ABSTRACT
Optic nerve choristomas are very rare lesions. They are characterised by the presence of fat cells, mesodermal collection of fibrous tissue and smooth muscle, and atrophic optic nerve tissue. Although the condition can be diagnosed on histology, it is not always possible to carry out surgical exploration for tissue diagnosis in view of the apical location of the lesion and the inherent risk to vision from surgery. Detailed neuro-imaging becomes vital in these cases for diagnosis. We report a case of a patient with an optic nerve choristoma diagnosed on neuroimaging, review the current literature related to this condition, and discuss the key clinico-radiological features of optic nerve choristomas.
KEYWORDS: Optic nerve choristoma, optic atrophy, MRI
Choristomas consist of histologically normal tissue occurring in an ectopic location. Optic nerve choristomas are very rare lesions, with few cases previously reported in the literature. We report a case of a patient with optic nerve choristoma diagnosed on neuro-imaging, and review the current literature related to this condition.
Case report
A 36-year-old female patient first presented at the age of 16 with painless, slowly progressive reduction of vision in her left eye. She had no relevant past medical history. Clinically she had a left optic neuropathy, with reduced visual acuity (6/12 unaided, 6/9 pinhole), reduced colour vision (Ishihara 1/13 plates), global visual field depression on automated perimetry, a left relative afferent pupillary defect, and left optic atrophy. Examination of the right eye was normal.
Magnetic resonance imaging (MRI) at presentation revealed a lesion extending from the left side of the chiasm to the mid left orbit, that was intimately associated with the optic nerve itself. The lesion was partially hyperintense on T1-weighted imaging and demonstrated no significant enhancement on T1 fat-saturated post-contrast imaging. Over an 18-year period, the lesion remained stable on serial imaging (Figures 1 and Figures 2), although the visual acuity in her left eye gradually deteriorated to 6/38.
Figure 1.
Composite of axial T1-weighted images:(a) Pre fat-suppression and contrast: High signal intensity seen due to the fat content of the choristoma. (b) Post fat-suppression and contrast: Contrast enhancement is not seen Composite of MRI axial T2-weighted images: (c) Pre fat-suppression: High signal intensity (d) With fat suppression: Signal disappears with fat suppression
Figure 2.
Axial pre fat-suppression T1-weighted images: a. From 2009 (earliest MRI available). b. From 2019
Discussion
The previous literature on optic nerve choristomas has not described the long-term outcomes of this rare condition.1–4 To the authors’ knowledge, our case represents one of the longest follow up of optic nerve choristoma in the literature to date.
Histologically, optic nerve choristomas are characterised by the presence of atrophic optic nerve tissue closely associated with dense fibrous tissue and adipose cells. It is the fat component of the choristoma within the dural sheath of the optic nerve, that accounts for the characteristic high signal intensity seen on T1-weighted MRI, and this disappears on fat-saturated T1-weighted imaging.2 It should be noted that fat is also bright on T2-weighted imaging (albeit to a lesser extent) and will suppress on fat-saturated T2-weighted imaging.
Scattered sheets of smooth muscle are present within the dense fibrous tissue. The smooth muscle appears differentiated and is arranged in broad sheaths with intersecting fascicles.1,3,4 The cells proliferate within the optic nerve sheath and extend into the optic nerve septa, leading to enlargement of the optic nerve.2,3
Radiologically, the intra-canalicular portion of the optic nerve was involved in all previously reported cases of choristomas. They show an indolent course, leading to widening of the optic canal without bony destruction.2
Where reported, tissue biopsies confirm the presence of choristomas with adipose tissue and smooth muscle, and histological examination remains the gold standard for diagnosis.1,3,4 However, it is not always possible to carry out surgical exploration for tissue diagnosis in view of the apical location of the lesion and the inherent risk to vision from surgery, as with our patient. Detailed neuro-imaging becomes vital in these cases for diagnosis, as although a differential diagnosis remains without histological confirmation, the constellation of findings make choristoma the most likely diagnosis in our patient.
The main differential diagnoses of optic nerve choristomas on radiology are optic nerve sheath meningiomas and gliomas. The high signal intensity seen on T1-weighted imaging due to the fatty tissue in choristomas will not be present in these other two conditions, which tend to be iso- to hypointense on T1-weighted imaging.5 Whilst meningiomas and gliomas may display variable T2 hyperintensity, this will not suppress on fat-saturated T2, as is the case for choristomas.
Additionally, similar to normal intraorbital fat, choristomas are less likely to show post-contrast enhancement on fat-saturated T1 imaging.2 This differs from optic nerve sheath meningiomas, which show enhancement on post-contrast T1 imaging, and become more apparent on fat-saturated images, leading to the ‘tram-track’ appearance of the nerve surrounded by tumour.5 The findings also differ from optic nerve gliomas, which tend to show variable enhancement on post-contrast T1-weighted imaging.5 Table 1 shows a summary of the MRI findings in optic nerve choristomas, optic nerve gliomas and optic nerve sheath meningiomas.5
Table 1.
Summary of MRI findings in selected optic nerve pathologies
| Lesion | MRI T1-weighted images | MRI T2-weighted images |
|---|---|---|
| Optic nerve choristoma | Pre fat-suppression and contrast: High signal intensity seen due to fatty component in choristoma Post fat-suppression and contrast: No contrast enhancement |
Pre fat-suppression: High signal intensity With fat suppression: Signal disappears |
| Optic nerve glioma | Pre fat-suppression and contrast: Iso- to hypointense Post fat-suppression and contrast: Variable enhancement |
Pre fat-suppression: Usually hyperintense; there may be fluid signal hyperintensity within sheath around mass With fat suppression: No change to signal |
| Optic nerve sheath meningioma | Pre fat-suppression and contrast: Iso- to hypointense Post fat-suppression and contrast: Homogenous enhancement |
Pre fat-suppression: Iso- to hyperintense With fat suppression: No change to signal |
In conclusion, we have presented a rare case of an optic nerve choristoma with long term outcome. We have also highlighted some of the key clinical and radiological features that can be used to diagnose optic nerve choristomas. This is especially important where tissue diagnosis is not possible.
Declaration of interest statement
The authors have no financial disclosures or conflicts of interest in the article.
References
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