Surgery and outcomes of six patients with intradural epidermoid cysts in the lumbar spine

Epidermoid cysts are rare benign neoplasms within the neuraxis, which are commonly located in the intracranial region and account for less than 1% of all intraspinal tumors [1, 2]. In 1835, Cruveilhier was the first to describe epidermoid cysts, calling them tumeurs perlées (pearly tumors) [3, 4]. Epidermoid cysts can be congenital or acquired. Congenital epidermoid cysts are frequently found in association with spinal dysraphisms such as syringomyelia, dermal sinus and spina bifida, while the most common etiology for an acquired cyst is repeated lumbar puncture [2, 5].

Although epidermoid cysts can be extradural, intradural or extramedullary, or intramedullary in the spine, the tumors are often intradural and extramedullary in the lumbosacral region [6]. The symptoms of an epidermoid cyst are usually non-specific. Neurologic symptoms such as progressive paraparesis, motor-sensory complaints and sphincter troubles cause great distress [2, 4, 5, 7, 8]. Patients with an epidermoid cyst usually suffer for a long time with symptoms, for an average time of 6 years, due to their slow-growing nature [2, 4].

Because it is an indolent benign tumor, an epidermoid cyst can be cured by complete excision. However, complete resection is difficult to achieve because its capsule adheres to the spinal cord or nerve roots, so subtotal resection is usually the more common surgical objective [9].

In this series, six cases with an epidermoid cyst in the lumbar spine underwent surgery in our center. This retrospective study was performed to analyze the clinical characteristics and suggest treatment options for epidermoid cysts in the lumbar spine.

Epidermoid cysts in the spine are thought to arise from one of two possible origins: congenital or acquired [10]. Congenital epidermoid cysts, which are more common and often associated with other spinal dysraphisms, are caused by the anomalous implantation of ectodermal cells during closure of the neural tube between the third and fifth week of embryonic life [2, 11, 12]. The number of acquired cysts has decreased significantly in recent years. They are attributed to the displacement of epithelial tissue secondary to a previous lumbar puncture or trauma [13‐15]. In our series, the epidermoid cysts in all six patients were regarded as congenital because they had neither suffered trauma nor received a lumbar puncture. Surprisingly, we also did not find evidence of other spinal abnormalities.

Due to the rarity of epidermoid cysts, there are no descriptions of the clinical features in the literature. In 1992, Roux et al. reviewed 47 cases with intramedullary epidermoid cysts of the spinal cord and summarized that: the average age was 34, the mean length of time between the onset of symptoms and diagnosis was 6 years and the ratio of males to females was 1.35:1 [2]. Our cohort has a similar age demographic (range: 15 to 56 years; mean: 37.7 years), much shorter duration of symptoms (range: 2 to 120 months; mean: 29.7 months) and a strange sex ratio (one man and five women). The shortened duration of symptoms is thought to be the result of significant progress in imaging technology, especially the wide application of MRI.

The symptoms and signs of an intraspinal epidermoid cyst are directly associated with the size and site of the tumor but are similar to those of other space-occupying lesions in the spinal column [4, 16]. Non-specific symptoms and signs such as numbness, weakness, spasticity, paraparesis of lower extremities and defecation disorders pose challenges in pre-operative diagnosis. Ependymomas, metastasis, astrocytomas and dermoid cysts are alternative pre-operative diagnoses. With the help of MRI, ependymomas, metastasis and astrocytomas can be ruled out, but distinguishing an epidermoid cyst from a dermoid cyst relies on a pathological examination. For the six cases in our series, the long duration of localized pain and neurologic deficit as well as defecation disorders are non-specific and provide little help in the pre-operative diagnosis. Physical examinations and medical histories revealed no evidence of trauma or lumbar punctures.

MRI is an effective tool for the diagnosis of an intraspinal epidermoid cyst. X-rays often show a normal image and CT can only display the location and outline of the tumor. On MRI, the tumor is usually isointense or hypointense in T1-weighted images and hyperintense in T2-weighted images and the cyst content has the same signal as the cerebrospinal fluid [17‐19]. But these signal characteristics are not always constant and some variations have been reported in the literature [18, 20, 21]. The MRI results for five patients in our series were consistent with the typical imaging findings reported in the literature while for one patient out of the six there were unusual images (Figure 1).

Pathologic examination is vital for the final diagnosis of an epidermoid cyst. Histologically, epidermoid cysts are composed of lined stratified squamous epithelium supported by an outer layer of collagenous tissue. Progressive desquamation of keratin from the epithelial lining toward the interior of the cyst produces a soft white material rich in cholesterol crystals [2, 22‐24]. The absence of skin adnexa is the key to differentiate one from a dermoid cyst [2, 24].

A cyst contains considerable amounts of fat and little cholesterol, which may produce a vigorous inflammatory reaction if the cyst ruptures. Two cases reported in the literature illustrate this point [2, 17, 19, 25].

We found an interesting phenomenon. All the cases of intraspinal epidermoid cysts in our series and cases in the literature were isolated and single, while the dermoid cysts in the spine with a similar pathogenesis and pathological manifestation often had satellite lesions at adjacent levels [2, 9]. According to Willis, epidermoid cysts and dermoid cysts are two subtypes of epidermal inclusion cysts, which are neoplasms derived from sequestered skin [26]. Several theories have attempted to explain the pathogenesis of epidermal inclusion cysts. The classic, widely quoted theory set out by von Bostroem in 1897 expounded that those were developmental errors of the neural tube, occurring between the third and fifth week of embryonic life [12]. A newborn rat model developed by Van Gilder and Schwartz had skin homografts implanted along the neuraxis, which transformed themselves into epidermoid cysts or dermoid cysts. This work added weight to the theory of von Bostroem [27]. The theory also considers that the timing of the event (early or late) determines the type of tumor (dermoid or epidermoid) that will form [9, 12]. We have reviewed other theories for the pathogenesis of both types of tumor and regretfully found that no theory had tried to explain the phenomenon we found. In our opinion, the timing of developmental errors and the involvement of skin adnexa may be related to the difference between these two similar types of tumor. Perhaps, multiple epidermoid cysts in the spine, which are rarer, have not yet been found and reported.

Given their indolent, benign nature, asymptomatic epidermoid cysts should be managed conservatively. Surgical excision is essential for lesions with symptomatic progression and where imaging shows that neural elements are compressed [9, 22, 24, 28]. Although radiotherapy has been used in one case with repeated relapses of epidermoid cysts, achieving a relatively good result, this treatment should only be used for patients who refuse surgery or are inoperable for medical reasons [29]. Undoubtedly complete excision without neural damage is the goal of treatment. Emptying of the cyst material can be performed easily but the intimate adherence between the capsule and the spinal cord makes this goal difficult. So subtotal excision tends to be the more common surgical pattern for avoiding possible neural damage [10, 18, 19, 22, 23, 30]. Subtotal excision also causes great distress for patients and doctors because debris from the tumor may cause an early relapse of symptoms and the spread of cyst contents can cause foreign body reactions and severe complications [2, 17, 19, 23, 28, 29].

Total excision, which requires complete removal of the tumor without rupturing the capsule, is the perfect form of complete excision. It has higher technical requirements and comes with a higher possibility of neural damage. Microsurgical techniques can help to achieve total excision and subsequently decrease the chance of a relapse. In our series, tumors in all six cases were located in the lumbar spine where total excision is easier to achieve and neural damage is more likely to be avoided using microsurgical techniques. The encouraging results are that radicular pain nearly disappeared, patients suffering from a neurologic deficit and defecation disorders recovered well and no patients suffered a relapse during their follow-up periods.

In spite of the indolent, benign nature of epidermoid cysts, local recurrence is not uncommon after incomplete surgical excision [2, 17, 19, 23, 28, 29]. Metastatic lesions of an epidermoid cyst have never been reported. Malignant transformations have not been found in the spine but there have been a few reports of intracranial cases with a malignant transformation [31‐35]. Symptomatic relapsed cases should be retreated by surgical excision. Because of the formation of scar tissue resulting from previous interventions, complete excision is more difficult for relapsed cases and palliative approaches with the aim of relieving symptoms are often implemented.

Although they are rare benign tumors, epidermoid cysts can occur intraspinally and cause severe neurological deficits with a long duration of symptoms. MRI is an effective tool for diagnosis with characteristic isointense or hypointense regions in T1-weighted images and hyperintense regions in T2-weighted images. However, pathologic examination is still the gold standard for final diagnosis. With the help of microsurgical techniques, total excision may lead to complete recovery for patients with symptomatic lesions. When total excision is impossible, subtotal excision can be adopted to avoid damaging neural function. Extreme care should be taken to minimize the residual and prevent the contents of the cyst spreading into the subarachnoid space, which can lead to prolonged chemical meningitis and subsequent arachnoiditis.

The pathogenesis of epidermoid cysts is still not very clear and the phenomenon that all intraspinal epidermoid cysts are isolated and single cannot be fully explained. More research is needed to clarify the pathogenesis of epidermoid cysts, which will also aid treatment.

Written informed consent was obtained from each patient for publication of this study and the accompanying images.

HY, DZ and ZW are the co-first authors.