Journal of Neuro-Ophthalmology:
Washington University School of Medicine (CC), St. Louis, Missouri; and Department of Neurosurgery, Beijing Tiantan Hospital, Capital Medical University (KT, MZ, SL), Beijing, China.
The authors declare that there are no conflicts of interest in this study. No external funding, apart from the support of the authors' institution, was available for this study.
Address correspondence to Song Lin, MD, Department of Neurosurgery, Beijing Tiantan Hospital, Capital Medical University, No. 6 Tiantanxili, Beijing 100050, China; E-mail: email@example.com
A 51-year-old woman underwent craniotomy for removal of a pituitary tumor. At surgery, anomalous tissue was found projecting forward from the anterior angle of the optic chiasm. Termed “the chiasmal spur,” this anomaly has been described previously in 5 patients, all as an incidental finding at autopsy. Our case is the first instance of the chiasmal spur being discovered and photographed intraoperatively.
A 51-year-old woman reported oligomenorrhea, fatigue, and decreased vision in her left eye. Examination revealed normal acuity and visual fields bilaterally. The optic discs were normal in appearance. Brain MRI demonstrated a pituitary tumor, and the patient underwent a left transfrontal craniotomy.
During surgery, both optic nerves were exposed and it was discovered that additional tissue extended anteriorly 1 cm from the midline surface of the chiasm (Fig. 1). This tissue formed a spur midway between the optic nerves and was slightly adherent to the pituitary tumor. The spur tapered to a rounded point before reaching the sphenoid bone and appeared continuous with the optic nerves and chiasm. The pituitary adenoma was subtotally resected with care taken not to damage the spur in order to avoid any possible visual loss. The postoperative course was uneventful, and the patient's acuity and visual fields remained normal.
In 1961, Ellis et al (1) reported 4 cases of this optic chiasmal anomaly at autopsy and coined the term “chiasmal spur.” The patients' causes of death appeared neither to be related to each other nor to the presence of the spur. In each instance, the optic nerves appeared normal, and the spur extended 0.4-1.0 cm from the anterior margin of the chiasm (Fig. 2). The relationship between visual function and the spur was not documented in any patient. Histological analysis revealed myelinated nerve fibers arranged in a parallel longitudinal pattern (Fig. 3). In 2 of the reported cases, an additional process extended 0.2 cm from the posterior margin of the chiasm at the midline. Ellis et al (1) pointed out that in 1934, Volland (2) had first described this anatomic anomaly as an incidental finding at autopsy in a patient with bronchial carcinoma.
The etiology of chiasmal spur is uncertain. Ellis et al (1) reviewed Wilbrand's observation made in 1904 that axons from the inferonasal aspect of the optic nerve diverged into the contralateral nerve before reaching the chiasm. (3) This fiber bundle has become known as Wilbrand's knee. The authors speculated that the chiasmal spur was formed by isolation and fusion of Wilbrand's knee from each optic nerve.
In 1997, Horton (4) demonstrated in both primates and humans that Wilbrand's knee does not exist. Rather, this arrangement of retinal fibers forms gradually over a period of many years following monocular enucleation (Wilbrand's 2 patients had each suffered loss of an eye). Presumably, this shift in fibers occurs due to shrinkage and distortion of chiasmal anatomy. In addition, Lee et al (5) reported 3 patients who underwent resection of one optic nerve at the optic nerve-chiasm junction, and postoperatively, no perimetric evidence was found to support the existence of Wilbrand's knee.
A more likely explanation for chiasmal spur formation relates to development of retinal axon distribution within the optic chiasm. Since histologic analysis of the chiasmal spur revealed myelinated nerve fibers, it is likely that the anomaly found in our patient consists of neural tissue as well. The determination of crossed and uncrossed fibers within the optic chiasm is a complex process, which is not fully understood (6,7). Within the retina, gene expression of specific transcription factors and guidance receptors regulate the uncrossed projections. However, the means by which retinal axons are guided across the midline are still unclear. In addition, there appear to be transcription factors that affect development of the ventral diencephalon, which, in turn, also plays a role in axon divergence at the chiasm. Presumably, some aberration in the genetic programming of transcription factors and receptor proteins could lead to formation of the chiasmal spur.
While the chiasmal spur has been reported previously, our case is noteworthy as it is the first documented and photographed in the living patient. The presence of normal acuity and visual fields in our patient suggests that this anomaly has no detrimental effect on visual function. Given previous histologic findings that the spur contains myelinated nerve fibers, it would be prudent to avoid damaging it at the time of surgery.
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