Dermal sinus is a congenital anomaly in the form of a fistula, through which the skin communicates with the nerve structures of the spinal cord and brain. Pathology can affect any segments of the craniospinal tract, manifested by skin signs (deepening, hyperpigmentation, hypertrichosis), neurological disorders (lower paraparesis, pelvic organ dysfunction, cerebrospinal hypertension), often complicated by meningitis, abscess. Diagnosis of dermal sinus is carried out using clinical and neurophysiological techniques, CT, MRI, ultrasonography of the central nervous system, radiography of the skull and spine. The basis of treatment is radical surgery.
Dermal sinuses are a fairly rare congenital anomaly of the central nervous system. The prevalence of spinal forms among newborns is within 1:2,500. This indicator may not reflect the true frequency of the pathological condition in the population, since many cases remain unrecognized. Cranial forms are even rarer – about 100 cases have been described in the literature. The disease is detected in patients of all age groups, including the elderly, but more often fistulas are found in children under 5 years old. Most studies do not show gender differences, some authors report a prevalence of pathology among women (1.7 times).
The exact causes of dermal sinus remain unknown. The development of congenital anomalies is associated with the influence of hereditary and environmental factors on the fetal body. The first are caused by genetic mutations (autosomal, X-linked), trisomies, aberrations of 4, 5, 13, 18, 22 and other chromosomes. If there is a defect in one of the parents or relatives, the probability of its occurrence in the child increases to 2-5%. There are the following factors affecting the fetus between 3 and 8 weeks of gestation:
- Toxic. The development of the neural tube is disrupted by intoxication with petroleum products, alcohol and drugs, the action of ionizing radiation (living in ecologically disadvantaged areas, working with radiation sources). The connection of pathology with the use of certain medications by the mother is revealed – anticonvulsants, cytostatics, salicylates, etc.
- Intrauterine infections. Acting during critical periods of fetal development, pathogenic pathogens, especially TORCH infections, have a teratogenic effect. This leads to the appearance of not only gross malformations, but also small anomalies (dysembriogenesis stigmas), which include dermal fistulas.
- Somatic diseases. The intrauterine development of the fetal central nervous system is affected by some diseases of the mother: decompensated diabetes mellitus, thyroid pathology, obesity. A significant role is played by hypo- and vitamin deficiency – folic acid deficiency during pregnancy.
Significant causes are considered to be perinatal injuries, intrauterine hypoxia, an increase in the mother’s body temperature against the background of fever or the use of hot baths in the early stages of pregnancy. If a woman becomes pregnant at the age of 35 and older, the probability of dysmorphic abnormalities increases by more than 3%. The identification of one or more factors is the basis for including a pregnant woman in a high-risk group of having a child with a neural tube defect.
The separation of the cutaneous ectoderm and neuroectoderm occurs in the first 2 months of intrauterine development. When a neural tube is formed, a mesoderm is inserted between the two leaves, which is the source of bones, cartilage and muscles separating the skin from the brain tissues. The incompleteness of these processes causes the formation of a fistula course connecting the dermis with nerve formations. If spinal dysraphy affects only the ectoderm, the sinus is limited by the gap from the skin to the spinal canal. The defect of mesoderm closure is accompanied by an anomaly of the brain and vertebrae.
Nasal sinuses occur when the skin is incompletely separated from the dura mater by a process of the frontal bone between 50 and 60 days of embryogenesis. A pathological neuroectodermal connection is formed through the blind hole, ending from the nose to the anterior cranial fossa. For many spinal dysraphia, a defect of the posterior semicircle of the spinal canal is typical (incomplete closure of the arch). The dermatomic level of the lesion corresponds to the spinal segments with which the connection has been established, i.e. the course extends two or more vertebrae above the cutaneous opening.
The fistula channels are lined with a multilayered squamous epithelium, surrounded by connective tissue fibers. Cellular elements (nerve, ganglion, meningeal), fatty deposits, blood vessels, cartilage tissue can be found in them. The length of the passages varies: some reach the deep fascia or the outer meninges, 2/3 of the fistulas are embedded in the subarachnoid space, ending in cysts. The latter contain ecto- and mesodermal derivatives, including accessory structures: hair follicles, glandular cells, smooth muscles.
Abnormal strokes can occur on any part of the craniospinal axis – from the nose to the coccyx. Most of them are considered in the structure of the syndrome of latent spinal dysraphism or splitting of the vertebral arches (spina bifida occulta). Taking into account localization in modern neurology and neurosurgery, the following forms of pathological messages are distinguished:
- Spinal. They can be lumbosacral (up to 41%), lumbar, sacrococcygeal, thoracic, cervical. Spinal sinuses are attached to the dura mater, nerve elements of the cone, ponytail, terminal thread.
- Cranial. Occipital (85%), frontal (nasal) and parietal forms are described. Cranial fistula passages penetrate into the cranial cavity, occipital ones can reach the drain of the venous sinuses, communicate with cysts in the cerebellum or IV ventricle.
According to the literature data, spinal forms of this pathology occur about three times more often than cranial ones. In accordance with the histological structure and the supposed features of embryonic development , some authors suggest using the following classification of spinal fistula passages:
- Long. They end at the level of the dura mater, subdural or subarachnoid space. They have a fibrous structure and are often associated with spinal anomalies.
- Short. Pathological passages are usually located above the plate of the vertebral arch, end in soft tissues, rarely reach the epidural space. There are epithelial or non-epithelial.
Fistulous passages can end blindly, dermoid or epithelial cysts (extra-, intradural, intramedullary). The sinuses are usually single and are located along the median line of the body. There are reports of lateral, double or multiple fistulas in literary sources. According to the clinical course, pathology can be uncomplicated or complicated.
This type of congenital anomalies is characterized by a wide range of clinical manifestations – from asymptomatic course to severe neurological disorders. Many manifest themselves only by external skin signs, in some cases the latter are so inconspicuous that the malformation becomes an accidental finding during neuroimaging examination or is revealed much later – at the stage of complications.
Spinal dermal sinus
Spinal dermal sinuses are most often localized in the lumbar and lumbosacral zone. When examined, a funnel-shaped depression or a gaping hole of a pathological canal running in the cranial direction is found on the skin of the patient’s back. Such changes are often observed against the background of hyper- or hypopigmentation zones, capillary hemangioma, a bundle of coarse hair (local hypertrichosis). The size of the foci varies from 3-5 mm to 4-7 cm. With inflammation of the fistula, the surrounding tissues swell, the skin turns red, a gray cloudy discharge is released from the outer opening.
Clinical manifestation is possible in a wide age range. Unlike skin changes, which are usually a cosmetic problem, neurological disorders associated with myelodysplasia, the presence of cysts and the inflammatory process are progressive. At birth, they are absent, subsequently manifest and increase. Along with minimal deficiency, sluggish paraparesis of the lower extremities develops with hypesthesia, hyporeflexia and muscle atrophy. In half of the patients, a violation of the function of the pelvic organs in the form of urinary and fecal incontinence is detected.
The concomitant syndrome of the fixed spinal cord, which occurs when the hemocirculation in the lower parts is disturbed due to their tension against the background of spine growth, is characterized by an increase in neurological disorders. Symptoms are supplemented by pain in the lower back and legs, gait disorders, orthopedic defects. The latter include shortening of the limbs, clubfoot, curvature of the spine (kyphosis, scoliosis). Spinal fistulas are also associated with diastematomyelia, meningomyelocele, and teratomas.
Cranial dermal sinus
Cranial fistulas localized in the nasofrontal area are clearly visible during normal examination. They are defined as a small dimple, sometimes associated with abnormal hair growth or the release of a yellowish-transparent liquid. Occipital or parietal sinuses are hidden under the natural hair cover, so they are much less noticeable. They are usually detected against the background of episodes of recurrent meningitis during a detailed examination with shaving of the head.
Neurological symptoms occur when the canal of the dermal sinus penetrates intracranially and ends in a cyst. Volumetric formations provoke local compression of nerve structures and an increase in intracranial pressure, which is accompanied by headaches, vomiting. During the examination, signs of ataxia, nystagmus, edema of the optic disc are detected. Cranial forms can be associated with other anomalies: fronto-nasal dysplasia, oral-facial-finger syndrome type I, hemifacial microsomy.
Fistula passages are the entrance gates for bacteria to enter the spinal canal and the cranial cavity – 61% of patients are diagnosed with neuroinfection in the form of secondary purulent meningitis, myelitis, intramedullary, extra- and subdural abscesses. The breakthrough of the cyst contents or the release of dermal elements into the cerebrospinal fluid leads to irritation of the meninges with the appearance of aseptic meningitis, often recurrent. An increase in intraspinal and intracranial cysts is accompanied by the progression of neurological deficits, hydrocephalus, and bone deformities.
Preliminary diagnosis of the dermal sinus is carried out on the basis of clinical data – a combination of characteristic skin signs, increasing neurological symptoms and concomitant inflammatory phenomena. Neuroimaging methods allow to verify pathology by determining the localization and direction of the fistula, its connection with nerve tissues, and the presence of associated anomalies:
- Neurosonography. Ultrasound examination of spinal defects is used as a screening in children of the first year of life, before ossification of the posterior structures of the spine. Ultrasound determines the subcutaneous course and cysts, but has low sensitivity and specificity, does not allow to obtain detailed anatomical data, does not reveal complex anomalies.
- Magnetic resonance imaging. It is considered the method of choice in the neurodiagnostics of pathology. The fistula course is visualized as a linear decrease in signal intensity in T1 and T2 modes when it is amplified from the surrounding soft tissues. MRI of the brain and spinal cord makes it possible to detect cysts, abscesses and other associated disorders, to differentiate dermoid formations with epithelial ones.
- CT scan. Provides maximum information about the anatomical features of bone structures: non-hardening of the arches, abnormal vertebrae, septa. Visualization of the fistula channels themselves is difficult. Dermoid cysts on CT are visible as rounded hypodensive formations with capsular calcification without signal amplification from the brain tissue.
- Radiography. Conventional radiographs of the skull and spine in the anteroposterior and lateral directions are useful for detecting disraphic changes, but the absence of such changes does not exclude dermal fistulas. Radiography is sometimes used as an auxiliary tool for preoperative planning.
In a comprehensive examination of a patient, neurophysiological methods are used to clarify functional disorders: electroencephalography, analysis of somatosensory evoked potentials, electroneuromyography. The use of invasive diagnostic procedures is limited: fistulography is associated with the danger of infection through the fistula canal, lumbar puncture is risky due to the threat of cyst rupture or abscess. Regardless of the results of neuroimaging, intraoperative diagnostics is indicated for all patients.
In the practice of a neurologist, spinal fistulas should be differentiated from epithelial coccygeal passages (pilonidal cysts), pseudodermal sinuses, meningomyelocele. With cranial defects, encephalocele and nasal gliomas are excluded. Intramedullary and intracranial cysts must be distinguished from other volumetric formations (tumors, abscess). Anomalies complicated by the inflammatory process are distinguished from neuroinfections of a different etiology.
Treatment tactics are determined by the localization of the defect, its clinical manifestation, concomitant anomalies, and the presence of complications. In many cases, in order to achieve optimal results, a complex effect on the pathological process is practiced. It is extremely important to provide timely neurosurgical care, on which the outcome of the disease directly depends.
In newborns with a lesion of the coccygeal zone, dynamic observation is possible. Such a fistula usually ends at a distance from the membranes of the brain, so the risk of infectious and inflammatory complications is minimal. In other cases, conservative therapy is given an auxiliary role. Such techniques are unable to eliminate a structural defect, but they help reduce the risk of adverse consequences of its course, prepare for surgery, and provide postoperative management of patients. The following methods can be used:
- Pharmacotherapy. An active inflammatory process around the fistula, meningitis or abscess is an indication for systemic antibacterial therapy, which can also be preventive. Sometimes corticosteroids are prescribed by a doctor’s decision before surgery.
- Physical therapy. The presence of a neurological deficit requires early postoperative rehabilitation, which may include massage, physical therapy, physiotherapy. This helps to improve the functionality of patients.
If the pathology affects any segments above the coccygeal, radical intervention is recognized as the only effective method of treatment – microsurgical excision of the dermal sinus and removal of cystic extensions. In parallel, correction of vertebral and cranial anomalies is performed. The operation is recommended to be performed before the development of neurological deficits or purulent complications – in early childhood (as soon as the child can transfer it) or immediately after diagnosis.
Delayed intervention is indicated with an active inflammatory process in the fistula area, which requires preliminary antibiotic therapy. Intramedullary abscesses are opened by posterior median myelotomy, drained, washed with antiseptic solutions. After the operation, a control neuroimaging study is performed, which is repeated annually or as needed.
Prognosis and prevention
With timely radical removal of the dermal sinus (with a cyst or intramedullary abscess), a favorable prognosis is noted with high chances of preserving and improving neurological functions. The delay in diagnosis contributes to the aggravation of disorders with the development of a permanent disabling defect. Incomplete excision of abnormal tissues is associated with the risk of relapse and infectious complications. Primary prevention measures may include pregnancy planning, elimination of provoking factors, and perinatal diagnostics.