 |
|
Acquired Retinal Myelination in Neurofibromatosis 1
Arch Ophthalmol. 2002;120:659-661.
INTRODUCTION
Myelination of the anterior visual pathways begins centrally at the
lateral geniculate body and is completed at term. This process normally terminates
at the lamina cribrosa, but it occasionally may extend into and beyond the
peripapillary retinal nerve fiber layer. Congenital retinal myelination is
not uncommon, occurring in 0.3% to 0.6% of the population as an isolated developmental
anomaly, or rarely as part of a generalized disorder, such as the Goltz-Gorlin
syndrome, or the syndrome of anisometropia high myopia, and amblyopia.1 Although recent evidence suggests that the retinal
myelination may progress in such cases,2
acquired retinal myelination is exceptionally rare.3
We describe 2 cases in which myelinated retinal nerve fibers appeared
in children with neurofibromatosis 1 (NF1) and optic nerve glioma. In both
cases, spontaneous improvement in visual function preceded the retinal myelination.
Report of Cases
Case 1
An 8-year-old boy was referred to us with reduced vision and optic disc
swelling in the right eye. There was a maternal family history of NF1, and
he had cutaneous features of NF1 and Lisch nodules. The right eye was proptosed,
with optic disc edema, opticociliary shunt vessels, and a relative afferent
pupil defect (RAPD) (Figure 1).
Corrected visual acuity was reduced to 20/80 OD, and a magnetic resonance
imaging (MRI) scan confirmed the presence of an optic nerve glioma. Visual
field testing, which was reliably repeatable in this child, showed an enlarged
blind spot and peripheral constriction on the right side. No treatment was
initiated, and annual follow-up was arranged.
|
|
|
|
Figure 1. Swollen optic disc with opticociliary
shunts (case 1).
|
|
|
Within a year, visual acuity 20/30 OD, the RAPD resolved, and the visual
field enlarged. The disc edema resolved, and the shunt vessels disappeared.
Four years after the initial visit, examination showed myelinated nerve
fibers adjacent to the right optic disc (Figure 2). Visual acuity was stable at 20/30 OD, and the MRI findings
were unchanged. The retinal features have remained unchanged for 4 years.
|
|
|
|
Figure 2. Pale optic disc with myelinated
nerve fibers (case 1).
|
|
|
Case 2
An 8-year-old boy was referred with reduced vision and a pale left optic
disc. He had features of segmental NF1, with café-au-lait spots on
the left chest wall and the imaging features of a left optic nerve glioma
extending to the optic nerve head on MRI scan (Figure 3). Corrected visual acuities were 20/15 OD and 20/30 OS
with left optic atrophy. A full systemic evaluation was performed, but no
treatment was initiated. Visual acuity remained stable during the next 2 years,
and then gradually improved to 20/20 OS. Five years after the initial visit,
he had developed segmental myelination of the peripapillary retinal nerve
fibers in the left eye (Figure 4).
These findings have remained unchanged for 2 years.
|
|
|
|
Figure 3. Magnetic resonance imaging scan
showing optic nerve glioma (case 2).
|
|
|
|
|
|
|
Figure 4. Acquired myelination adjacent
to optic disc (case 2).
|
|
|
Comment
Myelin is deposited in the central nervous system by oligodendrocytes.
The 2 major glial cells of the central nervous system (the type 2 astrocyte
and the oligodendrocyte) are derived from stem cells known as oligodendrocyte
precursor cells, which actively produce myelin prenatally. Myelination ceases
at birth as the oligodendrocytes mature into adult type cells and the biochemical
stimulus for myelination (notably platelet-derived growth factor) is withdrawn.4
The mechanisms of termination of the myelination at the lamina cribrosa
are not clearly understood. This process is important, as opaque nerve fibers
would affect vision. Regulatory proteoglycans have been shown to determine
where the retinal ganglion cell body ends and the axon begins. This ganglion-axon
junction is at the level of the lamina cribrosa in the human optic nerve.
Myelination is limited to the axon, and therefore does not extend beyond the
lamina cribrosa.5
Certain animals such as rabbits lack a lamina cribrosa, and myelinated
retinal nerve fibers are a normal feature of their eyes. The lamina cribrosa
may therefore act as a physical barrier to keep oligodendrocytes out of the
retina.
There is experimental evidence of a physical barrier at the ocular end
of the rat optic nerve, and a similar mechanism has been postulated in humans.6
Myelination could occur postnatally if the barrier were disrupted and/or
the oligodendrocytes were stimulated. The barrier may be directly distorted
by the tumor as in case 2, or disrupted by disc edema in case 1. Previously
described cases of acquired retinal myelination have occurred following resolution
of papilloedema2 or associated with optic
disc drusen,3 where there would be disruption
of the lamina cribrosa.
Stem cell proliferation in NF1 occurs due to local inactivation of a
tumor supressor gene.7 In the anterior visual
pathway, this process produces optic glioma—characteristically an indolent
tumour. It is recognized that visual function in optic gliomas can improve
spontaneously, with improvement in the scan appearance.8
The improvement in vision in this case was most likely not due to tumor regression,
but to axonal remyelination by oligodendrocytes locally reactivated by the
same process that triggered the optic glioma. If the lamina cribrosa is also
disrupted, remyelination could extend into the retina, with the process stopping,
as the stem cell proliferation is self-limiting.4
These 2 cases, therefore, provide an insight into the complex dynamic
of focal tissue growth disorders in NF1. Tumor growth is not exponential,
but modified by focal repair mechanisms.
AUTHOR INFORMATION
Manoj V. Parulekar, FRCS;
John S. Elston, BSc, MD
Oxford, England
Corresponding author and reprints: John S. Elston, BSc, MD, Radcliffe
Infirmary, Oxford Eye Hospital, Woodstock Road, Oxford OX2 6HE, England (e-mail: Mary.Spearman{at}orh.anglox.nhs.uk).
REFERENCES
1. Lee MS, Gonzalez C. Unilateral peripapillary myelinated retinal nerve fibers associated
with strabismus, amblyopia, and myopia. Am J Ophthalmol. 1998;126:853.
FULL TEXT
| PUBMED
2. Ali BH, Logani S, Kozlov KL, Arnold AC, Bateman B. Progression of retinal nerve fiber myelination in childhood. Am J Ophthalmol. 1994;118:515-517.
ISI
| PUBMED
3. Jean-Louis G, Katz BJ, Digre KB, Warner JE, Creger DD. Acquired and progressive retinal nerve fiber layer myelination in an
adolescent. Am J Ophthalmol. 2000;130:361-362.
FULL TEXT
|
ISI
| PUBMED
4. Tang DG, Tokumoto YM, Raff MC. Long-term culture of purified postnatal oligodendrocyte precursor cells:
evidence for an intrinsic maturation program that plays out over months. J Cell Biol. 2000;148:971-984.
FREE FULL TEXT
5. Perry VH, Lund RD. Evidence that the lamina cribrosa prevents intraretinal myelination
of retinal ganglion cell axons. J Neurocytol. 1990;19:265-272.
FULL TEXT
|
ISI
| PUBMED
6. French-Constant C, Miller RH, Burne JF, Raff MC. Evidence that migratory oligodendrocyte-type-2 astrocyte (O-2A) progenitor
cells are kept out of the rat retina by a barrier at the eye-end of the optic
nerve. J Neurocytol. 1988;17:13-25.
FULL TEXT
|
ISI
| PUBMED
7. Lau N, Feldkamp MM, Roncari L, et al. Loss of neurofibromin is associated with activation of RAS/MAPK and
PI3-K/AKT signaling in a neurofibromatosis 1 astrocytoma. J Neuropathol Exp Neurol. 2000;59:759-767.
ISI
| PUBMED
8. Parsa CF, Hoyt CS, Lesser RL, et al. Spontaneous regression of optic gliomas: thirteen cases documented
by serial neuroimaging. Arch Ophthalmol. 2001;119:516-529.
FREE FULL TEXT
SECTION EDITOR: W. RICHARD GREEN, MD
|
