A Better Method for Detecting the Risk of Vision Loss in Craniofacial FD
Optic neuropathy or vision loss is a major concern for people with craniofacial fibrous dysplasia (FD). Because surgical correction can lead to further complications, management and treatment for FD related vision loss has been a challenge and source of debate for decades. A previous study found that FD lesions closing around the optic canal rarely lead to permanent vision loss. Some researchers suggest that vision loss in FD is more complicated than compression of the optic nerve canal alone. For example, excess growth hormone in FD patients can lead to bone enlargement in the skull. In craniofacial FD that enlargement can mean facial asymmetry or proptosis (protrusion of the eyeball) which also can stress or damage the optic nerve. A recent study led by Dr. Alison Boyce and a team of researchers at the NIH examined these contributing factors and suggested an improved method of monitoring craniofacial FD to prevent vision loss. “This research suggests that in many cases we can detect optic neuropathy very early, before patients have symptoms,” said Dr. Boyce “Most–but not all–cases of optic neuropathy in FD start in childhood, so monitoring should start early.” This paper provides normative data for kids, which is really helpful for clinicians monitoring craniofacial FD.
Surgeons rely on a method known as computed tomography to identify patients who have optic canal narrowing and decide whether that narrowing is severe enough to call for surgical intervention. However, research suggests that most patients that present with optic canal narrowing on computed tomography remain asymptomatic and see normally over time. This seems to indicate that this type of radiographic analysis of the optic canal may not be effective in predicting vision loss. However, computed tomography still remains common practice for FD vision loss, and Dr. Boyce’s team saw that new, more precise diagnostic tools were necessary.
The NIH team examined the effectiveness of a closely related diagnostic tool, optical coherence tomography (OCT), in diagnosing vision loss in FD. OCT is widely used to supplement ophthalmologic examination in addition to computed tomography. This method uses imaging techniques to evaluate optic nerve atrophy by measuring the thickness of axons, which deliver the electrical signals between the brain and the optic nerve. This is also known as retinal nerve fiber layer (RNFL). OCT RNFL has been shown to detect changes in RNFL thickness prior to vision loss, so it may also follow that RNFL thickness could serve as a biomarker, or a biological measurable indicator for the development of vision issues in FD patients.
In order to assess their hypothesis, the researchers studied patients enrolled in the NIH natural history study who were clinically diagnosed with craniofacial FD and had neuro- ophthalmic examinations, including OCT, after 2008. They then performed a number of visual evaluations on the subjects, like visual acuity, visual fields, color vision, contrast sensitivity and optic nerve atrophy. Participants who had related vision disorders but not associated with vision loss were excluded from further analysis. The participants then had their optic canals evaluated with CT scans. From this, optical canal cross sectional area (to measure for narrowing), optic nerve length and globe displacement (to measure for proptosis) were all assessed.
From the 70 patients who met the selection criteria, researchers found that overall RNFL thickness diagnostic tool outperformed the current practice of computed tomography measurements of optic canal narrowing and optic nerve stretch. Subjects with vision loss exhibited a greater decrease in RNFL thickness with each year of age than subjects with normal vision. Subjects with vision loss also exhibited decreases in RNFL thickness during adolescence. Overall, RNFL thickness accurately identified vision loss within this population that was studied.
This has significant implications for future care of the condition, given that there is a notable difference in rate of RNFL thinning in patients with and without vision loss. With this insight, medical providers may be better suited to distinguish between patients at risk for vision loss and provide medical intervention before irreversible damage can occur.“People with craniofacial FD should consider bringing this article to their ophthalmologists. Although OCT is pretty widely available, not every office either has one or uses it routinely,” explains Dr. Boyce, “Since doing this study I’ve been asking patients who are establishing care with new ophthalmologists to make sure ahead of time that OCT is available in the practice.”
Deep thanks to Sabina Wahed, FD/MAS Alliance intern who prepared this post. You can find the research article in full here.