Optical Coherence Tomography for the Management of Fibrous Dysplasia of the Skull Base with Optic Nerve Involvement

Background The skull base is the most common site of the craniofacial skeleton to be affected by fibrous dysplasia (FD). A significant proportion of such FD patients present with compression of the optic nerve, which can cause visual impairment, but most patients initially do not experience vision loss. Controversy exists regarding whether to proceed with a prophylactic surgical decompression of the optic nerve or to elect for “watchful waiting.” Optical coherence tomography (OCT), a physiologic imaging modality widely used to assess the condition of the retinal nerve fiber layer (RNFL), has been useful in the monitoring of other compressive lesions on the optic nerve including pituitary tumors and craniopharyngioma, and may have a beneficial role in managing patients with FD involving the optic nerve.

Methods Six patients with suspected optic nerve compression who had received OCT imaging as part of their neuro-ophthalmic exam were reviewed over a 2-year period at a tertiary medical center. Patient records were primarily evaluated for the presence of optic neuropathy grouped by either normal age-adjusted RNFL thickness (above the fifth percentile) or abnormal (below the fifth percentile) as measured by OCT. Other visual exam measures including visual acuity, visual field defects, and optic cup-to-disc ratio and radiographic measures including optic canal size and the presence of optic nerve encasement or compression on computed tomography (CT) were also analyzed.

Results Two patients were found to have mild optic neuropathy in one eye each. The mean (± SD) RNFL thickness on OCT was 93.1 ±  12.0 μm, with 3 of 12 eyes falling below the age-adjusted fifth percentile. Abnormal age-adjusted RNFL thickness was associated with the presence of optic neuropathy (p < 0.05, see [Table 1]). Visual field defects, optic canal size, and optic nerve encasement differed by the presence of optic nerve compression on CT (p < 0.05), but the occurrence of optic neuropathy did not differ by optic nerve compression (p = 0.45).

Conclusion Based on the case series of this rare condition, abnormal RNFL thickness as measured by OCT better predicted the presence of optic neuropathy than CT alone. OCT may therefore be a valuable imaging modality to monitor FD patients for development of optic neuropathy during periods of conservative “watchful waiting” to determine the best timing for intervention if necessary.

Table 1 (P083) Presence of optic neuropathy by age-adjusted optical coherence tomography RNFL thickness percentiles and by optic nerve compression on CT imaging

	Age-adjusted RNFL thickness		p-Value	Optic nerve compression		p-Value
	Bottom 5% (n = 3)	Top 95% (n = 9)		Present (n = 6)	Absent (n = 6)
Optic neuropathy	2 Mild, 1 No	0 Mild, 9 No	0.045	2 Mild, 4 No	0 Mild, 6 No	0.45

Abbreviation: RNFL, retinal nerve fiber layer.