What is Optic Nerve Coloboma?
Coloboma comes from the Greek word ‘koloboma’ which means missing, shortened, or cut off part. It’s a birth defect that can affect different parts of the eye such as the iris (colored part of the eye), lens, choroid (layer containing blood vessels), retina (back of the eye), and the optic nerve.
Eyelid colobomas create a gap in the full thickness of the eyelid. While these can appear anywhere on the eyelids, they’re most likely to be found where the inner and middle parts of the upper eyelid meet. These can happen due to accidents, surgeries, or can be present at birth. They may stand alone or occur along with other facial deformities or syndromes. It is important to protect the cornea (front layer of the eye) in these cases, so eye drops and ointments for lubrication may be used, along with moisturizing chambers. Sometimes, it may be necessary to use patches. If needed, surgery may be done to improve the cornea’s coverage. The exact surgical procedure depends on the size and location of the defect.
Iris colobomas, on the other hand, affect the lower, inside quadrant of the iris. This is due to the embryonic optic fissure, a seam in the developing eye, failing to close during the fifth week of gestation, leading to a pupil that looks like a keyhole. Sometimes, a bit of iris can form over the coloboma to create a “bridge coloboma”. These are usually present in both eyes and can result in an uniquely-shaped cornea. If the cornea’s shape is normal, a surgical cause of the coloboma may be present. Iris colobomas could be associated with other eye deformities and conditions like glaucoma (damage to optic nerve due to high pressure in the eye), nystagmus (rapid eye movements), or strabismus (crossed or misaligned eyes).
These iris colobomas can cause sensitivity to light, visual distortion, and double vision and may also be undesirable for aesthetic reasons. To address these issues, cosmetic contact lenses with an artificial pupil or surgical repair may be used. Artificial iris prosthetic devices are being explored, especially in the presence of an artificial lens implant. Intraocular lenses painted with an iris may be implanted after removing a cloudy lens due to cataract (a condition that clouds the lens of the eye), or foldable artificial irises may be inserted through a small incision.
Associated issues like limbal dermoids (benign growths on the cornea) could occur as well, and these look simple but should be handled with care as it poses a risk of penetrating the eyeball. Surgical removal should only be done when a lamellar graft (cornea or sclera or both) is available, as this can replace the removed tissue. The remainder of the original article focuses on optic nerve colobomas.
What Causes Optic Nerve Coloboma?
When an eye is developing, a type of tissue called the neuroectoderm moves towards the surface tissue and forms a structure called the optic vesicle. This vesicle then folds in on itself to form an optic cup on top and towards the body, and an optic fissure at the bottom and away from the body. The part of the vesicle closest to the body forms the optic stalk.
As the eye further develops, the edges of the optic fissure start to come together and merge, leaving behind a small hole, the optic disc, for an artery called the hyaloid artery. This process of the optic fissure closing up starts around the fifth week of a baby developing in the womb and typically finishes around the seventh week.
However, if this optic fissure doesn’t close up fully, it results in a condition called colobomas. Since the optic fissure closes up last at the bottom, colobomas are usually found in this area. Depending on how big it is and where it’s located, a coloboma can lead to anything from no noticeable effect on vision, to complete loss of vision. It can occur as a result of a mutation in any of the 39 genes linked with colobomas, or it can occur randomly.
Risk Factors and Frequency for Optic Nerve Coloboma
The condition is found in about 0.14% of all people. In half of these cases, both sides of the body are affected.
Signs and Symptoms of Optic Nerve Coloboma
When the area of eye responsible for creating images (retino-choroidal coloboma) fails to develop properly, it can result in a weakening of the choroid and the outer layer of the eye (sclera), causing a bulging or protrusion (staphyloma). If the main area responsible for sharp central vision (fovea) is affected, it can have a significant impact on vision. However, if the fovea is not involved, even with large affected areas, vision can be relatively normal.
Children with prominent differences in one of the optic discs, the areas in the eye where the optic nerve enters, usually start showing symptoms during preschool years, which are seen as sensory esotropia – a type of crossed-eyes condition. The condition might not affect the child’s visual acuity, as seen in optic disc pit, or optic disc drusen, which are little yellow or white deposits in the optic disc. Optic nerve colobomas can be associated with several other eye conditions:
- Microphthalmos – having small eyes
- Iris coloboma – a gap in the iris (colored part of the eye)
- Ciliary coloboma – defects in the ciliary body of the eye
- Lens notching
- Retinal detachment – separation of the retina from its attachments to the back of the eye
- Neovascular membranes – new, abnormal blood vessels growing in the eyes
- Macular holes – small holes in the macula, the part of the retina responsible for central vision
Rhegmatogenous retinal detachments can occur due to breaks in the thin layer over the coloboma. A successful treatment for patients with optic disc coloboma and retinal detachment is vitrectomy with silicone oil.
Testing for Optic Nerve Coloboma
Colobomas, a type of eye abnormality, can occur in a variety of conditions, including:
* Treacher Collins syndrome – a condition passed down in families which affects facial development.
* Cryptophthalmos – a condition involving abnormal development of the eyelids.
* Cat eye syndrome – a genetic disorder resulting from a specific abnormality in chromosome 22 where eye colobomas typically appear vertical, like a cat’s eye.
* Patau syndrome (also known as trisomy 13) – a severe genetic disorder.
* Fraser syndrome – a rare congenital disorder characterized by a variety of malformations.
* Manitoba Oculotrichoanal syndrome – a rare condition unique to people of Northern Manitoba Aboriginal descent.
* Goldenhar syndrome and First arch syndrome – two conditions that affect the development of the eye, ear, and spine.
* Franceschetti syndrome – a genetic disorder characterized by abnormalities of the head and face.
* Amniotic band syndrome – a condition where parts of the fetus (developing baby) become entangled in fibrous string-like amniotic bands in the womb.
* CHARGE syndrome – a complex genetic disorder that affects many areas of the body. The name represents different symptoms: Coloboma, Heart defects, Atresia (blockage) of nasal, Growth retardation, Genital abnormalities, and Ear abnormalities.
Other diseases like renal coloboma syndrome, Aicardi syndrome, Solomon syndrome, Noonan syndrome could also cause colobomas in the eye.
When coloboma is suspected, the ophthalmologist may check the entire eye including the eyelids, eyebrows, conjunctiva (a clear layer covering the front of the eye), cornea, sclera (the white part of the eye), the lacrimal system (which makes tears), the lens, the iris, choroid (a layer of blood vessels in the eye), and the optic disc (the spot where the optic nerve attaches to the eye).
Interestingly, children born with colobomas involving the optic nerve (which transmits visual information from the eye to the brain) can still have normal sight, provided their foveal anatomy (the part of the eye responsible for sharp central vision) develops normally. However, a more severe coloboma can increase the chances of the retina detaching from the back of the eye, which requires urgent medical attention. A variety of factors including abnormal eye structures or optic nerve cysts can also increase the likelihood of retinal detachment and severe vision impairment.
Clinically, doctors see large optic nerve changes often on the lower part of the eye. These changes can occur in one or both eyes equally. Patients might also have smaller than average eye size or optic nerve cysts that are connected to the subarachnoid space, which is the area between the brain and the thin tissues that cover it. Sometimes these cysts grow and cause pressure on the optic nerve.
Optic coherence tomography (OCT) is a non-invasive imaging test that enables doctors to visualize the eyes’ optic nerve changes in greater detail than using only an eye examination. It helps distinguish between different optic nerve anomalies. This imaging technique can help monitor for new growth of unusual blood vessels near the optic nerve and can differentiate between multiple eye disorders.
Treatment Options for Optic Nerve Coloboma
Patients with bilateral ONC, meaning they have this condition in both eyes, should have an imaging scan of their head. This is because these patients are more likely to have abnormalities in the middle of their brain. Other steps for treating these patients include using sunglasses to help with light sensitivity, which is called photophobia. Treatment for a condition called anisometric amblyopia may also be needed if the patient has poor vision.
Doctors who specialize in eye health, known as ophthalmologists, should consider the possibility of a genetic disorder or issues with the formation of the brain and nervous system. This is especially true for patients who have a small eye with a defect in its development, known as colobomatous microphthalmia, and are also experiencing delays in their development or have other abnormalities.
Research has found that ocular coloboma, or an eye defect that occurs when a gap forms in one of the structures of the eye during development, often goes hand-in-hand with abnormalities in the brain and nervous system, urinary and reproductive systems, or skeleton. A study by Van Dalen and colleagues found these kinds of systemic anomalies almost exclusively in patients with bilateral ocular colobomatous changes, meaning changes to both eyes related to this defect.
What else can Optic Nerve Coloboma be?
When diagnosing optic nerve head anomalies, different conditions can be considered. One of these is optic nerve pits, which are not caused by issues with foetal optic fissure closing, but instead are a result of an early developmental issue. These pits can lead to a condition called maculopathy, which in some cases can result in poor vision, sometimes as bad as 20/200. There are different theories about where the fluid within the retina, causing maculopathy, comes from, with some suggesting it’s either fluid from around the brain and spinal cord, or fluid from within the eye itself.
Morning glory disc anomaly (MGDA) and optic nerve coloboma are distinct from each other. MGDA presents with a specific set of symptoms, such as a conical-shaped optic disc, a tuft of nerve fibre at the centre, and pigment around the area, while optic nerve coloboma doesn’t have these features. The exact cause of MGDA is unclear, but it may be due to a failure during the closure of the optic stalk, which is a part of the eye’s development. People with MGDA often have poor vision, eye misalignment problems, and are prone to retinal detachment due to tears at the disc border. If they have MGDA, they should get a full neurological examination as there may be associated brain and nervous system conditions.
When comparing optic nerve hypoplasia, a condition causing poor vision in children, and optic nerve coloboma, the former is more common. Children having this problem in both eyes usually have involuntary eye movement, while those with it in one eye can present with misaligned eyes. Visual acuity for these patients can range from 20/20 to no light perception. The characteristic sign is the appearance of a “double ring” around a small optic nerve. Other physical abnormalities may be associated with this condition, especially if both eyes are affected. Septo-optic dysplasia, also known as deMorsier syndrome, can occur in these cases and is associated with developmental delay and other brain and hormone regulation issues.
Megalopapilla is another condition, where the disc area of the eye is larger than normal but the rim area and nerve fibre layer are typical. It may be a natural variant of a normal optic disc. However, for some, visual acuity may decrease.
Possible Complications When Diagnosed with Optic Nerve Coloboma
The effect that optic nerve colobomas, a type of birth defect, initially have on a person’s vision can vary quite a bit; some people may have normal vision despite this condition. However, as the disease progresses, visual acuity or sharpness can decrease due to complications such as fluid build-up and retinal detachment at the optic disc pit, resulting in tissue decay or development of abnormal blood vessels beneath the retina.
People with an optic nerve head coloboma often exhibit a greater extent of abnormal tissue than those with optic pits. This defect, together with a poorly differentiated retina and pigment layer, an irregular lamina cribrosa (part of the optic nerve’s structure), and an enlarged scleral canal (the white part of the eye), potentially creates communication channels between the vitreous cavity (the eyeball’s filled space), the subretinal space, the subarachnoid space (a space around the brain and spinal cord), and the orbital space (eye socket). As a result, a retinal detachment can occur through such defect.
Retinal detachment treatment in patients with optic nerve colobomas can be challenging as there is a high chance of relapse. The hole close to the optic nerve cannot be directly sealed with laser, diathermy, or cryotherapy. However, if the vitreous body in the eye is pulling on this hole, inducing a posterior vitreous detachment may alleviate this tension and allow the retina to reconnect as in optic pits treatment.
Common Complications & Treatments:
- Decreased visual acuity due to complications such as retinal detachment
- Tissue decay or abnormal blood vessels underneath the retina
- Communication channels between different eye regions may cause a retinal detachment
- Treatment challenges due to propensity for recurrence
- Treatment options like inducing a posterior vitreous detachment for relieving tension
Recovery from Optic Nerve Coloboma
If a child has a severe visual impairment in one eye, it’s important that they wear safety glasses as soon as they are independently active, and goggles when participating in sports. Amblyopia, a condition where vision doesn’t develop properly in one eye, may not always be treatable. However, trying part-time occlusion therapy, which involves covering the good eye to make the poorly sighted eye work harder, can help demonstrate to parents how poor the vision in the affected eye might be.
Strabismus, a condition where the eyes don’t line up in the same direction, can be managed with surgery if needed. Sometimes, prescribing glasses to correct far-sightedness in the child’s good eye could correct a specific type of strabismus called accommodative esotropia, where one eye turns inward.
Children with nystagmus, a condition causing the eyes to make repetitive, uncontrolled movements, can also be treated surgically, particularly if it causes them to excessively turn their head to better focus their sight.
Preventing Optic Nerve Coloboma
It’s important for patients to understand the potential risks associated with optic nerve colobomas, conditions that can lead to vision loss. This includes conditions like retinal detachments and choroidal neovascularization. Retinal detachment is when the thin layer at the back of your eye, which senses light and sends images to your brain, pulls away from its normal position. Choroidal neovascularization is when new blood vessels grow beneath the retina and disrupt vision. Both can have serious effects on your eyesight.
