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INTRODUCTION
Focal nondisjunction of the primary neural tube in its spinal cord region results in a normal or near-normal spinal cord, except for the presence of a tract anchoring the dorsal surface of the spinal cord to the base of a characteristic skin lesion. This tract, depending on its cellular constituents and its completeness in extent, can lead to tethered cord syndrome and/or collection of dermal sinus tissue with their dreadful effects. This review aims to give a concise and updated summary of the conditions that have been grouped under a common embryogenetic mechanism, including congenital spinal dermal sinus tract (CSDST), limited dorsal myeloschisis (LDM), and their mixed lesions [3,5,6,11,25,39,42]. A comprehensive study on these focal spinal nondisjunctional disorders (FSNDs) can be found in a recent book chapter titled “Limited dorsal spinal nondisjunctional disorders” [43].
EMBRYOGENETIC MECHANISMS
The basis for grouping these conditions as one entity
Primary neurulation can be broadly divided into four main stages, occurring sequentially at each axial level of the embryo, during that the neural plate goes through 1) formation, 2) shaping, 3) dorsal bending, and 4) closure of the neural groove to become the primary neural tube [2]. At the tissue level, the last stage actually consists of a complex sequence of three overlapping events [2,14,31,41] (Fig. 1) : 1) A broad overlapping surface epithelium (SE) and neuroepithelium (NE) junction of several cells’ thickness is formed at the paired neural folds. It occurs as the neural folds progressively elevate and converge towards the dorsal midline, and the NE cells enlarge in height and drag the f lattened SE cells onto their dorsal surface. 2) The two overlapping epithelia through progressive delamination become completely separate, known as disjunction. The delamination process starts with an inter-epithelial space in the middle part of the interface of the two epithelia. As the space expands, it acquires a crescent shape because the basal lamina of the two epithelia remains attached at the ventro-lateral extreme of their contact (ventral contact point). While the two neural folds are progressively approaching each other to close the dorsal midline gap, the inter-epithelial space extends further dorsally towards the dorsal midline. Ventrally, the basal lamina bridge at the ventral contact point finally breaks down, and the two epithelia separate there. A new basal lamina then forms on the “inter-epithelial surfaces” of each of the two epithelia. When the inter-epithelial space on each side reaches the dorsal-most meeting point of the two epithelia and separates them, it consummates disjunction. 3) During the same period, fusion of the two epithelial layers at the dorsal midline occurs. Fusion is preceded by apposition of the two opposing sets of epithelia at the tips of the two neural folds, and formation of intercellular adhesion at contact points, which are initially discontinuous from superficial to deep [33]. In normal embryos, completion of neural groove closure at an axial level is marked by the appearance of a continuous basal lamina under the SE across the dorsal midline, and a continuous sheath of basal lamina around the NE (primary neural tube) at that level [15]. There are two subtle details of these intercalated processes that are important to our embryogenetic hypotheses : first, fusion of the two epithelial layers likely proceeds independently of each other [41]. Secondly, although the exact timing of epithelial fusion and disjunction is unknown, intuitively, fusion of the epithelia must precede disjunction of the SE and NE.
Faults occurring at a focal point during these last phases of neural groove closure give rise to FSNDs, which are characterized by nondisjunction and incomplete fusion of a focal, or “limited”, segment anywhere along the future spinal cord down to the S1 or S2 cord level [9,18,19,27]. Depending on the aberrant behaviours of the various primordial cells involved in the neural groove closure, several anatomical phenotypes of the final malformation are seen. In addition, the matured features of the subtype malformations of FSND are determined by individual or combined errors of the primordial SE and NE cells, as well as the mesoderm and neural crest cells (Fig. 2) [31,32,37].
At the molecular genetic level, the mechanisms of primary neurulation have only been partially elucidated. The different stages of primary neurulation have different key molecular players [20]. In FSNDs, the underlying faulty molecular events are likely confined to those related to fusion of the epithelia and delamination/disjunction occurring at the dorsal midline. One group of regulator proteins that may be targets in the genesis of FSND are the Rho GTPases, including Rac1 and Cdc42. Both Rac1 and Cdc42 are involved in the regulation of cellular protrusions in SE during dorsal midline fusion [20,28]. As for the molecular mechanisms of nondisjunction, there is even scarcer information; one possibly point of aberration may occur during the caspase-dependent apoptosis of the cells at the SE-NE border [20,44]. There are two general points about defective molecular mechanisms in FSNDs : 1) to account for the “limited” extent of a FSND lesion, a causative molecular defect must involve a small clone of cells during neurulation; and the mechanisms concerned are probably cell-autonomous i.e., the presence of wild type cells nearby cannot correct the error within the affected cells [20]. And 2) the different phenotypes described below may be due to different molecular faults. For example, Rac1 and Cdc42 only involve SE fusion; thus the clinical phenotypes due to their aberrations could be different from those affecting proteins regulating NE fusion [20,28].
The embryogenetic mechanisms for the subtype FSND malformations, described in terms of morphological changes in the embryonic tissue, are as follows (Fig. 2) : 1) a CSDST develops, when fusion of the SE fails at a focal point, but the underlying NE fusion has at least been represented by some intercellular adhesions, and disjunction at this focal point does not occur. In this situation, closure of the primary neural tube immediately cranial and caudal to this focal nondisjunction spot is unhindered, but the gaping SE is persistently linked with the NE at this focal spot. This link, a midline gap in the converging SE and, below it, between the dorsal scleromyotomes in opposite sides of the embryo, remains very narrow. Further unimpeded development of the surrounding normal full-thickness dorsal myofascial tissues around the midline strip progressively sets the primary neural tube into its normal, primarily intraspinal location. However, a dorsomedian tract of SE tissue persists as the original link between the closed primary neural tube and the still slightly gaping epithelial surface. The tract is firmly anchored on the SE side because its component cells are still essentially part of the SE, but its deep-end attachment to the NE cells after closure of the neural groove may not be firm. The deep end of the tract could therefore be dislodged from the underlying neural tube, by cellular movements during normal development of the neural crest cells, meninges and scleromesoderm; so that the inner anchorage of the tract may end short of the spinal cord but on the meninges or even the musculofascial layers.
2) A LDM develops, when the fusion of the NE fails at a focal point but the overlying SE fusion has at least been represented by some intercellular adhesions, and disjunction at this focal point also does not occur. Like the reverse of the development of a CSDST, the SE gap is closed, but because NE fusion and disjunction never happen at the focal spot, the NE remains linked with the SE at the spot. This link, a midline gap in the converging NE and, above it, between the dorsal scleromyotomes in opposite sides of the embryo, remains very narrow. Further unimpeded development of the surrounding normal full-thickness dorsal myofascial tissues around the midline strip progressively also sets the primary neural tube into its normal, primarily intraspinal location.
However, a dorsomedian tract of NE tissue (vs. SE tissue in CSDST) persists as the original link between the focally gaping primary neural tube and the closed epithelial surface [24]. The tract is attached on the undersurface of the SE, and affects the integration of mesodermal tissue at that focal area to form normal full-thickness skin.
3) A tract with mixed LDM and CSDST develops, when fusion of NE and SE both fail and disjunction never occurs, often in an “orthodox” manner with the inner portion of the tract containing NE tissue while the outer tract consisting mainly of SE. The “pulling forces” along the outer and inner portions of the tract during embryogenesis will determine the relative proportion of the two kinds of tissues in the final configuration. However, a “conjoint” type may also occur, where the entire tract is lined by both SE and NE elements. These mixed entities are rare but may easily elude detection [5,11].
4) As for LDM with hidden dermal element - LDMs with dermal elements but without a sinus tract, the origin of the SE cells could be from pluripotent cells near the dorsal midline, or from SE cells somehow being included during the formation of the LDM stalk [5,25].
5) FSND with spinal cord lipoma : dorsal spinal cord lipomas have been known to be associated with LDMs, either directly adjacent to the LDM stalk or continuous with it. Since both transitional and dorsal lipomas are thought to arise from premature disjunction, during the same embryogenetic period as nondisjunction; it is not surprising that nondisjunction and premature disjunction disorders may coexist [25]. Equivalent parallel faults may also account for the coexistence of CSDST and lipomas.
And 6) FSND with split cord malformation : the fibroneural stalk or dermal sinus tract found in some cases of split cord malformation may in fact be the remnant the dorsal portion of an anomalous ecto-endodermal fistula resulting from aberrant early gastrulation [21,23], which is the embryogenetic basis for split cord malformation.
PATHOLOGICAL ANATOMY AND CLINICAL MANIFESTATIONS OF FOCAL SPINAL NONDISJUNCTONAL DISORDERS
Variations in the exact cellular types and histological configurations of FSND lesions account for the spectrum of nondisjunctional disorders, and partial atresia of a tract may explain certain variants of the full forms [8]. Concerning the documentation of the “level” of a FSND, it should reflect the spinal cord level of the initial causative nondisjunctional error, and accordingly, it should be the level of the laminar defect through which the fibroneural stalk or sinus tract passes.
Pure CSDST
Pathological anatomy
A dorsal midline dermal sinus tract (a narrow tubular structure lined by squamous epithelium) is the essential feature of a CSDST [13,40]. The size of the ostium in the skin of this tract is variable but usually small (Fig. 3). How deep the tract penetrates is also variable; over 60% end intradurally, and some are firmly attached to the spinal cord (Figs. 4-6) [43].
Regarding their location along the vertebral column, over 60% of CSDSTs are in the lumbosacral spine; the rest are distributed over the thoracic and cervical regions [43]. The shape of a sinus tract on the sagittal plane varies depending on its level of origin, because the spinal cord ascends along the vertebral column for a fair distance during development due to their discrepant growth rates. A lumbosacral CSDST typically takes on a V-shape with the apex pointing at exactly the laminar level of its nondisjunctional error; its subcutaneous tract descends caudally to reach the lamina, and from thence it ascends towards the thecal sac. With more cranial lesions, the subcutaneous tract becomes progressively more horizontal until it points cranially towards the dura in cervico-thoracic lesions. At the skin level, other skin stigmata may sometimes accompany the sinus ostium (Fig. 3). At the laminar level, the tract may pass through the interspinous ligament, or through a bifid spinous process or lamina. The tract then penetrates the dura but can also run between the dural layers for a short length before becoming intradural. Within the thecal sac, it may be adherent to the nerve roots or filum and, until proven otherwise, one should always assume all sinus tracts reach the spinal cord. Anywhere along the sinus tract, a dermoid cyst may form from existing keratin material, and it can even be intramedullary (Fig. 7). In a report of ten intramedullary spinal dermoid cysts, nine had a traceable CSDST [7]. The rare occurrence of spinal dermoid cyst without a sinus tract is probably due to isolated sequestration of pluripotent SE cells or atresia of the outer tract [8].
Histologically, a dermal sinus tract is lined by keratinizing stratified squamous epithelium (Fig. 8A and B). Other components in variable abundance include hair follicles and shafts, mesenchymal derivatives such as blood vessels and fibrous tissue (Fig. 8A-C), and occasionally even nerve fibers. Keratin material fills the lumen of the tract and the cavity of dermoid cysts (Fig. 8D). Sometimes, the lumen of part of the tract may be obliterated (Fig. 8C). In slender tracts, a transitional zone of epithelial to non-epithelial tissues can be observed over the tract’s deep end (Fig. 8E). Within the CSDST, inflamed granulation tissue containing mixed neutrophils, plasma cells, lymphocytes, and histiocytes is consistently found (Fig. 8A-C) [4,13,38]. It is due to chemically induced inflammation from keratin accumulation, and may also be secondary to bacterial infections from sinus tract’s communication with the skin surface.
Clinical manifestations
The age of presentation has a wide range; in most series, the mean is 3 years or below, with some patients first diagnosed in their 30’s or even 50’s [43]. The commonest presentation is skin stigmata, usually a cutaneous pit, frequently associated with pigmentation, haemangioma, skin tag, subcutaneous lipomas, or hypertrichosis (Fig. 3). Associated subcutaneous lipoma should arouse the suspicion for an associated spinal cord lipoma, and hypertrichosis for split cord malformation. In many published series, over 40% of patients had neurological deficits involving limbs and/or bowel and bladder. In infected cases and in patients harbouring large intradural epidermoid/dermoid cysts, neurological deficits may arise catastrophically [43].
In actual practice, there is uncommonly a definitive history of discharge from a sinus ostium, which is seen only in 25% of cases; and even rarer, inflamed skin around an obviously infected ostium or a deep-seated abscess, which occurs in less than 15%. Paradoxically, in some series, a history of recurrent meningitis or active meningitis is found in up to 40% of cases [43].
Pure LDM
Pathological anatomy
The two constant features of all LDMs are 1) a cutaneous stigma and 2) an underlying fibroneural stalk anchoring the spinal cord to the skin lesion (Fig. 9). The cutaneous marker, a pearly crater of abnormal skin, commonly known as a “cigarette-burn scar/mark”, is due to hindrance on normal skin development by the stalk attaching to the undersurface of the SE (Fig. 2). In all instances, the fibroneural stalk, extending from the deeper side of the abnormal skin, ultimately merges with the spinal cord. Only one example of a discontinuous stalk has been documented [8]. In all LDMs, the spinal cord is tethered to the surface myofascial tissue by the fibroneural stalk [22,34,35] and by the meningeal and other mesenchymal investments condensed around the stalk. The merge point of the stalk with spinal cord is always above the conus, indicative of this being due to faulty primary neurulation [25]. If there are lesions truly arising from secondary neurulation defects, but mimicking the morphology of a LDM, they should be classified as a different entity [9,27].
LDMs can be categorized by the external appearance of their skin lesion into flat (non-saccular) or saccular (Fig. 9). The flat LDM is recognisable either by a simple pin-point pit or a wider crater with “non-skin” squamous epithelium. Internally, the fibroneural stalk passes through the deep fascia, a bifid lamina/ the interspinous ligament, and the dura (Fig. 10). The intradural stalk of a lumbosacral lesion is seen, on magnetic resonance imaging (MRI), as a separate longitudinal structure dorsal to the filum and, more rostrally, dorsal to the conus (Fig. 10A); at the stalk-cord union (merge point), it a trapezoid shape on axial MRI (Fig. 10B). The fibroneural stalk is V-shaped on the sagittal plane, similar to the observation in CSDST, when it is in the lumbosacral region (Fig. 11); but it becomes progressively horizontal or slanting upwards when located more cranially (Fig. 12). The tethering effect to the cord can be very obvious when the fibroneural stalks are stout (Fig. 13), or when the cord is tented dorsally at the stalk-cord merge point (Fig. 14). In rare cases, the stalk appears to pull the cord archly dorsally towards the skin lesion to the extent that the cord seems to have deformed the overlying bone and myofascial layers (Fig. 15). In all these examples, the apparent dynamism of the tethering is obvious.
The saccular LDM arises with the initial embryogenetic architecture of a flat LDM, but the increasing hydrodynamic pressure of cerebrospinal fluid (CSF) soon changes its anatomical configuration drastically. CSF may be forced up along the slender dural sleeve surrounding the stalk, or through the potential space within the fibroneural stalk’s core to reach and distend the overlying skin into a fluctuant sac. Internally, the “sac” of saccular LDMs can have 3 types of content. First, the content is a segmental myelocystocoele : when the portion of the cord bearing the dorsal myeloschisis has an associated hydromyelia, the central lumen of the fibroneural stalk may be distended by CSF into a large myelocystocoele housed in an epithelium-covered sac (Fig. 16). This type is most commonly found in the cervical region (Figs. 17-19) [29,34-36]. Second, it has a basal neural nodule. When there is no hydromyelia, the fibroneural stalk and its central lumen remain compressed and narrow in its deeper course, but the superficial portion of the stalk swells into a fluid sac with basal neural nodules at its base, while retaining the original nondisjunctional attachment to the cutaneous epithelium (Figs. 20 and 21) [21,22,30,34]. Third, it has a slender fibroneural stalk traversing the CSF sac to reach its dome (stalk-to-dome) : least commonly in saccular LDMs without hydromyelia, the normal meninges around the neural tube extends to ensheath the fibroneural stalk, and projects to reach the SE. CSF squeezes into the dural fistula containing the fibroneural stalk and ultimately distends the thinner, less well-supported squamous epithelial membrane on the surface into a CSF-filled, skin-based but epitheliumcapped sac. Strands of the fibroneural stalk traverse the fluid cavity of the sac to reach the part of the dome bearing the edge of the crater, where the nondisjunction occurred (Fig. 22).
Depending on the fluid pressure and the thickness of the apical epithelium and adjacent skin, the sac wall varies from the coarse, purplish, corrugated cap in the not-so-turgid tubular structures in many cervical saccular lesions, to the translucent membrane topping a tense lumbar sac, and finally to the giant, diaphanous bubble. A transitional form between saccular and flat LDMs can be observed, due to transient increase in fluid pressure during straining, in flat LDMs with the intermittently ballooning central crater (Fig. 23).
As with CSDST, registration of the spinal level of LDMs has been far from ideal. The challenge is due to the great difference in the level of the skin lesion, the level where the fibroneural stalk passes through the lamina or penetrates the dura, and where it merges with the spinal cord. In the largest LDM series published [25], the vertebral level where the stalk merges with the spinal cord was chosen as the level of the LDM because it is usually the most unequivocal feature on MRI. The locations of the LDMs in that series are shown in Fig. 24, while the distribution of the types of LDMs is depicted in Fig. 25. Over two-thirds of LDMs in that series are located in the lower half of the spinal cord [25].
Histologically, the central feature of all LDM stalks is neuroglial tissue, a hallmark of the stalk’s origin from the NE. It is either in large elongated swaths containing scattered neurons (Fig. 26A), or in nests embedded in dense fibrous tissue (Fig. 26B). Also found in every stalk is a profuse network of peripheral nerves randomly admixed with the glial nests, but in some cases, nerves are seen emanating from a central core of neuron-containing glia likened to an abortive spinal cord (Fig. 26C). Large nodules of dorsal root ganglion cells are seen in some cases, attesting to the occasional entrapped neural crest stem cells during formation of the neural stalk (Fig. 26D). Pacinian corpuscles (Fig. 26E) seen amongst some of these nerves suggest they are indeed sensory axons. Evidence of mesenchymal condensation around the lengthening neural stalk is shown by the almost universal inclusion of numerous fibrous bands, skeletal muscle, fat (Fig. 27A), and prominent vascular channels sometimes in the form of a vascular glomus (Fig. 27B). Glioependymal tissue lines the sac cavities in cases of segmental myelocystocoeles (Fig. 27C). The cutaneous “cigarette-burn mark” has the histological appearance of a dermal layer with engorged vascularity, abundant nerve fibres, and an abnormal collagen fiber matrix. The unevenness of its surface is due to the ruggedness of the epidermis and dermis (Fig. 28) [16,17].
The question is sometimes asked what constitute the minimum criteria for a diagnosis of LDM. It has been shown that there are patients with clinical and radiological features of LDM (Figs. 29 and 30) and most of its histological features including periphery nerve fibers, but no glial tissue within the stalk (Fig. 31) [10,16,17]. In many of these patients, melanocytes are also a prominent feature. Since periphery nerves and melanocytes are neural crest derivatives, and neural crest cells are located in the primary neural tube over the dorsal midline, a nondisjunctional stalk might drag with it neural crest progenitor cells without neuroglial progenitor cells [2,30,31,37]. Thus the diagnosis of LDM can probably be applied in these patients with periphery nerve fibers but no glioneuronal tissue in the stalk.
Clinical manifestations
Most LDM patients also present at a young age. In a series with a total of 63 patients, the mean age at presentation of 56 children was 5.9 years; and that of seven adults was 28.2 years.
About half of LDM patients are neurologically intact at presentation, which underscores the importance of the cutaneous marker as an initial diagnostic clue [25]. The “pathognomic” cutaneous marker in both flat and saccular LDMs is a confined area of abnormal epithelium over the dorsal midline. In flat LDMs, the cutaneous lesion can be a conspicuous crater or a tiny pit. 1) Crater : the commonest skin abnormality in flat LDMs is a sunken crater on the flat skin surface made of pinkish squamous epithelium (Fig. 32A and B), often with elevated skin margin (Fig. 32C) and sometimes surrounded by a capillary haemangioma with irregular corrugated borders (Fig. 32D) or hyperpigmented skin (Fig 32E). There are occasionally long hair emanating from the crater (Fig. 32F), and some craters are edged by hooded overhanging skin (Fig. 32G). In several examples, the crater is adjacent to an area of wrinkly, over-stretched skin that periodically distends with CSF on dependent posturing or straining (Fig. 33A). Very rarely, the centre of the crater is adorned with a CSF-filled blister (Fig. 33B), which is transitional form between flat and saccular lesions. And 2) pit : the most subtle skin abnormality in a flat LDM is a small midline pit with no other unusual features (Fig. 34), easily missed on cursory examination, and might be confused with a CSDST ostium. Sometimes the pit situates within a capillary haemangioma. Pit lesions are usually found in low thoracic and lumbar cases.
Externally, saccular LDMs usually appear as a skin-based sac, but rarely as a translucent membranous sac. 1) Skin-based sac : the sac wall is thick, skin-based with a dome that is distinctly abnormal skin. The appearance of the dome cover can be roughly subdivided into three subtypes. One subtype has a wide top of purplish, raw-looking, thick stratified squamous epithelium (Fig. 35A). A second subtype has a much smaller, pale, discrete, puckered crater of squamous epithelium on the dome (Fig. 35B). A third subtype has a small, almost imperceptible patch of ultra-thin epithelium on the apex of the relatively delicate skin-based dome (Fig. 35C). And 2) membranous sac : this type should be managed urgently as an “open” spinal dysraphic lesion, but the fibroneural stalk should not be missed. An example is a tubular, CSF-filled sac made of a diaphanous membrane resembling thickened arachnoid, protruding through a 4 mm skin-lined dorsal defect (Fig. 36). The base of this sac has a shallow collar of skin similar to the skinbased sacs. The locations and types of the cutaneous lesions are summarized in Table 1.
It is noteworthy that other cutaneous markers of dysraphism such as hypertrichosis, capillary haemangioma, or misaligned gluteal crease are never seen alone in LDM without the quintessential epithelial crater or pit. The pearly midline crater in a non-saccular LDM thus remains the single most important diagnostic clue for LDM, especially before the development of neurological symptoms.
LDMs cause neurological deficits solely by their tethering effect, which vary in kind and severity by the spinal level of the LDM. In general, LDM patients tend to have milder disability than patients with other forms of dysraphic malformations such as split cord malformations and spinal cord lipomas. In Pang et al.’s series [25], half of the LDM patients had neurological deficits at presentation. About 10% of patients had significant weakness and neuropathic bladder, and the rest of the patients had mild or tolerable neurological or urological deficits, with relatively little hindrance to their lifestyle. The correlations between the types of deficits and locations of the LDMs are summarized in Table 2. The proximity of tension to the relevant cord segments does seem to correspond with the kind of deficits. For example, only cervical lesions produce hand and arm weakness; leg weakness is seen in only 9% of cervical lesions, but 22% in upper thoracic lesions, 38% in thoracolumbar lesions, and 50% in lumbar lesions; and bladder dysfunction is seen in approximately 15% of lower thoracic and lumbar lesions but not in cervical or upper thoracic lesions. Lumbar LDMs close to the conus are perhaps more treacherous because they more often implicate the bladder yet are more likely to be occult.
Similar to other cord tethering entities, the probability of neurological injury increases with longitudinal growth of the spine and with age. Pang et al. [25] demonstrated that older patients with LDMs had a tendency to present with more severe neurological and urological disabilities; and that infants and young children were more likely to be neurologically normal (Figs. 37 and 38). Neurological deterioration was observed in all four adolescents who had had longitudinal follow-up and delay of surgery of 1 to 9 years.
Mixed LDM and CSDST, LDM with hidden dermal elements, and parallel LDM and CSDST in close proximity
In 2013, lesions definitely composed of the histological findings of both LDMs and CSDSTs were first described [25]. They are rare compared to the pure forms of LDMs and CSDSTs. In a series of 75 LDM cases, there were five cases of this mixed type [5]. In another series of 51 cases that consisted of 40 LDMs and 11 CSDSTs, another five cases were documented [11].
Macroscopically, a mixed lesion of LDM and CSDST can mimic either of the pure forms. Even cystic type has been observed [11]. Thus, their recognition relies on histology. Three histological types have been documented. In the “orthodox” type, the dermal and neuroglial elements are in tandem in their respective embryologically orthodox order, i.e., an outer tract of CSDST and an inner tract of LDM elements (Fig. 39) [11]. In the “conjoint” type, the entire FSND tract is lined by both SE and NE elements (Figs. 40-42). The most treacherous mixed lesions have been observed in which the dermal elements form microscopic squamous epithelial islands within the neuroglial tissue of an otherwise proper LDM tract – “LDM with hidden dermal elements” (Fig. 43) [5,38]. Rarely, parallel LDM and CSDST tracts can co-exist in close proximity from skin to spinal cord (Fig. 44).
The most salient implication in clinical practice with the discovery of this mixed type is that a mixed lesion cannot be reliably exonerated at the time of surgery without benefit of histology. Thus, it seems prudent that in all cases of suspected LDM, the entire tract is removed from skin to spinal cord. In addition, all FSND patients should have a delayed post-operative MRI to rule out a recurrent dermoid cyst.
FSND with spinal cord lipomas, split cord malformations, and other dysraphic malformations
FSNDs have been observed to occur with other dysraphic or paradysraphic malformations such as spinal cord lipomas, myelomeningoceles, split cord malformations, and neurenteric cysts (Table 3 and Fig. 45). In such cases, the complex anatomy of the other malformations usually dominates the pathological anatomy of the composite malformation, for a pure FSND lesion is structurally more subtle unless it is a CSDST with a large intradural dermoid/ epidermoid cyst.
The clinical manifestations of these composite malformations also follow the usual course of the associated anomalies, especially when there are only LDM elements without dermoid cyst. However, if present, the dermal elements can have significant adverse impact due to their inherent risks of inflammation, infection, proliferation producing mass effect, and recurrence if not totally excised. One should therefore always be vigilant for hidden dermal elements when dealing with spinal dysraphic malformations.
EVALUATION
A thorough clinical history and physical examination are paramount to elicit the symptoms and signs of neurological deficits related to the types and spinal level of FSNDs, past or active infection, and clues to the existence of associated anomalies. Urological assessment including urinalysis, ultrasonography of the urinary system, voiding cystourethrogram, and also urodynamic studies should be performed.
MRI is the imaging technique of choice to delineate the details of pathological anatomy of the malformation especially regarding composite lesions. MRI findings suggestive of FSND are : 1) a tract linking the skin and the spinal cord even it does not appear continuous (Figs. 4-6, 10); 2) posteriorly tacked-up spinal cord (Figs. 10, 11, 13-15); and 3) a cystic lesion over the dorsal midline (Figs. 7, 12, 17, 21-23). When FSND is suspected on the MRI, the entire path of the tract must be traced from the skin through subcutaneous tissue, lamina, dura, and to the spinal cord. The constituents of the tract are interpreted as much as possible (Fig. 45) : any cyst along its course or in the vicinity is particularly noted (Figs. 7, 12, 17, 21-23), as is the presence of any associated anomalies especially spinal cord lipoma and split cord malformation. Lastly, the whole spinal axis should be surveyed for the rare coexistence of multiple FSNDs in the same spine (Fig. 46).
However, the reliability of MRI in delineating FSNDs is far from absolute. For example, a small tract can be below its resolution (Fig. 5) [12,38], and it cannot always differentiate a CSDST from a LDM, except when the CSDST becomes inflamed and exhibit abnormal enhancement of the tract, a constituent cyst, and adjacent meninges (Fig. 4) [4]. All are essential information for surgical planning.
MANAGEMENT
All FSNDs can cause functional impairment by tethering of the spinal cord, while CSDST or any of its mixed forms pose additional risks of inflammation, infection, mass effect, and even secondary hydrocephalus if left untreated. Early surgery should be performed in all patients with CSDST, and in symptomatic patients with LDM. In asymptomatic children with pure LDM, we also strongly recommend surgery to obviate the dreadful consequences of late and unrecognized tethering. Observation by serial MRI is probably only suitable for patients with equivocal MRI findings or perhaps in asymptomatic adults.
For CSDST patients with active infection and neurological deficits, urgent surgery covered with appropriate antibiotics should be done. However, if infection is not accompanied by neurological deficits, surgery should be deferred until the infection has been treated with antibiotics and local therapy.
Surgery
The principles of surgery in all FSNDs are to completely untether the spinal cord and to remove all epithelial elements if present. For pure forms, a narrow laminectomy for exposure is usually adequate. Laminoplasty is an option, and dural grafting is rarely necessary. In cases with a large intradural dermoid cyst, however, wider bony exposure is usually needed. The extent of longitudinal exposure must include the span between the skin lesion and where the tract joins the spinal cord, which is usually apparent where the cord outline suddenly becomes trapezoid instead of the normal ovoid. In uncertain cases, the skin should be widely draped to accommodate for extension of the incision. If the lesion is in the lumbosacral region, the filum terminale may be thickened and should be cut during treatment of the FSND, so that appropriate provision must be made for more caudal exposure.
Surgical technique for CSDST
The patient is laid prone, the sinus ostium identified, and the laminae of the planned laminectomy confirmed with radiograph (Fig. 47). Standard midline longitudinal skin incision with a small elliptical island around the ostium is made; the tract is then traced from superficial to deep through the subcutaneous layers and deep fascia, to reach the bifid spinous process or through the interspinous ligament. The laminectomy is then carried out carefully around the tract. Dissection of the tract should be done under magnification to minimize the possibility of leaving behind residuum [4]. The dura should always be opened unless the surgeon is absolutely certain that the tract ends outside the dura. When the tract goes intradurally, a cuff of dura may need to be excised with the tract, and the latter should be traced to its destination spot on the spinal cord. Often the tract becomes attenuated and loosely perches on the surface of the cord. Adequate bony exposure must be done without compromise to display the full extent of the tract. If a long tract truly spans many laminar levels, skip laminectomy technique should be considered in which some laminae between the tract’s dural entry point and its spinal cord attachment point are strategically kept intact. The CSDST tract can be carefully delivered from the laminectomy wounds (Fig. 48). Afterwards, primary closure of the dura is usually possible.
The sinus tract may expand along its course or terminate in a dermoid or epidermoid cyst. Extradurally located cysts are readily excised. For large intradural cysts, refined microsurgical techniques are required for their complete removal since the cyst wall is notoriously adherent to nerve roots and pia (Figs. 49 and 50). Microbial cultures from adjacent areas should be obtained, and post-operative antibiotics should be given until negative growth is confirmed.
Surgical technique for flat LDM
The surgical strategy for flat LDMs is similar to that for CSDST. After a standard midline skin incision, the skin crater or pit is excised and the stalk at the base of the skin lesion is carefully dissected out and traced through the defects in myofascial layers, and laminae or interspinous ligament (Fig. 51A). To provide good exposure of the stalk-spinal cord attachment for a safe excision, at least one level of laminectomy both rostral and caudal to the stalk-spinal cord merge point should be planned (Fig. 51B). Again, if the stalk spans multiple levels, skip laminectomy may be considered.
A midline durotomy is made, centred upon the entry point of the fibroneural stalk and extended according to the track of the stalk as demonstrated on the MRI. The usually slender stalk is most often attached to a discrete linear spot or cleft on the dorsal midline of the cord (Fig. 51C); it is simply cut flush with the cord surface (Fig. 51D and E). Any peripheral nerve twigs, blood vessels, and fibrous bands encircling the neural stalk are similarly cut. Once the intradural stalk had been disconnected from the cord, the entire stalk with its skin appendage is resected en bloc (Fig. 51F).
The LDM stalk may be exceedingly slender and attaches to the cord in a minute midline scar (Fig. 52), or the stalk flares out into a wider hold on the cord so that the cut edge on the cord resembles a gaping fish mouth (Fig. 53). The stalk may also contain a glomus of vascular channels (Fig. 54), or its deep end expands into tentacles of blood vessels that crawl on the cord (Fig. 55). Rarely, the stalk attachment is stout and deceptively complex, and the dorsal roots surround it like a cuff (Fig. 56). These attachments are cut flush as above but sparing the surrounding nerve root to reveal a large base of raw spinal cord (Fig. 56B). Rarely, it is necessary to approximate the pial edges of the large raw bed of a pure LDM to eliminate a potentially adherent surface susceptible to re-tethering (Fig. 57). The dura is closed primarily.
Surgical technique for saccular LDMs
The surgical technique for tackling the internal structures of LDMs is basically the same whether the LDM is saccular or flat. The minor differences between handling the two types lay in the initial superficial soft tissue dissection. For the saccular LDMs, a large skin ellipse is made at the sessile base of the sac to expose the dural funnel where the narrow dural fistula fans out to form the sac at the skin level (Fig. 58A). The dural fistula is then traced to the lamina level as with the flat LDM (Fig. 58B).
For the basal nodule type of saccular LDM, the sac is entered at the base to locate the basal neural nodule (Fig. 58C) and the underlying fibroneural stalk within the dural fistula, where it is traced to its attachment to the cord and removed (Fig. 58D and E). Saccular LDMs with very thick stalks that traverse the sac to reach the dome are exposed from the spinal cord side up towards the top (Fig. 59). The base of the stalk is then disconnected from the cord surface. In large saccular lesions with slender stalks that may be hard to find, the sac is opened at the dome and the fibroneural stalk is located at the base of the abnormal skin crater, then traced to the spinal cord surface where it is transected (Fig. 60). In saccular LDM with myelocystocoeles, the stalk may be longer than expected and its cut end can often be traced directly into the hydromyelic cavity of the cord (Figs. 61 and 62).
Surgical techniques for FSNDs associated with other anomalies
The operative strategy for these conditions is a combination of the above techniques for FSNDs with the specific techniques suitable for the associated malformations [25,26]. The technique for the associated malformation usually predominates (Figs. 63 and 64). The most salient point to note is the essential total extirpation of any dermal elements in a complex lesion.
OUTCOMES AND CONCLUSION
The clinical outcome of FSNDs is dictated by the presentation clinical status. With proper surgical techniques, surgery for FSNDs without large intradural dermoid/epidermoid cysts is usually uncomplicated; recurrence of dermal elements and re-tethering should be extremely rare; and most FSND patients without neurological deficits remain neurologically intact after surgery [1,25,42]. Over two thirds of patients with preoperative neurological deficits improve after surgery; about one third will not improve though remain stable (Table 4) [1,25,41]. In CSDST patients with large intradural dermoid/epidermoid cysts or active infections, the results are less salubrious [1,7,40,43]. These statistics thus highlight the importance of early detection and prompt surgical intervention in FSNDs.
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