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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.

Picture of a normal retina, reddish with blood vessels and optic nerve head

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.

Picture of a retina with small blood vessels and very small optic nerve head

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

 

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