Coloboma is a polyethological disease characterized by isolated or combined splitting of the iris, retina, choroid, optic nerve or eyelid. Common clinical manifestations of all forms of coloboma are a decrease in visual acuity, a feeling of pain in the eyes, asthenopic complaints. The specific diagnosis depends on the type of lesion and may include external examination, visometry, ophthalmoscopy, biomicroscopy, ultrasound in In-mode, CT, MRI. Conservative methods of coloboma treatment have not been developed. Surgical tactics depend on the form of the disease and may include peritomy, collagenoplasty, laser coagulation, vitrectomy.
A coloboma is an acquired or congenital pathology of the organ of vision, which manifests itself as a complete or partial splitting of its structures. The term coloboma was introduced by Walter in 1821, translated from Greek means “missing part”. The frequency of coloboma in the population is about 0.5-0.7 per 10,000 newborns. According to statistics, the iris coloboma is the most common among all forms (1:6000). Pathology occurs with the same frequency among men and women. Congenital cleavage of the choroid is one of the causes of blindness and visual impairment in childhood. The disease is most common in China (7.5:10000), the USA (2.6:10000) and France (1.4:10000).
Congenital coloboma is a genetically determined disease with a predominantly autosomal dominant type of inheritance. Variable mutations of the PAX6 gene are associated with a large number of eye malformations, including coloboma. At the same time, it has been proved that anomalies of DNA methylation of individual histones under the influence of environmental factors lead to epigenetic changes, which can also be a trigger of this pathology. Splitting of the structures of the eyeball is a consequence of a violation of the closure of the germinal fissure at 4-5 weeks of embryonic development. The cause of the congenital lesion may be infection of the mother with cytomegalovirus in the early stages of gestation.
Unlike other forms, the iris coloboma can be inherited both by autosomal dominant and autosomal recessive type. The autosomal dominant type is associated with a lesion of the PAX6 gene with localization in the short arm of chromosome 11. The type of mutated gene in the autosomal recessive transmission pathway has not been established. When a deletion of 24-DEL, NT1353 occurs, splitting of the iris is usually combined with microphthalmia. The development of the acquired form is influenced by factors such as alcohol abuse and hormonal imbalance. An iridoretinal coloboma is formed by mutation of the SHH gene of the long arm of chromosome 7, a chorioretinal coloboma is formed by a lesion of the GDF6 8q22 or PAX6 11p13 gene. The cause of the combined splitting of the optic nerve disc, retina and choroid is a mutation of the SHH gene with localization on chromosome 7q36.
Coloboma of the eyelids of acquired genesis often occurs due to tissue necrosis or scarring during traumatic injuries. Postoperative defects on the background of iridectomy with pathological neoplasms of the iris can also provoke this pathology.
From a clinical point of view, ophthalmology distinguishes coloboma of the iris, retina, choroid, optic nerve disc, lens and eyelids. When all the above structures are involved in the pathological process, a complete coloboma develops, with a less extensive lesion – a partial one. Splitting can be unilateral or affect both eyes. In a typical form, the defect is localized in the lower-nasal quadrant, which is due to the topography of the slit of the eye canal. Atypical coloboma is called with localization in other parts of the organ of vision. Clinical manifestations depend on the form of the disease.
With a coloboma of the iris, a pronounced cosmetic defect is formed, resembling a pear or a keyhole. Against the background of a normal or slight decrease in visual acuity, patients are unable to regulate the volume of light rays entering the retina. The result is a pathology of light perception. When the entire thickness of the vascular membrane is affected, the symptoms of scotoma (the appearance of darkening areas in front of the eyes) are added. Bilateral coloboma leads to the development of nystagmus. The disease can occur in isolation or be one of the manifestations of Charge or Ecardi syndromes.
The degree of decrease in visual acuity in coloboma of the optic nerve depends on the volume of the lesion. With an isolated defect, vision may correspond to the norm, in case of combination with microphthalmos or splitting of other structures of the eyeball, complete blindness is possible. Most patients report double vision, dizziness, binocular vision impairment in the form of strabismus. The disease is often accompanied by the development of a clinical picture of astigmatism of the myopic type (decreased visual acuity, distortion of the image in front of the eyes). The congenital form of pathology is often combined with Goldenhar, Down and Edwards syndromes, as well as epidermal nevus.
An isolated retinal coloboma is characterized by a latent course. Patients complain only when secondary complications develop in the form of ruptures accompanied by retinal detachment. The long-term existence of pathology leads to the formation of cattle. The anomaly is clinically manifested by areas of darkening in front of the eyes. When the lens is split, its refractive function is disrupted, which causes the formation of zones with different refraction. Clinically, the disease is manifested by symptoms of astigmatism. Coloboma of the eyelids, as a rule, is accompanied by incomplete closure, injury of the conjunctiva with eyelashes, the development of erosive defects. This pathology is often complicated by secondary conjunctivitis.
Methods of diagnosis of coloboma depend on the form of the disease. The coloboma of the iris is characterized by the appearance of a defect in the shape of a pear or a keyhole, which are visualized during external examination. During ultrasound biomicroscopy, the congenital form is in most cases accompanied by hypoplasia of the ciliary body. The ciliary processes have a shorter length and a larger width compared to the norm. The randomness of the fibers and the indistinctness of the structure of the zinc ligament indicates its underdevelopment. The degree of visual acuity reduction is determined by the method of visiometry.
With a coloboma of the optic nerve, a slight increase in its diameter is determined by ophthalmoscopy. Rounded light depressions with clearly bounded edges are visualized. The method of ultrasound scanning in In-mode or computed tomography (CT) allows you to detect deep defects on the posterior pole of the eyeball. In some cases, magnetic resonance imaging (MRI) determines hypoplasia of the intracranial optic nerve. After the age of 20, a regmatogenic retinal detachment is often formed. With progression in patients with pathological depressions in the optic disc, MRI signs of macular edema are determined, which often leads to rupture and detachment of the retina. Histological examination reveals concentrically oriented fibers of smooth muscles.
Coloboma of the vascular membrane during ophthalmoscopy is a white formation with scalloped edges. As a rule, the defect is localized in the lower parts of the fundus. Visometry determines myopia, the degree of which depends on the volume of the lesion. During biomicroscopy, the coloboma of the lens has the form of cleavage, which is located in the lower inner quadrant. The progression of pathology leads to pronounced deformation of the equator of the lens.
The tactics of treatment of coloboma depends on the form and extent of the lesion. With a slight splitting of the iris and the absence of violations of the function of the visual organ, surgical intervention is not indicated. With a decrease in visual acuity, it is necessary to perform peritomy with subsequent stitching of the edges of the iris. In order to prevent further progression of coloboma, the behavior of collagenoplasty is recommended. The purpose of surgical intervention is the formation of a collagen framework that prevents the progression of coloboma.
When the optic nerve is affected, laser coagulation is indicated only in patients with the formation of a subretinal neovascular membrane. Vitrectomy followed by laser coagulation of the retina is recommended when visual acuity decreases to 0.3 diopters with concomitant macular retinal detachment. In addition, the method of treatment of chorioretinal coloboma is endodrenation through an intermediate membrane with further laser photocoagulation of the retina around the marginal zone. With a pronounced coloboma of the lens, it is necessary to remove it with subsequent implantation of an intraocular lens. In turn, the defect in the form of splitting of the eyelid is eliminated with the help of blepharoplasty.
Prognosis and prevention
No specific preventive measures have been developed to prevent the development of coloboma. For the prevention of light perception disorders with iris coloboma, the use of mesh glasses or darkened contact lenses with a transparent center is recommended. Patients with primary manifestations of this pathology need to be examined twice a year by an ophthalmologist with mandatory visometry, biomicroscopy and ophthalmoscopy of the fundus. With a slight splitting of the structures of the eyeball, the prognosis for life and working capacity is favorable. Extensive damage can be complicated by a total decrease in visual acuity up to complete blindness, which leads to disability of the patient.