A representative CirrusTM optical coherence tomography (OCT) optic disc cube study (A) and serial CT scans (B) of a 9-year-old girl with subtle left optic neuropathy and a very slowly expanding cystic lesion abutting the left optic canal. A) The numbers in the two green circles in the RNFL (retinal nerve fiber layer) represent the single number comparison between the two eyes. Generally, the nerve fiber layer is considered thin when it is less than about 70 microns. Note that in the RNFL Deviation Map panels the optic cup (the area within the red circle) on the left (OS) (black arrow) is a bit larger than on the right (OD), also suggestive of axon loss. In the RNFL Thickness graph, note the differences between the left (dashed line) and the right (solid line) in the temporal (TEMP) region (asterisk), indicating that in this region retinal nerve fibers are thinner on the left. For children under 18 normative data for the Extracted Vertical Tomogram and the RNFL Tomogram are not available. B) Serial coronal plane CT images at approximately the same region are shown. The expansile cystic lesion is indicated with the solid white arrow, and the optic nerve by the dashed arrow. The findings indicate the presence of a slowly expanding lesion, the cystic, fluid-filled nature of which was confirmed on MRI. On clinical examination, there were subtle findings of left optic neuropathy in that she performed slightly worse on the Ishihara color test and the Pelli Robson test of contrast sensitivity in her left eye. There was no evidence of an afferent pupil defect. Photos also demonstrated subtle temporal pallor of her left optic disc. There were no objective changes in visual acuity. She has been followed clinically with neuro-ophthalmologic examination approximately every three months to assess for any significant progression, which would be an indication for surgical intervention. The findings on the OCT study confirm the clinical impression of a left optic neuropathy and are particularly useful when visual fields are not obtainable or particularly reliable (usually due to age-related inability to perform the test), as well as an objective measure for longitudinal follow-up. The nerve fiber layer findings on OCT can also be used to predict what visual outcome one might expect after a successful decompression surgery. If one were to find a field defect on examination, but the corresponding optic nerve retinal nerve fiber layer was preserved on OCT testing, it would be reasonable to expect full recovery of vision after surgery. However, if there were nerve fiber layer loss, recovery of vision would be unlikely as the findings most likely represent axons that have died back.