Spinal Cysts

Epidemiology

Spinal dermoids and epidermoids together account for 0.8% to 1.1% of primary spinal tumors. They are equally frequent in males and females. The majority of dermoid cysts (62%-84%) are intradural extramedullary lesions. Sixteen to 38% of dermoids are intramedullary. Extradural location is rare. About 5% of all dermoid and epidermoid cysts are multiple. The cranial to spinal ratio of dermoids is 6:1.

Clinical Presentation

Congenital dermoids may present in the newborn period or infancy. However, they may also grow very slowly and not cause symptoms until adulthood. Acquired lesions may cause symptoms years after lumbar puncture, surgery, or trauma. Patients commonly present with a several month history of backache or radicular pain and slowly progressive myelopathy. Because most spinal dermoid cysts arise in the lumbosacral area, they often present as a conus medullaris syndrome. Rupture of the lesion may be asymptomatic or cause acute aseptic (chemical) meningitis, with headache, seizure, hydrocephalus, and/or ischemic stroke (secondary to arterial vasospasm).

Pathophysiology

Dermoids are not true neoplasms. They arise from inclusion of dermis within adjacent tissue. The congenital form develops at the time of neural tube closure, between 3 and 5 weeks of fetal life. The acquired form develops after lumbar puncture, surgery, or trauma due to iatrogenic introduction of dermal elements into the spinal canal, most often into the subarachnoid space. The incidence of dermoids after myelomeningocele or meningocele repair is about 2%.

Congenital dermoids may be associated with a cutaneous “stigma,” such as hypertrichosis in the overlying skin, with a dorsal dermal sinus, spina bifida, diastematomyelia, syringomyelia, and/or intradural extramedullary or intramedullary lipoma. The most frequent of these is the dermal sinus, which results from focal nondisjunction of neural ectoderm from cutaneous ectoderm. However, dermal sinuses are present in only a minority of cases (20%). Most dermoids have no associated dermal sinus.

Pathology

Dermoid cysts can be connected with the skin through a dermal sinus tract. When present, the tract shows an external ostium that may appear as a dimple in or adjacent to a midline cutaneous hemangioma. There may be a small patch of thin sparse wiry hairs within the ostium. Infrequently, the ostium lies to one side of the midline.

The surrounding bone of the spinal canal may evidence dysraphism and signs of pressure remodeling with canal enlargement due to chronic mass effect.

Dermoid cysts are filled with thick caseous material, which is an oily mixture of breakdown products of hair, secretions of sweat and sebaceous glands, teeth and nails, liquid lipid, and cholesterol. One can also observe skin appendages and, rarely, calcifications ( Fig. 14-1 ). In contrast to epidermoid cysts, dermoid cysts are usually well demarcated from the surrounding tissue.

Dermoid cysts show stratified squamous epithelium plus adnexal appendages, especially hair follicles and sebaceous and sweat glands ( Fig. 14-2 ). On immunohistochemistry, the epithelial lining usually stains for cytokeratins and epithelial membrane antigen (EMA).

Dermoids may occur anywhere along the craniospinal axis but are most often in the midline. In the spine, most dermoids are lumbosacral (60%), followed by sacrococcygeal, 25%; thoracic, 10%; and cervical, 5%.

Imaging

Epidemiology

Together, spinal epidermoids and dermoids account for 0.8% to 1.1% of primary spinal tumors. They are more frequent in males. The distribution of epidermoids within the spinal canal is similar to that of spinal dermoids. About 60% of epidermoids are intradural extramedullary and 40% are intramedullary. Extradural epidermoids are very rare. Approximately 5% of all spinal dermoid and epidermoid lesions are multiple.

Clinical Presentation

Congenital epidermoids grow even more slowly than dermoids, so symptoms may not occur until age 30 to 50 years. Acquired epidermoids may not cause symptoms until 2 to 23 years after lumbar puncture, surgery, or trauma. Clinical symptoms are identical to those of dermoids, namely, back pain, radiculopathy, myelopathy (most often cauda equina syndrome), or symptoms of chemical meningitis from cyst rupture.

Pathophysiology

Epidermoids are not true neoplasms. Rather, they arise from inclusion of epidermis within the spinal canal. Enlargement of the cyst results from proliferation of the stratified squamous cells and from desquamation of keratinized debris into the center of the cyst. Congenital epidermoids develop at the time of neural tube closure, at between 3 and 5 weeks of fetal life. Acquired epidermoids develop after lumbar puncture, surgery, or trauma by introducing epidermis into the intradural space iatrogenically. In one study, 41% of all intraspinal epidermoids were caused by lumbar punctures. Acquired epidermoid cysts may also develop after (myelo)meningocele repair (13%-27%), which is about six times more frequent than dermoid cysts. Congenital epidermoids may be associated with hypertrichosis, posterior dermal sinus, spina bifida, diastematomyelia, syringomyelia, and intradural extra-medullary or intramedullary lipoma. Dermal sinuses are less frequent with epidermoids than with dermoids.

Pathology

Epidermoids may cause remodeling of the spinal canal just as do dermoids. Epidermal cysts are encapsulated lesions. A smooth, glistening surface (capsule) encloses waxy material representing a mixture of keratin and cholesterin. There is a “mother-of-pearl” appearance that is almost pathognomonic and that leads to the synonym “pearly tumor.” Often, no sharp border to the surrounding tissue is visible.

The cysts are lined by stratified squamous epithelium mounted on connective tissue. The center of the cyst is filled by concentric lamellae of desquamated keratin ( Fig. 14-3 ). The epithelial lining shows positive immunohistochemical staining for cytokeratins and EMA.

Epidermoids can occur anywhere along the craniospinal axis. Unlike dermoids, which are most common in the midline, epidermoids tend to lie off midline. Spinal epidermoids most commonly affect the lumbosacral levels.

Imaging

MRI

T1W and T2W MR images show a cystic lesion with signal intensity similar to CSF, that is, hypointense or isointense on T1W and hyperintense on T2W imaging ( Fig. 14-4 ). As a result, epidermoids may be very difficult to detect and display on standard MRI, especially when the lesion is small. If the cyst is large, causing enlargement of the subarachnoid space, eventually with deformation of the spinal cord, high suspicion has to be raised and additional sequences have to be performed. Diffusion-weighted imaging (DWI) provides a definitive diagnosis, because epidermoids cause diffusion restriction whereas free CSF and arachnoid cysts show no restriction of diffusion. Fluid-attenuated inversion recovery (FLAIR) and constructive interference in steady-state sequence/fast imaging employing steady-state acquisition (CISS/FIESTA) sequences and magnetization transfer techniques are also useful for depicting the more viscous nature of the epidermoid cyst. Occasionally, epidermoids show high density on CT, hyperintense signal on T1W MRI, and hypointense signal on T2W MRI (compared with CSF). The precise signal depends on the chemical state of the cholesterol and/or on the proportion of cholesterol versus keratin (see Box 14-1 ). Methemoglobin from bleeding can also cause high signal on T1W MRI. Such atypical imaging features may make it difficult to differentiate epidermoid cysts from lipomas or dermoids, unless fat saturation sequences are incorporated into the study. Because epidermoids do not contain fat, they retain their high signal on fat-suppressed images, whereas lipomas and dermoids do not. Epidermoid cysts usually do not enhance, although a thin rim of enhancement may occasionally be observed at the periphery of the lesion, possibly representing compressed parenchyma or granulation tissue. Epidermoid cysts may become infected, especially if they are associated with a dermal sinus. When the lesion shows a thick ring of enhancement, it may resemble an abscess, or have become one.

BOX 14-1

MRI of Epidermoid

PATIENT HISTORY

This 50-year-old woman presented with slowly progressive pain at the sacrum.

FINDINGS

The first MRI demonstrates a large sacral epidermoid with signal intensity slightly hypointense to CSF on T2W imaging (see Fig. 14-26A ) and slightly hyperintense to CSF on T1W imaging (see Fig. 14-26C ), representing xanthochromia or higher protein concentration. This feature helps to display the lesion better than typical epidermoids, which have signal intensities similar to CSF. No contrast enhancement is seen (gadoterate meglumine [Dotarem], 0.1 mmol/kg, was used). The CSF-like fluid-filled cystic lesion above the epidermoid communicates with the subarachnoidal space through a pedicle and represents an associated meningocele (see Fig. 14-26A to C ).

OUTCOME

The sacral canal was widened, and the sacral and coccygeal vertebral bodies were eroded. The epidermoid cyst extended anteriorly into the anal region, without breaking through the wall of the rectum. The cyst was completely removed, and the meningocele was repaired.

COMMENT

Two years later the patient developed a sacral fistula that discharged fluid. The anterior cyst filled with iodinated contrast medium on CT myelography (see Fig. 14-26G, I ) and proved to be a recurrent meningocele. The posteriorly located T1-isointense, T2-hyperintense cyst did not fill with contrast media on CT myelography and was histologically proved to be a recurrent epidermoid cyst. The thick contrast enhancement around the lesion (see Fig. 14-26F ) is a sign of inflammation. Note the scar tissue in the region of the laminectomy (see Fig. 14-26D-F, H ). The patient underwent a successful reoperation. The fistula and the epidermoid were removed, and the meningocele was repaired and covered with a muscle flap.