The term occult spinal dysraphism (OSD) encompasses a group of abnormalities that occur during the development of a human embryo, beginning in the third week of gestation. They are the result of incorrect “dysjunction” of the neuroectoderm with incomplete separation of the epidermis (overlying skin) from the neural tube (spinal cord and central nervous system) and neural crest (peripheral nervous system), aka “secondary neurulation”. These defects are often referred to as spina bifida occulta (OSDs) defects are distinguished from spina bifida aperta or myelomeningocele, which results from failure of formation of the neural tube (spinal cord) with resulting exposure of the spinal cord through a skin defect, aka “primary neurulation”.
The true frequency and prevalence of OSD in the population is unknown given the subtlety or absence of obvious clinical signs that renders this condition as largely overlooked and under-diagnosed It is observed about two times more common in females compared to males. Etiological factors, as with spina bifida aperta, might be folate deficiency during pregnancy or genetic factors, even though the relationship in OSD is not that well researched and established. It does seem clear, however, that having given birth to one child with known OSDs increases the risk of having a second child with similar defects.
Clinically, OSDs can be difficult to diagnose and that is why the diagnosis can be missed until later in life. Sometimes they are not even discovered until adulthood. Suspicion of OSD is certainly raised with observation of so called “neurocutaneous signs”. These are abnormalities on the skin usually at the lower midline of the back above the coccyx. It has been reported that in patient is spina bifida occulta the prevalence of neurocutaneous signs is approximately 80% (1).
These visible signs, can include a tuft of hair (hypertrichosis), a red lesion known as a capillary hemangioma, a skin opening known as a dermal sinus tract, an area of thin bluish skin also known as an atretic meningocele, a fatty mass under the skin called subcutaneous lipoma, a dermal appendage or retracted skine also known as dimple. Most commonly, two or more of these skin lesions are present in association with OSDs, however, they can be very subtle. Dermal sinus tracts are especially important skin lesions to recognize because they can extend to the spinal canal and represent an increased risk of serious infection such as meningitis and spinal cord abscess.
The term OSD implies the presence of one or more spinal cord anomalies, which can cause tethering of the spinal cord and possible neurological and bladder/bowel function deficits. The terms thickened or fatty filum terminale, filum lipoma, intramedullary lipoma, spinal lipomas, lipomyelomeningocele, lipomyelomeningocystocele, split cord malformations, inclusion tumor and/or dermoid cyst, and terminal syringohydromyelia are all different forms of OSD. They may be associated with a low-lying conus, which is the terminus of the spinal cord and this is all usually diagnosed by MRI. A detailed description of these abnormalities is beyond the scope of this review.
These lesions cause many different symptoms or can be completely asymptomatic, aka “incidental”. In infants, aside from the skin defects noted above, there may be very subtle changes in leg shape and length, especially in the feet including “clubfeet”. In children, as they learn to walk, asymmetry in the leg strength can become more obvious and symptoms related to bladder and bowel function can become more prominent. Most often, older children and adolescents are diagnosed with OSD after they initially present to their doctor with complaints of urinary incontinence and or leg pain and weakness after physical activity. In older children and adults, back pain and leg pain is more prevalent, often asymmetric. They mechanism by which those lesions affect the spinal cord and the nerves leading to neurological deficits have been attributed to a “tethering of the spinal cord” and mechanical forces and stretches applied to the spinal cord by any form of OSD. This leads to inflexibility of spinal cord movements in the spinal canal and ischemic damage (“stroke”). Occasionally, there is compression of the lower spinal cord, i.e. in the presence of a massive lipoma. This explains the often-observed progression of symptoms in symptomatic individuals as well as incomplete recovery despite surgical intervention.
Surgical treatment is indicated in symptomatic individuals, specifically if there is evidence of progression. It is more and more appreciated that surgical intervention can be beneficial for symptomatic adults. The symptoms, once present, generally continue to worsen as development continues and are more likely to be irreversible the longer they are present prior to treatment. Therefore, early diagnosis and intervention in the presence of symptoms is important. Surgery and the extend of surgery depends largely on the form of the OSD defect, however, the definitive treatment goal is “de-tethering of the spinal cord” which entails a microsurgical procedure with “separation” of the spinal cord from the mass or structure to release the mechanical stress. While there are risks involved with this kind of surgery, most neurosurgeons believe that preventative surgery is important to avoid the potential of irreversible neurological or bladder problems over time. The risks of the procedure certainly correlate with the complexity and the type of the OSD.
The cause of the association of OSD with Chiari I and syringomyelia is twofold. First, tethered cord can cause syringomyelia, usually located in the lower third of the spinal cord and termed terminal syringomyelia. Treatment is usually aimed at the OSD rather than the syrinx, although large syrinxes are sometimes drained or shunted at the same time. Second, there have been recent claims that a structurally abnormal filum terminale leading to tethered cord symptoms can be present without typical MRI appearance. This may or may not be associated with Chiari I. This is a highly contentious issue and is appropriately covered in the controversies section.
In summary, OSD is a family of defects that develop early in gestation that can lead to different neurologic, orthopedic, and bladder/bowel symptoms. Untreated, neurologic symptoms may continue to develop and can become irreversible. Once the defect is identified, appropriate investigation is necessary and early surgical correction is preferred by most surgeons. Often, the diagnosis is signaled by the presence of skin changes located in the middle of the back at the site of the underlying defect. Unfortunately, it is not always an obvious diagnosis and the diagnosis requires careful attention on the part of each patient’s doctor (pediatrician, general practitioner, neurologist and neurosurgeon). With timely diagnosis and appropriate surgical intervention, however, many of the problems associated with OSD can be halted or reversed.