Megalocornea is a rare genetically determined disease characterized by an increase in the diameter of the cornea. The severity of clinical manifestations varies from an asymptomatic course to such nonspecific symptoms as decreased visual acuity and the appearance of “fog” in front of the eyes. The main diagnosis of megalocorneia includes an objective examination, ultrasound in A-scan mode, pachymetry, refractometry, gonioscopy, tonometry, visiometry and calculation of the vitreal index. This pathology in most cases does not require special therapy. Only correction of visual acuity disorders and treatment of secondary complications are carried out.
Megalocornea is a rare non-progressive disease, the main sign of which is an increase in the diameter of the cornea. The first description of megalocornea as an X-linked disease was presented in 1914 by the German scientist B. Kaiser. Until that time, it was considered an abortive type of congenital glaucoma. Since the main way of inheritance of megalocorneia is X-linked, mostly men are ill (90% of cases). As a rule, pathology is diagnosed in early childhood. The disease is widespread everywhere. Disability of patients against the background of megalocorneia rarely occurs, in most cases it is associated with the development of secondary complications.
Megalocornea is a genetically determined pathology. The disease is usually inherited by a recessive, X-linked type. The cause of development is a mutation in the MGC1, MGCN genes. In this case, defective genes are localized on the long arm of the X chromosome (Xq 21.3-q22). In rare cases, this anomaly is inherited by autosomal dominant or autosomal recessive type.
The mechanism of megalocorneal development is based on violations of the early stages of embryonic development of the organ of vision. Normally, the shape of the eye glass changes from bell-like to spherical. In the case of megalocornea, the size of the diameter of the anterior opening of the eyeglass for a long period of time corresponds to the equatorial size. The consequence of these processes is that the diameter of the ciliary girdle with all the anatomical structures of the anterior part of the eye increases accordingly to the posterior segment of the eyeball. In turn, the endotheliocytes of the cornea are distributed normally, but over a longer length. This leads to an increase in the diameter of the cornea with its subsequent thinning in the central parts.
Megalocornea is a bilateral disease, usually not prone to progression. The main symptom of this pathology is revealed in early childhood during examination – the cornea is enlarged in diameter, with preserved transparency. Clinical manifestations of megalocornea depend on the severity of disorders in the structure and thickness of the cornea. In the absence of changes, visual functions remain within normal limits. Bilateral damage to the cornea is often accompanied by dystrophic disorders (mosaic dystrophy) and pigment spraying. Most often, patients experience a slight decrease in visual acuity (often both in the distance and near) and a feeling of distortion of objects in front of their eyes, which is associated with the development of myopia and astigmatism. Sometimes megalocornea is complicated by lens subluxation, which is caused by stretching of the ligamentous apparatus. In this case, patients show clinical symptoms of iridodonaise – trembling of the iris, which occurs with sudden eye movements.
The clinical picture associated with an increase in intraocular pressure is found only with such a complication as pigment dispersion syndrome with the subsequent development of secondary glaucoma. In this case, the pigment is redistributed from the pigment layer of the posterior leaf of the iris. Melanocytes settle on the structures of the posterior part of the eyeball. This leads to obstruction of the trabecular network of the anterior chamber angle with impaired outflow of intraocular fluid. Patients complain of fog in front of their eyes, painful sensations, decreased visual acuity, the appearance of “rainbow circles” when looking at a light source. In severe cases, stretching of the iris is possible, followed by hypoplasia of the stroma and increased pigmentation of the trabecular network. The result of these changes is the formation of the Krukenberg spindle.
Megalocornea can develop as an independent disease or occur against the background of such genetic pathologies as Aarskog, Marfan and Knist syndromes. The most common concomitant pathologies on the part of the organ of vision are ectopia of the lens and pupil, congenital myosis, pigmented glaucoma, subcapsular cataract. In turn, megalocornea can act as one of the symptoms of hydrophthalmos.
Diagnosis of megalocorneum is based on anamnestic data, the results of objective examination, ultrasound examination in A-scan mode, pachymetry, refractometry, gonioscopy, tonometry, visiometry and vitreal index calculation data. Most patients have a burdened hereditary history of this disease (maternal grandfathers are more often ill in the family, because mothers are carriers of the pathological gene). An external examination reveals an increase in the diameter of the cornea (more than 13 mm). The depth of the anterior chamber of the eye during gonioscopy in most patients exceeds the norm (2.75-3.5 mm).
By the method of visiometry, it is possible to establish a change in visual acuity, which may correspond to the norm or deviate somewhat towards weak myopia, hypermetropia. Astigmatism is also often determined. Intraocular pressure, measured by tonometry, often corresponds to the norm and ranges from 10 to 21 mm of mercury column. In the case of the addition of the clinical picture of secondary pigmented glaucoma, its increase is observed. The results of pachymetry, as a rule, indicate a decrease in the thickness of the cornea in the central part. When performing an ultrasound of the eye in the A-scan mode, an increase in the longitudinal size of the eyeball is observed. The vitreal index (the ratio of the length of the vitreous body to the longitudinal size of the eyeball) at an early age is lower than the standard indicators, and at an older age exceeds the norm.
Additional research methods in ophthalmology are keratotopography, keratometry, optical coherence tomography of the cornea (OCT). Keratotopography allows you to identify the pathology of sphericity, keratometry – a change in the curvature of the longitudinal meridians located on the surface of the cornea, as well as measure the radius of its curvature. With the help of OCT, a violation of the spatial relationship between the structures of the eyeball, an increase in its longitudinal size and depth of the anterior chamber of the eye, a violation of the functioning of the drainage system and deep changes in the cornea (keratitis, dystrophy, ulcers) is determined.
In the absence of impaired visual acuity, normal refraction and unchanged intraocular pressure, megalocornea does not require special treatment. When detecting refractive errors (myopia, hypermetropia, astigmatism), timely correction of vision with glasses or contact lenses is necessary. In this group of patients, surgical interventions are difficult due to insufficient pupil dilation, possible lens subluxation, high risk of rupture of the posterior capsule and displacement of the implanted intraocular lens. In case of secondary cataract development against the background of megalocorneum, cataract phacoemulsification is performed, followed by implantation of a special intraocular lens (IOL). The operation is performed through a corneoscleral incision. The technique of anesthesia is retrobulbar anesthesia.
Prognosis and prevention
Specific measures for the prevention of megalocornea have not been developed. With this pathology, patients need to undergo regular examination by an ophthalmologist, because after 30 years, patients with megalocornea are at high risk of developing subcapsular cataracts. Preventive examination should include mandatory visi- and tonometry. If an increase in the size of the cornea is suspected, it is necessary to measure its diameter as early as possible. The prognosis of megalocornea with timely diagnosis and correction of visual impairments is favorable for life and work capacity. Total vision loss up to complete blindness develops only against the background of secondary complications in the form of cataracts or glaucoma.