The term occult spinal dysraphism (OSD) encompasses a group of abnormalities that occur during the development of a human embryo, beginning in the second week of gestation. They are the result of incomplete or incorrect formation of the spinal cord, spinal column, and overlying skin. These defects are often referred to as spina bifida occulta . Importantly, OSDs are not the same as the more common type of spina bifida, known as spina bifida aperta or myelomeningocele, in which there are obvious defects not covered by skin.
Because OSDs are defects covered by skin, their true frequency in the population is unknown. It is known that OSDs are about two times more common in females compared to males. Additionally, the bulk of the knowledge regarding risk factors that lead to OSDs is based on extension of research into the genetically related group of disorders known as spina bifida aperta. 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 and that administration of folate beginning prior to and continuing during pregnancy greatly reduces this risk.
Clinically, OSDs can be difficult to diagnose and are often missed until later in life. Sometimes they are not even discovered until adulthood. Most commonly, they are suspected after detection of associated findings on the skin. These visible signs, usually located in the lower middle of the back indicating the site of underlying defects, 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 lipomyelomeningocele and others. Most commonly, two or more of these skin lesions are present in association with OSDs. 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 tight and/or fatty filum terminale, intramedullary lipoma, lipomyelomeningocele, split cord malformation, inclusion tumor and/or cyst, neurenteric cyst, and terminal syringohydromyelia are all different forms of OSD, which are 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. In infants, aside from the skin defects noted above, there may be very subtle changes in leg shape and length, especially in the feet. In children, as they learn to walk, asymmetry in the legs can become more obvious and symptoms related to bladder problems 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. If none of these are present, the diagnosis is often signaled in older children and adults by back pain.
Finally, the treatment for OSD depends largely on the cause. Generally, the cause of the symptoms is felt to be due to tethering or stretching and, occasionally, compression of the lower spinal cord. 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 to correct the lesion is the definitive treatment. 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 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 tight filum terminale without typical MRI appearance can 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 or general practitioner). With timely diagnosis and appropriate intervention, however, many of the problems associated with OSD can be halted or reversed. The association with syringomyelia is well documented, while the association with Chiari I is controversial.
- Iskandar BJ, Oakes WJ.Occult Spinal Dysraphism. In Albright AL, Pollack IF, Adelson PD (eds.): Principles and Practice of Pediatric Neurosurgery. Thieme Medical Publishers, New York, NY, 1999.
- Rauzzino MJ, Iskandar B, Oakes WJ. Occult Spinal Dysraphism. Contemporary Neurosurgery 1998;20(22):1-6.
- Iskandar BJ, Oakes WJ, McLaughlin C, Osumi A, and Tien RD. Terminal Syringohydromyelia and Occult Spinal Dysraphism. J Neurosurg 1994;81(4):513-519.