Definition and Description
Optic Nerve Hypoplasia (ONH) refers to the underdevelopment of the optic nerve during pregnancy. The dying back of optic nerve fibers as the child develops in utero is a natural process, and ONH may be an exaggeration of that process. ONH may occur infrequently in one eye (unilateral) but more commonly in both eyes (bilateral). ONH is not progressive, and is not inherited. ONH is one of the three most common causes of visual impairment in children.
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.
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.
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. In general the smaller the size of the optic nerve the less information can travel from the eye(s) to the visual parts of the brain.
Amblyopia can further reduce vision if one eye has better acuity than the other. In other words, the eye with the poorer vision may be poor in part due to amblyopia. The "stronger" eye becomes the dominant eye, and vision does not develop in the weaker eye because of this dominance.
The child's vision is characterized by a lack of detail (depressed field), but this lack of detail is not comparable to the blurred reduction in vision when a person removes her glasses. Children with ONH may be unable to locate objects in space precisely due to a lack of depth perception. Behaviors of some children with ONH may be due to associated medical conditions, such as inattentiveness and irritability due to low blood sugar levels (hypoglycemia).
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.
In most cases there is no known cause of ONH. Infrequently ONH has been associated with maternal diabetes, maternal alcohol abuse, maternal use of anti-epileptic drugs, and young maternal age (20 years of age or less), but these factors account for very few of the total number of cases. All races and socioeconomic groups seem to be affected by ONH.
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.