Keratoglobus is a pathology characterized by thinning of the cornea in the peripheral parts, followed by an increase in the radius of curvature and the acquisition of a spherical shape. It is clinically manifested by a decrease in visual acuity, double vision, lacrimation and photophobia. Diagnostics is based on anamnesis data, examination results, visometry, tonometry, ultrasound measurement of the longitudinal size of the eyes, pachymetry, ultrasound biomicroscopy and keratotopography. Contact vision correction is recommended in the early stages. Specific surgical treatment is reduced to through subtotal keratoplasty or UV-crosslinking.
Keratoglobus is a hereditary, rarely acquired disease in which the synthesis of elastin is disrupted, as a result of which the cornea takes the shape of a globe. Pathology was first described in 1907 by the Greek ophthalmologist A. Trantas. The prevalence is less than 1.7% among all corneal ectasia. Men are most often ill (83%), which is associated with an anatomical predisposition. In men, the cornea is normally 0.09 mm thinner on average than in women. Modern diagnostic methods in ophthalmology make it possible to detect keratoglobus from the age of 6 months, but in most cases the diagnosis is made at 6 years of age during a preventive examination. The disease often leads to the development of complications and can cause complete loss of vision.
The most common cause of keratoglobus development is a genetically determined defect in elastin synthesis. The ELM gene is responsible for the formation of this fibrillar protein. The occurrence of a mutation in certain loci of the ELM gene or a decrease in the number of desmosins leads to a violation of the formation of cross-links. At the same time, the strength threshold for the elastic fibers of the cornea decreases. The progression of dystrophic changes in the peripheral parts causes its thinning and loss of elastic properties. As a result, the cornea irreversibly acquires a spherical shape. According to the results of recent studies by R. S. Chuck, the anterior limbal lattice layer contains elastin and collagen, which provide strength, elasticity and stimulate the regeneration of the cornea, since they are located under the ball of stem cells. Pathology of this layer is also one of the triggers for the development of keratoglobus.
In rare cases, the violation of elastin synthesis has a systemic character. This leads to a combination of changes in the radius of curvature of the cornea with other connective tissue pathologies (Ehlers-Danlos syndrome type 4, Marfan syndrome, Rubinstein-Tabey syndrome). Also, the congenital form can develop against the background of “blue sclera” syndrome or Leber’s amaurosis. Acquired keratoglobus occurs less frequently. Etiological factors may be dystyroid ophthalmopathy, allergic keratoconjunctivitis or a history of chronic marginal blepharitis. In rare cases, keratoglobus is the terminal stage of keratoconus development, as indicated by the results of histopathological and immunohistochemical studies.
The change in the shape of the cornea leads to the fact that the depth of the anterior chamber of the eye becomes 2-2.5 times greater than normal. The progression of the disease is accompanied by an increase in the radius of curvature of the cornea, which leads to an increase in the longitudinal size of the eyeball. As a result, refraction is impaired, which is manifested by an increase in clinical manifestations of myopia and astigmatism. The cause of the development of acute keratoglobus is a rupture in the zone of the descement membrane or stroma.
From a clinical point of view, acute (corneal dropsy) and chronic keratoglobus are distinguished. Dropsy of the cornea is usually considered as a disease that occurs against the background of a chronic course of pathology. The first manifestations of keratoglobus develop in childhood. An early symptom is a progressive decrease in visual acuity, difficult to correct. Changing the shape of the cornea leads to a deformity of the eye, similar to astigmatism. Patients complain of double vision and distortion of the image in front of the eyes. This is due to a violation of binocular vision in patients with unilateral lesion. Bilateral keratoglobus is rarely manifested by symptoms of strabismus. As a rule, when both eyes are involved in the pathological process, anomalies of stereoscopic vision occur against the background of corneal edema.
An increase in the longitudinal size of the eyeball due to the spherical shape of the cornea is one of the risk factors for increased injury. It is harder for patients to close their eyes without additional effort. Corneal irritation is not accompanied by activation of the corneal reflex. In the later stages, the friction of eyelashes on the cornea leads to increased lacrimation. The formation of ulcerative defects or the addition of symptoms of keratitis is clinically manifested by photophobia.
Most patients report increased eye fatigue and the appearance of headaches with prolonged visual load. Keratoglobus can develop as an isolated disease or occur against the background of Van der Hoove syndrome or Leber congenital amaurosis. Clinically, Van der Hoove syndrome is manifested by blue sclera, osteoporosis and decreased hearing acuity. Classical Leber’s amaurosis is characterized by a decrease or loss of visual acuity without anatomical prerequisites. The combination of keratoglobus with these pathologies is prognostically unfavorable. The most common complication is keratitis.
Diagnosis of keratoglobus is based on anamnestic data, the results of physical examination, visometry, tonometry, ultrasound measurement of the longitudinal size of the eyes, pachymetry, ultrasound biomicroscopy and keratotopography. Patients often have a history of keratoglobus in one of their parents or close relatives. Less often, the disease develops against the background of other visual organ abnormalities or connective tissue pathology. When examining the cornea, a pronounced change in shape (spherical iris), scarring and erosive defects can be detected. The degree of visual acuity reduction during visiometry depends on the severity of keratoglobus. At the beginning of the development of the disease, a weak degree of myopia is diagnosed (up to -3 diopters). The progression of pathology without adequate treatment leads to a high degree of myopia (more than -6 diopters). As a rule, tonometry data indicate normal or slightly elevated intraocular pressure.
In acute keratoglobus, the method of ultrasound biomicroscopy makes it possible to visualize the rupture of the descemet membrane (more often vertical) in the central parts and stroma edema. In the chronic form of the disease, the thickness of the cornea in the center is normal or slightly reduced. There is a pronounced thinning on the periphery. The depth of the anterior chamber of the eye usually increases to 8-10 mm. Other anatomical structures of the eye (iris, lens) are within normal limits. Keratotopography is carried out in order to determine the curvature of the cornea in the central part (more than 3 mm) in 2 meridians (SimK1, SimK2), the sphericity index (below -0.26) and indicators that reveal changes in uniformity and optical properties.
For a more accurate diagnosis of changes in the thickness of the cornea, pachymetry is recommended. In this pathology, the thickness of the cornea in the limb area is less than 0.7 mm, in the central part – about 0.545 mm. The method of ultrasound measurement of the longitudinal size of the eyes reveals an elongation of the sagittal axis (more than 24 mm), which corresponds to the degree of myopia.
In the early stages of keratoglobus development, vision correction using contact lenses or glasses is effective. With a pronounced change in the radius of curvature of the cornea, surgical treatment is recommended. Previously, the tactics of surgical intervention were reduced to performing subtotal end-to-end keratoplasty. In this operation, the donor cornea is transplanted with its preliminary irradiation with a helium-neon laser. The graft is fixed along the periphery of the cornea. Due to the decrease in the thickness of the peripheral parts of the cornea in the postoperative period, it is necessary to exclude sudden head movements.
The modern method of treatment is ultraviolet cross-linking of corneal collagen. The mechanism of action of the technique is based on the effect of photopolymerization of stroma fibers when exposed to them with a solution of riboflavin, which acts as a photosensitizer. At the same time, collagen is cross-linked, thanks to which the stromal component is normalized. Surgical intervention is performed under regional anesthesia. After cross-linking, the cornea is irrigated with a saline solution followed by the use of antibacterial drops. In the early postoperative period, local use of antibacterial drops and oral administration of diuretics is recommended in order to normalize the volume of circulating blood. Glucocorticosteroids are indicated only after the restoration of the corneal structure.
Prognosis and prevention
Specific measures for the prevention of keratoglobus have not been developed, since in most cases this pathology is congenital. In order to prevent the recurrence of the disease in the postoperative period, active head movements should be limited and traumatic injuries should be avoided. With a burdened hereditary history, it is necessary to be examined by an ophthalmologist 2 times a year from the age of 6 months. Detection of myopia or astigmatism is an indication for early correction.
With timely treatment, the prognosis for life and working capacity is favorable. In severe cases, the disease leads to complete loss of vision and disability of the patient. When keratoglobus is combined with other connective tissue pathologies, as well as when it occurs against the background of blue sclera syndrome or Leber congenital amaurosis, the prognosis is usually unfavorable.