Morning glory disc anomaly (MGDA) is a congenital optic disc dysplasia (1). The term reflects the morphologic similarity of the condition to the flower of the morning glory plant (2). The incidence of the anomaly is unknown, although most authors agree that it is rare (3). It consists of an enlarged, funnel-shaped, excavated disc surrounded by an annulus of chorioretinal pigmentary disturbance (4). Although MGDA may be isolated, numerous other ocular abnormalities are associated with it. Non-ocular associations include hypertelorism, basal encephalocele, agenesis of the corpus callosum, facial hemangiomas, renal anomalies, reversible and irreversible carotid artery narrowing, and hypopituitarism (1,4,5). Chiari type І malformation has not previously, to our knowledge, been described in association with MGDA. We report a single case of this association.
A 44 year-old woman with no visual complaints consulted an ophthalmologist for a routine eye examination. Owing to the finding of an abnormality in the right optic disc, the patient was referred for neuro-ophthalmologic consultation.
Best-corrected visual acuity was 20/40 in the right eye and 20/20 in the left eye. There was a relative afferent pupillary defect in the right eye. She identified 7/10 Ishihara color plates in the right eye and 10/10 in the left eye. Extraocular motility was full with normal ocular alignment. Intraocular pressures were 14 mm Hg in both eyes. The anterior segment appeared normal on biomicroscopy. In the right eye, ophthalmoscopy revealed an enlarged, funnel-shaped, excavated disc, a spoke-like configuration of retinal vessels emanating radially from the disc edge, and a surrounding annulus of chorioretinal pigmentary disturbance (Fig. 1A). The macula and peripheral retina were normal without any suggestion of retinal detachment. The ophthalmoscopic appearance of the left eye was normal.
There were no abnormal facial features (flattened nasal bridge or cleft lip) or neurologic or endocrine symptoms to suggest basal encephalocele. Brain MRI was normal except for a 12 mm caudal displacement of the cerebellar tonsils through the foramen magnum into the cervical spinal canal, typical of Chiari type І malformation (Fig. 1B).
We report a single patient with MGDA and Chiari I malformation, an association not described previously.
MGDA typically occurs in women (6) and is unilateral (7,8). Visual acuity in the affected eye is usually between 20/200 and counting fingers, although patients with 20/20 acuity or no light perception have been reported. The diagnosis of MGDA is usually made clinically (9). The typical ophthalmologic findings are an enlarged optic disc with a funnel-shaped scleral defect, an elevated peripapillary tissue annulus referred to as a chorioretinal pigmentary disturbance, and pale, whitish, fluffy glial hyperplasia overlying the optic disc. Additional features include radially arranged straight, narrow retinal vessels (9). MGDA is not typically an inherited condition or part of a multisystem genetic disorder, although it has been reported as part of the renal-coloboma syndrome and trisomy 4q (7).
There is controversy regarding the etiology of MGDA. Some believe it to be a form of optic disc coloboma resulting from defective closure of the embryonic fissure. Others interpret the central glial tuft, vascular anomalies, and scleral defects, together with the histologic findings of adipose tissue and smooth muscle within the peripapillary sclera, to signify a primary mesenchymal abnormality (1,8). It is clear from histopathologic studies that retinal pigment epithelium is present in the scleral defect and that a mesodermal hypothesis is more likely (10). Another theory proposes that an abnormal enlargement of the distal optic stalk during development of the eye allows the inner layer of the optic cup to enter, causing an excavation at the entry site (8).
Chiari I malformation is a downward descent of the cerebellar tonsils and is regarded as part of a pathologic continuum of increasingly severe hindbrain maldevelopments (11). Chiari described three types of this entity, of which type I is the most common (12,13). It is defined as herniation of one or both cerebellar tonsils at least 5 mm below the level of foramen magnum (6,11,12,14,15). Diagnosis is established by sagittal MRI of the craniocervical junction (6,12). Other MRI findings may include reduced length of the clivus, retroflexion of the odontoid process, compression of the fourth ventricle, syringobulbia, small or absent cisterna magna, and obliteration of the retrocerebral cerebrospinal fluid (CSF) spaces in the posterior fossa (12,14). Milhorat et al (12) reported the last finding in all 364 patients in their study. Chiari type I malformation is distinguished from Chiari type II and Chiari type III malformations, which include not only downward displacement of the lower cerebellum and medulla into the spinal canal but also other complex anomalies of the brain and spinal cord (16).
Chiari type I malformation usually becomes symptomatic in the second or third decades of life, although it may remain asymptomatic (6,11,17). The symptoms reported by most patients with Chiari type I malformation are typically nonspecific and nonlocalizing, such as headache, retrobulbar pressure, blurred vision, floaters and flash lights, photophobia, intermittent diplopia, dizziness, disequilibrium, poor coordination, hoarseness, chronic cough, sleep apnea, dysphagia, palpitation, numbness, pressure in the ear, decreased hearing or hyperacusis, vertigo, and oscillopsia (11). Objective findings such as nystagmus (classically downbeat on vertical gaze), cerebellar signs, and cranial nerve deficits occur in only a minority of patients (11). Owing to these nonspecific signs and symptoms, approximately 50% of patients have been told that they suffer from a psychogenic disorder before a Chiari 1 is diagnosed (11). On the other hand, no direct correlation has been observed between symptoms and the anatomic severity of herniation, so that the pathophysiologic characteristics of the disease cannot be explained by the abnormal anatomy of the craniospinal junction alone (17). Clinical manifestations vary among patients with similar amounts of tonsillar ectopia (17).
The presence of tonsils in the foramen magnum is believed to alter CSF flow (18). CSF flow studies using cine flow MRI have demonstrated that many patients with a symptomatic Chiari type I malformation have abnormal CSF flow at the foramen magnum (19). The newly formed CSF is displaced from the compressed subarachnoid spaces of the posterior cranial fossa into available spaces within the supratentorial and spinal compartments. Such displacements affect CSF compliance and can alter the normal damping effect of an open CSF system, which induces changes in the venous volume and pressure occurring with respiration, Valsalva maneuvers, changes in posture, and cardiac cycle. Although the CSF displacement could play a role in the suboccipital headache, retrobulbar pain, visual phenomena, and vertigo, the most obvious CSF-related symptoms are those attributable to syringomyelia (12).
The resolution of comitant esotropia and papilledema after posterior fossa decompression is further evidence that abnormal CSF flow has a pathogenetic role in Chiari malformation (6,11). Because this condition may be progressive, it is recommended that patients with Chiari type I malformations found incidentally receive regular neurologic and imaging follow-up including cine flow MRI to determine the status of CSF flow at the level of craniocervical junction (20,21).
The association of Chiari type II and Chiari type III malformations with embryologic defects of the brain and spinal cord has established these lesions as primary neural anomalies (12,14). However, there is clinical and experimental evidence that chronic tonsillar herniation in Chiari type І malformation could be attributable to underdevelopment of the occipital bone and overcrowding of the cerebellum within an undersized posterior cranial fossa (12,22). Results of recent morphometric studies are consistent with this view, and Nishikawa et al (23) suggested that the fundamental defect might involve underdevelopment of the occipital somites originating from the para-axial mesoderm.
The proposition that Chiari type І malformation is a disorder of mesodermal origin is supported by the following findings: 1) neurologic examinations and MRI of the brain and spinal cord have failed to provide any evidence of neuroectodermal defects; 2) the neural abnormalities are attributable to the secondary effects of chronic tonsillar herniation; 3) the incidence of neural anomalies among close relatives of affected individuals is similar to that in the general population; and 4) the MRI findings of reduced height of the supraocciput, increased slope of the tentorium, hypoplasia of the clivus, and osseus abnormalities at the craniovertebral junction are consistent with a defect of the para-axial mesoderm (23).
The association of MGDA with Chiari type І malformation in our patient supports the concept of a primary mesenchymal defect as the etiology of both abnormalities. Speculation about the embryology of MGDA points to the mesoectodermal dysgenesis of the optic nerve occurring in the first trimester (5). This timing also coincides with the timing of midline brain defects and formation of bony parts of the posterior fossa (5).
We do not think that the association of MGDA and Chiari type I malformation is accidental. Both conditions are rare and are likely to be mesodermal dysgeneses. These abnormalities have been independently reported in the renal-coloboma syndrome (7,24,25).
The authors thank Dr. Mohammad Hadi Bagheri, neuroradiologist and head of the radiology department of Shiraz University of Medical Sciences, for his assistance in the review of MRI of our patient and preparation of this article.
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