|
Optic Nerve Hypoplasia
(ONH)
The optic nerve is the main nerve that
travels from the eye to the vision parts of the brain. The optic nerve
from each eye partially crosses at the optic chiasm and then travels to
the visual cortex where the information from each eye is processed for
vision and for "seeing".
Although the eye doctor can’t see the
optic nerve directly, he/she can see the head of the optic nerve by
looking into the eyes with the help of an instrument called an
ophthalmoscope. In addition, a brain-imaging device known as an Magnetic
Resonance Imaging scanner or MRI can take a "picture" of the optic
nerve. Via use of an ophthalmoscope and by utilizing an MRI, the eye
doctor can get a good estimate about the size of the optic nerves in a
patient. A picture of a normal retina and optic nerve head is shown in
Figure 1.
Figure 1. Fundus photograph of a
normal optic nerve head and retina. The photo shows the pinkish optic
nerve head and blood vessels that exit/enter the optic nerve head to
provide nurishment to the retina.
Figure 2. Retinal photograph of optic
nerve hypoplasia. Note the much smaller size of the optic nerve - which
is the small white spot in the center of the photo surrounded by a dark
ring of pigmentation. This patient also had aniridia (abnormal pupil).
Hypoplasia means underdevelopment. In
this case, optic nerve hypoplasia means an underdeveloped optic nerve in
either one or both eyes. When the eye doctor looks into the eye, a
patient with optic nerve hypoplasia will have a small optic nerve head,
suggesting that the optic nerve is smaller than normal. An MRI of the
front part of the brain and orbits can provide a picture of the actual
size of the optic nerve from each eye. When the optic nerve head or
optic nerve is smaller than normal the patient is said to have optic
nerve hypoplasia. In ONH, the optic nerve is small and often appears
pale. Also, there may be a yellowish-white ring of pigment around the
optic nerve head, a condition sometimes referred to as the "double ring
sign".
It is important to know that in a
normal eye, the optic nerve is composed of about one million optic nerve
fibers that represent the visual field of the patient. In other words,
there are optic nerve fibers responsible for certain parts of the field
of vision; for example, some optic nerve fibers represent central
vision, that part of vision used for reading, and other optic nerve
fibers represent more side vision or peripheral vision. In optic nerve
hypoplasia, the missing nerve fibers may have represented central vision
in the patient and, as a consequence, the patient may have very poor
central vision and be considered legally blind (equal to or worse than
20/200 visual acuity). On the other hand, the missing nerve fibers may
represent peripheral vision in which case the patient could have normal
visual acuity (e.g., 20/20 visual acuity) but have some peripheral field
loss. As a consequence, patients with optic nerve hypoplasia have a wide
range of visual acuities from light perception only (LP, only can
discriminate light from dark) to perfectly normal visual acuity. Also,
the actual size of the optic nerve or optic nerve head is not a good
indicator for how much vision a patient may have or a good indicator of
visual impairment: some patients with real small optic nerves may have
normal or near normal visual acuity while other patients with about the
same size of optic nerves may only have light perception. In general,
however, the smaller the size of the optic nerve the less information
can travel from the eye(s) to the visual parts of the brain.
Causes of Optic nerve hypoplasia
The cause(s) of optic nerve hypoplasia
is (are) not known. We have seen drug addicts and "health food nuts" all
have babies with optic nerve hypoplasia. It is generally believed that
optic nerve hypoplasia is the result of some type of trauma or event to
the fetus, possibly occurring during approximately 10 – 12 weeks
gestation, when the optic nerves are starting to form and make
connections to the other vision parts of the brain. Apparently, the
individual nerve fibers that comprise the optic nerves either never form
or form normally and then through a process analogous to pruning die-off
resulting in smaller than normal optic nerves. One or both eyes may be
affected.
While the cause (s) of optic nerve
hypoplasia are not known, there are several studies that have found a
correlation between certain factors in the mother and a child born with
optic nerve hypoplasia. However, it is very important that the reader
recognize the difference between showing a correlation between certain
factors and knowing the cause of optic nerve hypoplasia. A correlation
does NOT MEAN it caused the condition. In other words, just because
there is a correlation does not mean that that particular factor caused
the optic nerve hypoplasia. Any ways, optic nerve hypoplasia (ONH) has
been correlated with the following:
-
Preterm birth – about 20% of
infants with ONH were premature
-
Alcohol consumption – a large
percentage (about 90%) of infants with fetal alcohol syndrome have
ONH
-
Ingestion of quinine during the
first weeks of pregnancy (for Malaria)
-
Ingestion of phenytoin, a medicine
used for seizures and cardiac problems
-
Maternal diabetes
-
Congenital cytomegalovirus (CMV)
infection
-
LSD consumption
-
No known risk factors – about 70%
of patients with optic nerve hypoplasia do not have any of the above
risk factors
Other Things Associated with Optic
nerve Hypoplasia (ONH)
A patient may have optic nerve
hypoplasia in one or both eyes, or it may be associated with other
neurological or visual problems. Optic nerve hypoplasia is often
associated with other brain defects including absence of the septum
pellucidum (a membrane that separates the front part of the lateral
ventricles of the brain), agenesis of the corpus callosum (connects the
left and right sides of the brain) and dysplasia of the anterior third
ventricle. These defects are sometimes referred to as "midline defects".
ONH may also be associated with the infant having nystagmus, where the
eyes constantly move back-and-forth or the infant may have "wondering"
eye movements in which the patient appears to be continuously looking
around but fails to respond to visual stimuli. An infant with optic
nerve hypoplasia that has nystagmus or wondering eye movements will
generally have worse vision that an infant with ONH who does not have
such eye movement problems.
Although more rare, sometimes patients
with ONH will also have poor muscle tone (hypotonia), mental
retardation, seizures and deficiencies of ACTH, ADH and/or prolonged
bilirubinemia.
Sometimes a patient will be described
as having "Septo-optic dysplasia" or "de Morsier’s syndrome" both of
which refer to a patient with optic nerve hypoplasia who has poor
vision, nystagmus and short stature. The midline defects or defects of
the middle part of the brain may lead to growth problems in the infant
with ONH such that the infant may grow too much or may not growth enough
resulting in short stature. If an infant with ONH has midline brain
defects, he/she must be followed closely by an Endocrinologist to
monitor and treat any growth problems.
In general, if the infant with ONH has
a significant loss of vision in both eyes the infant may develop a
misalignment of the eyes, a condition called strabismus. Typically one
eye will turn-in (esotropia) but sometimes one eye will turnout (exotropia).
In general, it is the eye with the worse vision that will turn-in or
out. The presence of strabismus in an infant with ONH is not a good
sign, since it suggests that one or both eyes have such poor vision that
they have a hard time working together. Depending on the circumstances
and severity, the ophthalmologist may recommend eye muscle surgery for
the misaligned eyes.
A patient with severe ONH will
sometimes have fixed, unchanging pupils. If the pupils are fixed or/and
always dilated, this is usually a poor sign and an indication of a
severe visual loss. The reason for the fixed/dilated pupils in an infant
with ONH is because the light information received from the eye cannot
reach the vision centers of the brain that are responsible for changing
pupil size. If the optic nerve fibers are not present than there is not
way for the light information from the eye to reach those brain centers
responsible for changing pupil size.
Rehabilitation
There is no cure or treatment, per se,
for ONH. A baby with ONH should be followed by an eye doctor, by an
Endocrinologist if applicable and may need to participate in early
vision rehabilitation/stimulation programs. In general, an infant with a
significant vision loss will show a delay of from approximately 3 – 9
months on certain tasks because of the vision loss or possibly due to
other factors related to the vision loss. The purpose of early
intervention programs and early vision stimulation programs is to
minimize the impact of the vision loss of general development. Just
because an infant has reduced vision does not necessarily mean that the
infant will be delayed or slow on other developmental tasks such as
orientation and mobility (walking, crawling). However, it is the purpose
of such rehabilitation efforts to minimize the impact of the vision loss
on these developmental milestones.
Other Factors and Considerations
ONH is now one of the major causes of
vision loss in infants and accounts for about 15 - 25% of infants with
serious vision loss. Over the years we have seen hundreds of patients
with ONH and the incidence of the disease seems to be rising. While some
suggest the incidence of ONH has been increasing since the 1970s, others
argue that the reason why more and more patients are being diagnosed
with ONH is that eye doctors are more attuned to the disease/condition
and more likely to offer it as a reason for the vision loss. It is
important to note that while we have seen several hundred patients with
ONH, we do not recall a family with more than one child with the ONH. So
whatever the reason for the ONH, it appears to be a fleeting, difficult
problem to solve or get a better hold of in terms of uncovering the
reason or cause of the condition.
There has been significant research
underway in trying to identify the genes responsible for ONH. Both
animal studies (mice and rats) as well as genetic studies in humans with
ONH appear to be promising. Some of this research is being carried-out
by scientists funded by the Ohio LIONS Eye Research Foundation in Ohio.
Hopefully, someday through research we’ll be able to screen fetuses for
ONH and offer strategies for treatment and prevention.
-L. E. Leguire
Ph.D., MBA
|
