of intermittent diplopia, and FND can cause bizarre, seemingly inexplicable neurologic symptoms. To see how well these possible diagnoses fit Ms V, we review the 5 core features of MG and FND compared with her evaluation (Table 1 and 2).⁴ A quick review of the right column leads us to conclude that the diagnosis of MG should not be made based on the information provided.

FND is sometimes regarded as a “waste-basket” diagnosis for anyone whose symptoms are considered too bizarre and unfamiliar.⁵ This is a mistake. Functional disorders, as much as organic ones, have their characteristic features, and the diagnosis of FND should not be made unless those characteristic features are present, which is not the case for Ms V.

In the absence of a better explanation, it is preferable to assign a descriptive diagnosis (eg, intermittent vertical diplopia). Descriptive labels have several advantages over anatomic or etiologic descriptions at this point. Firstly, a descriptive diagnosis helps to prevent premature closure—acceptance of a diagnosis before credible alternatives have been ruled out. Secondly, a descriptive diagnosis avoids the erroneous impression that we understand more about the etiology and localization of the problem than we do in reality. Thirdly, it is less tempting to try out unwarranted therapies for a descriptive diagnosis. For example, if it were suggested that Ms V had possible MG, this could lead to a trial of prednisone or immunosuppression with considerable risk of complications and little hope for success. Finally, a well-chosen descriptive diagnosis can double as an effective search term in our quest for etiology.

A search for causes of intermittent diplopia leads to a relatively short list of potential diagnoses in addition to MG. Among the contenders are demyelinating diseases (eg, Uhthoff phenomenon), restrictive ophthalmopathies (eg, Graves disease), decompensating strabismus, drug effect (eg, antiseizure medications), convergence spasm, convergence-retraction nystagmus, ocular neuromyotonia, Brown syndrome (ie, restriction of the superior oblique trochlea-tendon complex), eyelid myokymia, and superior oblique myokymia (SOM).⁶

Discussion

SOM fits well in the case of Ms V and is characterized by episodes of sudden-onset, rhythmic contractions of the superior oblique muscle of 1 eye leading to torsional eye movement, monocular oscillopsia, and vertical diplopia, especially on downward movement.⁷ Visual loss, dyschromatopsia, headaches, photophobia, periorbital swelling, and redness of the ocular surface do not occur. SOM is typically idiopathic, presumed to result from neurogenic hyperexcitability of the superior oblique muscle. In rare cases, SOM may be the result of trochlear nerve compromise at the nerve root exit zone by a superior cerebellar artery, cyst, brain tumor, or dural arteriovenous fistula (AVF).

First, Do No Harm

Even when we are not always able to arrive at an accurate etiologic or anatomic diagnosis, we can always strive to uphold the time-tested principle primum non nocere. This applies to diagnostics as much as to therapeutics.