Double vision can be an uneasy complaint. When a patient walks in with one eye out the door, it can stop us in our tracks. When was the last time I did a cover test, you might think? A drop of sweat begins to trickle down your forehead as you attempt to remember what you learned in third-year boards prep. Understanding how to perform a problem-focused exam can turn a diplopia complaint from chaotic to manageable. 

While there are many reasons that a patient can have double vision, there are a few that present with high frequency. Navigating these diagnoses requires knowledge of two major things – physiology and prisms. Today, your friendly, neighborhood, BV/VT doc is here to fill you in on some tips about diplopia management. In the next series of articles, we will address some common culprits of double vision and how to treat them. Let’s begin with a diagnosis that has lurked around my practice on many occasions – Congenital Fourth Nerve Palsy.

Pathophysiology

Cranial nerve anatomy is not just a forgotten course from optometry school. In fact, understanding the course of the nerves that connect to the eye muscles can make a huge impact in revealing the root of the problem. For example, when multiple muscles are impacted you may predict that the problem could be in an area of the brain where several nerves meet for lunch, such as the cavernous sinus. 

Fun fact: the trochlear nerve is the only cranial nerve to exit from the posterior midbrain. It does criss-cross applesauce as it moves anteriorly such that the right nucleus innervates the left superior oblique muscle and the left nucleus innervates the right superior oblique muscle (Figure 1). As you may recall, the primary function of the superior oblique muscle is intorsion, the secondary is depression, and the tertiary is abduction. Therefore, one might predict that an innervation abnormality may result in an eye which demonstrates excyclotorsion, elevation, and reduced abduction.

Statistics

Statistics give your patients a piece of mind. How abnormal am I, they wonder? Well, that’s a whole different story, but the incidence of 4th nerve palsies is 5.73 per 100,000. The incidence is higher in males than females. It is presumed that this may be because head trauma occurs more frequently in males.

The most common etiology of an isolated 4th nerve palsy is congenital, followed by trauma and vascular. Approximately 75% of all trochlear palsies are congenital. Although the mechanism is not exactly known, it is postulated that it may occur due to hypoplasia of the 4th nerve nucleus, muscle fibrosis or abnormal insertion of the superior oblique muscle. 

Clinical Presentation

Patients with congenital 4th nerve palsy typically present to your office later on in life due to decompensation. Oftentimes, they report diplopia with particular activities where the primary action of the superior oblique is maximized. For example, in my clinic, multiple patients have reported seeing double when they are driving and looking over their shoulder to change lanes (Figure 2). Patients with congenital palsies will rarely report a torsional component to their diplopia, unlike those who have an acute onset.

In your clinical exam, case history is key to reveal onset, frequency, and longevity of the condition. A patient with a congenital 4th nerve palsy may present with a habitual head tilt on the contralateral side (Figure 3A). This is because head tilt stimulates the vestibular-ocular-reflex(VOR) driven ocular counter-roll which activates the ipsilateral intorters and contralateral extorters(Figure 3B). Without the superior oblique functioning appropriately in its primary (intorsion) and secondary(depression) actions, the superior rectus over-acts to intort and elevate the eye, thereby increasing the deviation on ipsilateral head tilt. 

To counteract an increase in deviation, patients will adopt a head tilt to the contralateral side of the palsy. These tilts can be evident in childhood photographs, old driver’s licenses, myspace profile pictures etc. The longevity of the head tilt may also lead to facial asymmetry and hypoplasia on the unaffected side.

Relevant testing

The following is a list of some heavy hitter tests for a suspected congenital 4th nerve palsy. Although fundus photography can sometimes show excyclotorsion of the paretic eye, there have been some reports of a paradoxical excyclotorsion in the non-paretic eye. So, for our discussion let’s focus on the definite. 

1. Visual acuities: Congenital 4th nerve palsies may present with or without amblyopia in the affected eye. However, amblyopia is only a viable diagnosis after longevity of the condition has been confirmed.
2. Pupils: Pupils should be normal in congenital 4th nerve palsies. If abnormalities are present, such as contralateral horner’s syndrome, dilated pupil or an APD a visual field and MRI must be performed to rule out brain lesions or bleeds.
3. Extraocular Motilities/Versions: Motility can help us decipher the latency of the deviation. In a congenital 4th nerve palsy there is typically an over-action of the inferior oblique due to reduced inhibition from the superior oblique over a long period of time (Figure 4). In an acute 4th nerve palsy you will see a significant underaction of the superior oblique (more on why later).

   

4.  Cover test: Cover test in 9 fields of gaze helps identify comitance. In a longstanding deviation, a phenomenon called spread of comitance occurs (will explain later) which causes the deviation to look similar in many gazes. In an acquired palsy the deviation will be worse in the gaze which involves the primary action of the paretic muscle. In the case of a superior oblique palsy, this will be evident with head tilt on the ipsilateral side.
5. Fusional ranges: The literature reports that patients with congenital 4th nerve palsies have expanded fusional ranges so we may think large supra AND infra ranges during testing, right? Not exactly. True, patients with congenital 4th nerve palsies have expanded fusional ranges. However, any small amount of prism which exacerbates the compensating range can induce diplopia. What does this mean? For example, a person with 10 diopter right hyperphoria needs to compensate for 10 diopters to be aligned. If this patient is aligned in your chair intermittently, you already know they have expanded fusional ranges without any actual testing; 10 diopters is above and beyond the normal compensating range. So if they have been compensating for this deviation long term, and are now decompensating they are letting you know that they are maxed out on how much they can handle. Therefore, a base up prism, even 1 or 2 prism diopters, which further exacerbates hyperphoria will break down the patient because they cannot tolerate any more stress to their system. 
6. Parks-Bielschowsky 3-step test: This test typically works best in an acquired hypertropia since the spread of comitance has not occurred yet. 

Step 1 will begin by isolating the hyper eye in primary gaze. On covertest this eye will move down to regain fixation. 

Step 2 will identify which gaze increases the hyper deviation. If the superior oblique is involved the contralateral gaze will worsen the deviation due to the overaction of the inferior oblique.

Step 3 will determine which head tilt worsens the deviation. In a 4th nerve palsy the ipsilateral head tilt increases the deviation.

In longstanding, congenital, deviations you may not see this pattern as explicitly as you do in acquired deviations. This is due to the spread of comitance phenomenon.

Table 1

Spread of Comitance

Remember Hering’s law of equal innervation and Sherrington’s law of reciprocal inhibition? Let’s recap. These laws illustrate eye muscle movements based on yoked and reciprocal innervation. Hering’s law dictates that yoked muscles receive equal innervation to move the eyes into a desired gaze. For example, the right superior rectus and the left inferior oblique receive equal innervation to move the eyes up and to the right (Figure 5). Sherrington’s law states that each antagonist muscle receives reduced innervation to allow the eyes to carry out a desired action. In the same scenario, the left superior oblique, which is the antagonist of the left inferior oblique, would receive reduced innervation such that the eye could move up and to the right. 

In a superior oblique palsy, the inferior oblique antagonist will have overaction since the superior oblique is not functioning to keep it in check. The inferior oblique then requires less innervation to move the eye up and in because it has an extra kick from the poor acting superior oblique. If less innervation is received by the inferior oblique, consequentially less innervation will be provided to its yoked agonist, the superior rectus of the non-paralytic eye. And thus the cycle of spread of comitance begins. Each muscle will adjust its action and innervation based on the consequence of the initial paretic superior oblique muscle to equalize the deviation in all gazes. 

 In a longstanding, congenital, deviation you will still see an overaction of the inferior oblique. However, the underaction of the superior oblique muscle may appear minimal due to the spread of comitance. In an acute deviation, you would see a significant underaction of the superior oblique because spread of comitance has not occurred.

Treatment

Oftentimes a patient with a congenital fourth nerve palsy becomes symptomatic due to an overall vergence imbalance. This means that a patient with a decompensated vertical phoria may also have a poor ability to converge. As you can imagine it is a lot harder to converge when there is a horizontal and vertical misalignment vs. a horizontal misalignment alone. Therefore small to moderate amounts of vertical prism which align the eyes can aid with overall function. Our goal is to use the least amount of prism which allows for fusion, this is to improve compliance and minimize distortion. So, where to begin?

The Worth 4 dot or Maddox rod are excellent tools because they can help us understand sensory and motor fusion in free space. Our goal is to find the least amount of prism (typically ranging from 3 to 6 prism diopters) which allows for enhanced sensory fusion. Trial the prism and assess motor range. Compensating range (tolerance of base up prism) should now be expanded because the patient will have some help from the neutralizing prism to carry the load of their large heterophoria. 

The next step is to evaluate stereopsis such as wirt circles. When two eyes point at the same object in space, we know where that object is located due to depth perception. Therefore, if a suitable amount of prism is used which maximizes depth perception then that prism is likely to be successful in treatment. 

Finally, assess horizontal alignment and near point of convergence through the desired prism and note any additional improvement. Trial the prism in a trial frame and have the patient assess comfort at all distances. Prism should be split equally between the two eyes when more than 2 prism diopters are prescribed to avoid cosmetic rejection. The patient should be evaluated in 8 weeks after full-time wear of the prism glasses to reassess symptoms and progress. 

That’s all for now folks! I hope this recap will help you navigate your exam and management for 4th nerve palsies with a little ease. Until next time, when a different culprit of diplopia comes forward.