Dermoid cyst splitting the spinal cord.

Dear Sir,

With the advent of easily accessible Magnetic resonance imaging (MRI), the identification of even singular embryological aberrations which usually lie on a rare end of entire spectrum neural tube defects (NTDs) has increased, although the overall incidence is decreasing.[1] Split cord malformations (SCMs) with classic type I (bony spur) and type II (fibrocartilagenous) bands can sometimes be associated with dermoid cysts, teratoma, neurenteric cysts, and lipoma. It depends on what part of endomesenchymal tract differentiates. Here, we describe a case of complex SCM with dermoid cyst splitting the cord.

A 4-year-old boy presented with a tuft of hair and small sinus (non-discharging) over the lower back in midline. There was incontinence of urine since 1 week. However, he had no difficulty in walking. MRI scan showed SCM at the L1 level with low lying cord [Figure 1]. A standard laminotomy was carried out. Laminae were not hypertrophied. Thin fibrous band was seen as entering the dura through a tiny opening and merging with an intradural cyst. On opening the dura, a thin-walled cyst was seen mushrooming between split cords and a dividing fibrovascular band was seen as stalk of the cyst [Figure 2]. The cyst contained cheesy white material that was confirmed on histopathology as dermoid. Postoperatively the patient did not have any new deficits. On follow-up of 6 months, the patient had become continent.

Figure 1

(a) Coronal MRI showing split cord. (b) Sagittal MRI showing low lying cord with septum extending dorsally from skin and subcutaneous tissue across the spinal cord and vertebral bodies ventrally. Tract appears to be hyperintense at places, suggesting the presence of fat. (c) Axial MRI showing split cord

Figure 2

(a) The dermoid cyst (D) is seen on the dorsal aspect of the split cord. (b) Dermoid cyst (D) along with the endomesenchymal tract to reveal the two hemicords (HC). (c) After decompressing the cyst, the endomesenchymal tract (ET) was seen entering ventrally. (d) Two HC are seen after excision of the tract

The landmark theory of unified embryogenesis by Pang describes the presence of accessory neurenteric canal as a committal step in the genesis of SCM. Primitive neurenteric canal, which connects the yolk sac and amnion, is an unlikely cause of split cord because it opens into Hensen′s node which must lie opposite future coccyx. All split cords are always rostral to the coccyx.[2] This leads to the conceptualization of accessory fistula which joins the endoderm and ectoderm. The mesenchyme surrounding it condenses to form an endomesenchymal tract. Practically, endoderm seldom survives inside spinal cord; thus it is mesenchymal development that further decides the features of midline structures.[3] The formation of endomesenchymal tract early (< post-ovulatory day [POD] 21) or late (>21 POD) and uninvolvement or involvement of meninx primitiva determine the formation of SCM II or SCM I, respectively.[3] The endomesenchymal tract can be divided into anterior, intermediate, and posterior segments.[4] The persistence of anterior endodermal end can form intestinal duplication, neurenteric cysts. The persistence of intermediate part divides notochord and neural placode. The division of notochord leads to bifid or hemivertebrae, and neural placode leads to hemicords, i.e., SCM. The posterior end persistence leads to superficial lesions such as angiomas, hypertrichosis and/or deeper lesion such as dermal sinus, dermoid cyst or epidermoid cyst. The index case represents a dermoid cyst which was found splitting the spinal cord. The embryological reasoning for such splitting can be explained on the basis of unified theory.

The incorporation of dorsal ectoderm in the endomesenchymal tract in its posterior end can lead to differentiation of dermal elements and formation of dermoid, which represents an encysted part of dermal sinus entering from dorsal dural defect. This dermoid will be attached to fibrovascular stalk which is a remnant of endomesenchymal tract.[3]

Neurenteric cysts,[5] teratoma,[6] lipoma,[4] etc., has been reported sparingly as splitting lesion. Occurrence of such anomalies along with SCM is called complex SCM. Dermoid cysts are a common association with NTD. However, the dermoid which itself is splitting the cord and forming an SCM II has been reported and embryologically described by Pang. Isolated clinical presentation of such an entity has rarely been reported. In a series of 300 SCMs by Mahapatra AK, 5% have been shown to be associated with SCM, but whether they were splitting the cord or were present as separate tethering lesion has not been specified.[1]

This case is unique as it represents a rare occurrence of a classic description given by Pang.