Monofixation syndrome (MFS) is an eye condition marked by peripheral fusion in the absence of central fusion. It's a binocular vision disorder often defined by an eye misalignment (strabismus), a unilateral mild vision reduction (amblyopia) and a degree of depth perception (stereopsis).
Also Known As
- Primary MFS
- Secondary MFS
Causes & Risk Factors
Primary MFS can occur during normal development. The disorders that prompt a suppression blind spot (scotoma) in the central retina but permit peripheral fusion can also produce primary MFS. Secondary MFS often arises after extraocular muscle surgery to treat horizontal strabismus.
MFS risk factors include:
- Unilateral astigmatism
- Anisometropic amblyopia
Signs & Symptoms
Primary monofixation syndrome is often detected during a routine eye examination. The child's eyes have a small misalignment that is usually invisible to the naked eye. Usually, the patient's vision is mildly reduced in the non-dominant eye, and the child doesn't use both eyes together to achieve a high degree of depth perception. The child is said to have peripheral instead of central fusion.
Symptoms may include:
- Decreased stereovision
- Unilaterally reduced vision
- Small heterotropia with larger heterophoria
- Unilateral, facultative central foveal suppression scotoma with peripheral binocular fusion
The specialist can use sensory tests, such as Bagolini striated lenses and the Worth 4-dot test, to diagnose the disorder. S/he may also perform the 4-prism diopter base-out test to examine for a central suppression scotoma.
S/he can conduct cover-uncover testing for younger patients who can't undertake sensory testing.
MFS treatment usually seeks to address the amblyopia related to it. It includes patching therapy, prisms and eye muscle surgery. Patching or penalization therapy may be recommended where there is significant amblyopia and the patient is young enough for possible improvement. The doctor may use prisms to correct any refractive errors or ameliorate cataracts.
Eye muscle surgery may be advisable where the eye alignment is unstable, noticeable, or causing double vision.
Secondary MFS requires no treatment because monofixation is an excellent surgical outcome in strabismus surgery.
Prognosis & Long-term outlook
MFS can remain stable over decades in many patients, while a small percentage of patients may experience higher-grade stereoacuity improvement in sensory status. Some patients can also deteriorate and develop double vision, reduced stereovision, and larger tropia over time. Often, surgical intervention can decrease the tropia and restore secondary MFS.
Prevention & Follow Up
Patients with stable MFS require routine follow-up care. If the patient has had amblyopia treatment, the frequency of follow up visits depends on the treatments' intensity.