REPLY
=====

* Michael R. Watters
* John C. Stears
* William T.C. Yuh
* Bradford S. Burton
* Anne G. Osborn

We would like to thank Dr. Tekkök for his letter regarding our report (1) describing five cases of transdural spinal cord herniation and a review of 25 additional cases from the medical literature. A companion article (2) published in the same issue of *AJNR* described a case, as well as provided references for two cases (3, 4) from literature not included in our review. Dr. Tekkök provides literature references for three other cases (5–7) published prior to the submission of our article, as well as three cases reported subsequently (8). Collectively, this adds 10 additional cases, and we concur that such information would be of interest to readers of the *AJNR.*

Dr. Tekkök is correct to note that a history of remote trauma may be obtained from some patients, occurring decades prior to clinical presentation. Whether such is “irrelevant or trivial,” as suggested by Dr. Tekkök, is impossible to determine with present information. We chose to classify cases in which a history of accidental trauma was described by the patient (or author) as *post-traumatic,* and considered the possibility that such trauma precipitated the initial dural tears.

Most reported cases of transdural spinal cord herniation do not occur acutely after an identifiable event, although such has been reported (7). The majority occur without apparent provocation (which we and some others had termed as *spontaneous,* others as *idiopathic,* herniations). It has been proposed (2, 5, 9–13) that such patients have dural openings on a congenital basis, initially asymptomatic, although subclinical trauma unrecognized by the patient is also possible. The role minor or major trauma play in facilitating the development of spinal cord herniation through established dural defects remains to be determined and may not be as important for patient management as clinical and imaging features. Clinical presentations with myelopathic signs characteristically do not appear until after age 30 years. Clinical deterioration once symptoms begin is gradually progressive for another 1 to 6 years, until surgical correction of the transdural cord herniation has been made. A similar delay of several decades or more seems equally plausible for patients with a remote history of trauma, initially asymptomatic (clinically too “trivial” to cause symptomatic spinal cord injury), but perhaps sufficient to tear the dura. Such proposed mechanisms would permit the initial dural opening to have existed for many years prior to the onset of symptoms, regardless of what subsequent processes result in cord herniation through an established dural defect. Whether or not all patients who have dural openings (whether congenital or acquired) who survive past age 30 will develop symptomatic transdural herniation is unknown. An isolated dural defect is probably less obvious on imaging studies than when associated with spinal cord displacement, and imaging studies are less likely to be obtained during asymptomatic periods. Perhaps such cases will be identified in the future to define the natural history of the condition better.

Whether a dural tear occurs spontaneously, post-traumatically, or congenitally eventually may be shown to affect the natural history and course of the transdural cord herniation. We have thus far found only limited clinical differences. Our review of 30 cases, and the additional 10 cases in references provided by Dix et al (2) and Dr. Tekkök, indicate that the absence of a history of remote trauma is more often associated with female gender, and more likely to manifest clinically as a Brown-Séquard syndrome. When a history of remote trauma (not acutely associated with myelopathic symptoms and signs) is uncovered, the patient is more likely to be male, and more likely to manifest diffuse myelopathic signs. Our intent was not to resolve the mechanisms of the dural opening or transdural herniation of the cord, but to provide imaging data to facilitate clinical diagnosis and possible surgical intervention. To our knowledge our report of five cases is the largest series to date, as is our review of 25 previously reported cases, and is a broad review of cases of possibly differing etiologies. We appreciate the additional case references and comments provided by Dr. Tekkök, and acknowledge the limitations that case studies afford regarding pathologic and etiologic mechanisms. We suggest as a goal the earlier identification of transdural herniation. The increasing and disseminated knowledge of the process may lead to earlier evaluations. We suggest prospectively improved MR studies, better understanding of the MR details (as we have attempted), and additional operative and histologic detail.

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