Signs and Symptoms
Tilted disc syndrome (TDS) is a unilateral or bilateral congenital optic disc anomaly that may be discovered in patients of any age, with an incidence of 2% in the general population.1 There is neither a sexual predilection nor an identifiable hereditary pattern.1
The ophthalmoscopic appearance is variable.2 In TDS, the disc appears to be rotated about its axis with the long axis of the disc approaching the horizontal meridian in extreme cases. Instead of a vertically oriented disc, the nerve fibers appear shifted so that the superior portion of the disc seems to be positioned in the superior nasal quadrant, giving the disc a D-shaped appearance.3,4 In many cases, the major retinal vessels emerge from the disc, immediately run nasally, then abruptly turn and course temporally in the traditional vascular branching pattern. This vascular anomaly is termed situs inversus.3,5,6
Despite varied appearances, there are some consistent findings in TDS. The most consistently encountered is a conus in the inferior and inferior nasal aspect of the peripapillary retina contiguous with the optic disc. In some cases this anomaly, termed Fuch’s coloboma, can involve the inferior aspect of the disc with apparent rim thinning or obliteration with a pseudoglaucomatous appearance. This inferiorly located conus is associated with significant ectasia as well as staphylomatous formation within this localized area.1,3,7,8 The colobomatous formation may extend inferiorly outward from the disc and manifest as hypoplasia of the retina, retinal pigment epithelium and choroid, appearing as a lightly pigmented fundus.1-5 Other findings encountered with TDS include myelinated nerve fibers, lacquer cracks, choroidal folds, foveal retinal detachment and retinoschisis and peripapillary choroidal neovascular membranes with subretinal hemorrhages.1,9,10-13
Visual acuity is unaffected in TDS; however, visual field loss is common. The most commonly encountered visual field defect is a superior temporal scotoma.1,14-18 In cases where TDS is bilateral, this can appear as superior bitemporal scotomas suggestive of chiasmal compression.19 However, in TDS, the visual field defect is unchanging and does not respect the vertical hemianopic line as it would in a chiasmal compressive mass, thus helping to distinguish the two conditions.14-18 Other potential visual field defects include arcuate scotoma, nasal contraction and an enlarged blind spot.16
The most commonly encountered refractive error in patients with TDS is myopic astigmatism at an oblique axis.1,6,16 There has been conjecture that the refractive error results from fundus alterations seen in TDS.5 However, it has been seen that clinically significant lenticular astigmatism was present in TDS patients.20 In another report, researchers found that in the majority of tilted disc cases, astigmatism was mainly corneal, suggesting that morphogenetic factors in the development of the tilted disc might possibly influence the corneal development in such a way as to result in corneal astigmatism.21 It has been noted that color vision abnormalities, consisting of red-green, blue and mixed defects were found in eyes with TDS.22
Contrary to popular belief, there is no actual tilting or rotation of the disc in TDS, even though the disc may appear to be rotated by as much as 90 degrees about its axis. TDS actually represents a congenital coloboma due to incomplete closure of the embryonic fetal fissure at six weeks gestation.23 During development, the eye first appears in the form of the optic sulci in the fourth week of gestation. The optic vesicle forms from growth of the optic sulci towards the surface ectoderm. As the optic vesicle reaches the surface ectoderm, it invaginates to form a goblet-shaped optic cup. Incomplete closure upon invagination could result in a coloboma potentially involving the disc, retina and RPE.2,23
The inferior aspect of the disc (and adjacent fundus) has a congenital absence of tissue.3,4,8,24 Automated perimetry has disclosed reduced mean deviations in this and other areas of the visual field. Perimetric findings also support the theory that TDS is a variant of optic nerve hypoplasia.25 The colobomatous formation affects the shape of the chorioscleral canal due to a deficiency in the choroid, neural retina and RPE. As such, the nerve fibers will be concentrated in the superior and superior temporal aspect of the disc, while the inferior and inferior nasal section will be deficient in axons.3,4,8,24 This gives the nerve a D-shape with the flat edge along the area of the conus. The congenital absence of tissue in the inferior nasal aspect of the nerve may be significant enough that the patient will have a corresponding superior temporal visual field defect that does not respect the vertical hemianopic line.14-18
More recently, OCT has revealed that there is a sloping of the lamina cribrosa posteriorly from the upper to the lower part, a protrusion of the upper edge of Bruch’s membrane and choroid in eyes with TDS. The abnormalities detected by swept-source OCT and 3D MRI analyses indicate the possibility that the essential pathology of TDS is a deformity of the inferior globe below the optic nerve.26 OCT images of the optic discs show a protrusion of the upper edge of Bruch’s membrane and choroid at the nasal edge of the optic disc, with the retinal nerve fiber tissue herniating into this protrusion and bent superiorly, possibly contributing to visual field defects.27
The staphylomatous and ectatic formations caused by the incomplete fetal-fissure closure producing the conus also theoretically stretch the tissues, permitting secondary lacquer crack formation. These breaks in Bruch’s membrane may lead to the development of choroidal neovascular membranes with subsequent subretinal hemorrhages.2,7,9 Additionally, OCT has demonstrated that the subfoveal choroid is relatively thin and the subfoveal sclera thickened in some eyes with TDS.28 These changes have been associated with choroidal neovascularization and serous retinal detachment.
As TDS is a congenital anomaly, there is no management for the finding itself. In cases where choroidal neovascular membranes form as a result of TDS, the visual outcomes tend to be quite good, in that the membranes are very responsive to photocoagulation or demonstrate no progression, and may even involute without treatment.9 One report of a single patient indicated that intravitreal Lucentis (ranibizumab, Genentech) was effective in the management of choroidal neovascularization at the border of an inferior staphyloma associated with tilted disc syndrome.29 However, another report on three patients with TDS-related choroidal neovascularization showed no visual benefit after intravitreal Avastin (bevacizumab, Genentech) treatment.30 Serous retinal detachments secondary to TDS respond poorly to intravitreal Avastin treatment.31,32
The most important factor in managing TDS is proper diagnosis. The heaped-up axons in the superior aspect of the nerve in TDS are frequently misdiagnosed as either disc edema or papilledema. Also, the inferior nasal conus and possibly colobomatous extension into the disc is frequently misdiagnosed and treated as normal tension glaucoma. Further, the superior temporal defect in TDS can be confused with chiasmal compressive disease, especially when TDS is bilateral.
• There is a varied ophthalmoscopic appearance to TDS. However, the most diagnostic feature of TDS is the inferiorly located conus.
• The main differentiating factors between the visual field defect in TDS and chiasmal compressive disease is that the field defects in TDS are nonprogressive and do not respect the vertical hemianopic midline. However, depending upon the perimetric technology used, the defect may seemingly respect the vertical hemianopic line.
• TDS is often misdiagnosed as disc edema, papilledema, normal tension glaucoma and pituitary tumor.
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