Michael is a reserved six year old child whose family optometrist referred him for a second opinion. A review of prior records indicated that his pediatric ophthalmologist had previously diagnosed esotropia and referred him for an MRI. Here is Michael’s habitual head position when looking straight ahead.
As you can see he rotates his head to the left about 30 degrees, favoring gaze right. The most common reason for doing this is an abduction deficit, in this case a very limited ability to abduct the left eye. As you can see, when we move a target from midline toward his left field, Michael cross-fixtes with the right eye to follow the target while the left eye remains locked up on the midline. Interestingly he does not report diplopia.
This shows that even at age 6, relatively deep suppression can already set in to override conscious awareness of a double image. This is consistent with the fact that Michael experiences a level of strabismic amblyopia, with best visual acuity reduced to a variable 20/40 through the left eye with effort, yet remaining close to 20/20 with the right eye. There is no refractive component to the amblyopia, as his manifest refraction is isometropic at +0.75 -0.50 cx180 OU.
Michael’s suppression is selective, however, as he has full adduction ability of the left eye permitting fusion in right gaze on free space targets such as a Worth 4 Dot. Here is a video clip of Michael’s horizontal gaze in real time.
So is Michael’s problem in abducting the left eye is due to a congenital left lateral rectus paresis – or in this case close to a paralysis, or is it a sixth cranial nerve paresis? The MRI that Michael had showed perfectly symmetrical lateral recti in both eyes, but a congenitally small and thinned sixth cranial nerve innervation to the left lateral rectus. If the muscle has a limited range of motion, one might undertake vision therapy with an analog to what physical therapy can accomplish. But in the presence of a maldeveloped nerve, the prospect for vision therapy alone to improve the situation is very limited.
In Michael’s case we decided to encourage eye muscle surgery if the pediatric ophthalmologist is confident that both function and cosmesis can be improved. While Michael is achieving well in sports and schools, his demands at age six are relatively meager. He is becoming increasingly self-conscious about his appearance, and both children and adults are beginning to make comments. I advised the referring optometrist that we would follow his post-surgical status closely to make sure he maintains an optimal outcome. I don’t feel he needs pre-surgical amblyopia therapy, but I anticipate that he will need professional guidance as his brain and body sorts out his new eye alignment. This will likely include optometric vision therapy, factoring in lenses and prisms, but may also include physical therapy or chiropractic intervention given his current postural reflex skews.
Addendum: One could appropriately diagnose Michael as having Duane Syndrome Type 1, though the narrowing of the palpebral fissure and retraction on lateral gaze is very subtle. Management of his case will remain the same, but for diagnostic purposes that’s a reasonable label.