A congenital dermal sinus (CDS) is a tract lined by stratified squamous epithelium found in or very near the midline anywhere from the nasion to the coccyx. with none, however, being reported to enter the skull in the region occupied by the superior sagittal sinus. The tract may end just below the skin surface or may extend to the conus medullaris or the central canal of the spinal cord from the back, the fourth ventricle from the occipital region or the crista galli from the nasion. Excluding dimples in the sacrococcygeal region, the incidence of CDS appears to be approximately 1 in 2500 to 3000 live births.

Embryology and Pathology

A CDS is a form of dysraphism. These lesions result from a failure of normal midline fusion between the third and fifth weeks of gestation and appear to reflect abnormal adhesions between the ectoderm destined to form the neural tube from that of the dermis, with elements of the latter becoming trapped within or near the developing neural tube. Depending on the degree of incomplete separation, the tract may end in the subcutaneous tissue or may extend any distance inward to its ultimate embryologic level, which would be the conus medullaris for lesions located in the lumbosacral region, the central canal of the spinal cord for those at a higher level, the fourth ventricle for those in the occipital region, and the crista galli for lesions in the nasal area. As is true with other forms of dysraphism, a CDS frequently, though not invariably, has one or more associated cutaneous markers. CDS can also be found with other forms of dysraphism.

A CDS in the lumbosacral region or higher on the spinal axis occurs during the process of neurulation when the neural groove closes to form the neural tube. Neurulation forms the spinal cord to the conus medullaris, which represents the area of the posterior neuropore. As is true with other forms of dysraphism, CDSs most frequently are found in the lumbosacral region, with other locations along the spinal axis being quite rare. A CDS ­ or in reality a dermal dimple - in the low sacral or coccygeal area appears to form by a different embryologic process. referred to as canalization of the tail bud. The ultimate embryologic level for these dermal dimples is the fascia over the lower sacrum or coccyx. Embryologically it is not possible for the tract to extend to the subarachnoid space (SAS), in fact, the orientation of these tracts tends to be caudal rather than rostral.

A CDS has both dermal and epidermal elements since the primitive ectoderm has the capacity to form all components of the skin. The epidermal component consists of a well-differentiated stratified squamous epithelium, that of the dermis includes hair follicles, sebaceous glands and sweat glands. Embryologically one would not anticipate finding a teratoma associated with an isolated CDS and in fact this association seems to be quite rare. The CDS can peter out into a connective tissue band or nodule or may enlarge to form one or more dermoid cysts. Keratin debris from the desquamating epithelium and secretions from the sebaceous and sweat glands can produce a slowly expanding encapsulated mass containing material that can vary from pearly white and waxy to yellowish and buttery in color and consistency. The capsule of the dermoid is usually quite adherent to the surrounding neural structures and becomes even more densely adherent in the face of inflammation from infection or from the fatty acids of the degenerating material within the capsule, should they gain access to the SAS. Although dermoid cysts can be found at one or more locations along the sinus tract, they more commonly occur at its terminus. All cysts associated with a CDS should embryologically be dermoids; however, some confusion in this regard does exist. Some reports, especially in the older literature, do not correctly differentiate between dermoids and epidermoids associated with CDS. In addition, tissue sent for histopathologic examination may not represent the lesion in its entirety or the dermal elements may have been destroyed by inflammation. Malignant change in these lesions appears to be exceedingly rare.

Since the connection between the dermal ectoderm and neuroectoderm is small during early embryologic development, the disturbance of the mesoderm condensing about the tract is minimal. As a result the distortion of the adjacent supporting structures of the spine or cranium is insignificant unless associated with another dysraphic condition.

Clinical Manifestations

Of special consideration is a dimple or sinus in the lower sacrum or coccyx - a finding reported to be present in 2 to 4 percent of all infants. Because these lesions result from a different embryologic process than a CDS located in the lumbosacral region or above. the embryologic termination of dimples in the lower sacral and coccygeal region is in the fascia beneath. Their orientation is straight or caudal, a fact that can be demonstrated by stretching the surrounding skin either superiorly or inferiorly. Infection of the central nervous system is not a consideration with sacrococcygeal dimples as they cannot extend to the SAS. A pilonidal or  "hair­nest" cyst is an acquired lesion most commonly found at the same location as a sacrococcygeal dimple or sinus. It appears that many, if not the majority of pilonidal sinuses of young adulthood represent an inflammation of a pre-existing sacrococcygeal sinus. In a child it would be appropriate to excise an infected sinus, but it does not seem justified to remove all asymptomatic sinuses or deep dimples at this location, since only a very small fraction later develop into pilonidal sinuses.

As a result of increasing awareness, more CDSs are being recognized in early infancy before they become symptomatic from infection or mass effect. Concomitant cutaneous stigmata are very often present and may include angiomata, abnormalities of pigmentation, hypertrichosis, abnormal hair pattern, subcutaneous lipomata, skin tags, and rarely an additional sinus tract. Occasionally another dysraphic condition, such as a lipomatous malformation, may accompany a CDS. The opening of the sinus may be so small as to escape detection except by close inspection. Hair may be seen to protrude from the orifice. Debris or purulent material may drain from the site. The midline should be examined very carefully in a child who has had one or more episodes of unexplained meningitis, especially if the infecting organisms are staphylococci or from the coliform group.

The depth to which the CDS penetrates and whether or not it is associated with one or more dermoid cysts will determine the clinical manifestations. A CDS often goes unnoticed until the patient presents with infection, either meningitis or an extra- or intradural abscess. Infecting organisms include Staphylococcus aureus, S. epidermidis, Escherichia coli and Proteus species. Gram-negative bacteria are more frequently the causative organisms in lum­bosacral lesions. Multiple organisms may be present. Even without infection, the contents of a dermoid cyst, especially the fatty acids, spilling into the SAS can produce a chemical meningitis and dense arachnoiditis. The usual hallmarks of a local infection may be present or may be completely absent. even when a deeper - lying abscess or meningitis exists. Hydrocephalus can develop secondary to increased resistance of cerebrospinal fluid (CSF) absorption as a result of one or more bouts of bacterial or chemical meningitis.

A progressively enlarging dermoid cyst along the sinus tract in the spinal canal will eventually result in compression of adjacent neural structures with the conus medullaris most frequently affected, followed by the cauda equina and only very rarely the spinal cord above the level of the conus medullaris. Symptoms will depend on the level of the compression. Reports also mention that the conus medullaris can be low-lying and tethered as a result of a CDS alone. Usually, this finding is rare unless the CDS is accompanied by an additional dysraphic lesion. Intracranial dermoid cysts from a nasal location rarely show effects from compression of adjacent neural structures, while those associated with an occipital CDS would resemble any other posterior fossa mass.

Since there is little, if any, abnormality of the supporting structures, spinal deformity is not a factor in a patient with a CDS unless such develops subsequently from a neurological deficit as a result of infection or neural compression.

Diagnosis

Probing the sinus tract or injecting it with a radiopaque contrast medium is worthless as a diagnostic procedure and is contraindicated because it might incite inflammation or cause neural damage. A lumbar puncture is indicated when meningitis is suspected. Examination of the CSF may yield epithelial cells, but their presence or absence has no bearing on subsequent therapy. X-ray studies of the spine may show incomplete fusion of the neural arch or may be completely normal as is usually the case if the CDS enters through the intraspinous ligaments between adjacent spinous processes or creates a bony defect so small as not to be seen. A large dermoid cyst can widen the spinal canal like any other similarly located mass. Skull films may be normal or may show an oblique, inferiorly directed canal through the bone near the external occipital protuberance. If an extradural dermoid cyst of sufficient size is present, the inner table of the occipital bone may be eroded. An enlarged foramen cecum may be present with a nasal CDS. There is little need, however, to obtain plain x-rays of the spine or skull in the workup of a patient with a CDS. Computed tomography (CT), CT myelography, and magnetic resonance imaging (MRI) have greatly advanced the ability to image these lesions, with MRI being the initial image of choice. With MRI it is possible to trace a tract in the spinal region as it extends into the subcutaneous tissue, but because of the small size of the tract and the fact that it can deviate ever so slightly from the midline, there is no way of guaranteeing how far the tract has penetrated. The diagnosis of a CDS is clinical: imaging studies only support it. A negative imaging study does not exclude the need to explore the tract to its end; however, such studies can be helpful in ascertaining whether the CDS is associated with one or more dermoid cysts or with additional abnormalities such as a lipomatous mass. Interpretation of imaging studies can be difficult following bacterial or chemical meningitis which produce an arachnoiditis. A dimple in the sacrococcygeal region does not carry the possibility of communicating to the SAS. Therefore, imaging studies are not indicated.

Treatment

Treatment of a CDS is to surgically explore the tract to its terminus and excise it along with any associated dermoid cysts. This may prove to be very difficult following inflammation. A sinus tract extending to the spinal dural surface requires an intradural exploration, possibly to the level of the conus medullaris, as the tract can continue into the SAS even when imaging studies are normal. In the presence of meningitis it is best to delay surgical intervention until the patient receives a course of appropriate antibiotic therapy, assuming a stable neurological course. An abscess, be it subcutaneous. extra-, or intradural, requires drainage in addition to antibiotic therapy and is a surgical emergency if the patient exhibits rapid progressive loss of neurological function. In the presence of an abscess it may be advisable to limit the procedure to drainage of the purulent material, the removal of the tract or cyst capsule being delayed until the infection is cleared. It is not known if corticosteroids are of benefit under these conditions, although they do decrease the inflammatory response if dermoid cyst contents are spilled into the SAS. It does seem best, though, to administer prophylactic antibiotics at the time of CDS removal, even when no overt infection is evident. Good lighting and magnified vision, using either high-powered loupes or the operating microscope. are indicated to facilitate removal of the tract and, if present, any cyst capsule, which usually is adherent to nerve roots, conus medullaris, or spinal cord depending on the level. Good visualization becomes even more important following inflammation, when arachnoid adhesions make the dissection even more difficult. Surgical judgment is important in evaluating the risk of creating neurological impairment in order to remove viable capsule, particularly from within the conus medullaris. Rather than trying to remove the capsule in its entirety, bipolar coagulation or laser energy carefully applied to the capsule remnants left in place is likely to prevent regrowth with only slight risk of neurological sequelae.

An occipital CDS will be directed inferiorly and, if intracranial, will cross below the level of the tentorium. These CDSs are frequently associated with a dermoid cyst that might lie extradurally ­or if intradural, within the cerebellum, fourth ventricle, or even the cisterna magna. A CDS in the nasal area may require both an intra- and extracranial approach.

The most important factor influencing the outcome in CDS is total excision of the lesion in early infancy prior to the development of infection or neural compression.