Figure 1: Illustration of the Sacral Canal’s Bony Structure. Positioned posteriorly along the midline of the pelvis, beneath the lumbar spine and above the coccyx, the sacrum comprises five fused segments (S1 to S5). The sacral canal, a bony conduit, resides centrally within the sacrum.
Figure 2: Localization of the Dural Sac’s Bottom. Typically situated at the S1 level, the dural sac connects to the cranial cavity and is filled with cerebrospinal fluid, akin to a tall water bucket. Due to human bipedalism, the static water pressure at the dural sac’s base is quite high, approximately 1/9 of a standard atmospheric pressure.
Figure 3: Diagram of Nerve Roots. Inside the dural sac, the nerve root (depicted in yellow) is exposed. Upon emergence, it is enveloped by a layer of dural sac tissue (highlighted in green), which acts as an extension, akin to a sleeve continuing from a garment. Notably, the interior of the nerve root sleeve is devoid of cerebrospinal fluid, resembling a snug-fitting sleeve.
Figure 4: Formation of Sacral Cysts. Congenital weakness at some nerve root’s exit point at the dural sac’s base allows cerebrospinal fluid, under immense pressure, to push into the nerve root sleeve. This expansion leads to the enlargement of the bony sacral canal. Simultaneously, the cyst’s internal nerve roots are stretched, while adjacent external nerve roots are compressed, resulting in pain and urinary and fecal dysfunction. It’s evident that a sacral cyst is not a real cyst or a tumor but rather a cerebrospinal fluid leak. Thus, “resecting” a sacral cyst is impossible; only repairing the leak is feasible.
Figure 5: Ideal Surgical Strategy for Sacral Cyst . (1) Given that the disease originates from congenital weakness at the nerve root sleeve’s exit, which cerebrospinal fluid enlarges into a leak, the core surgical technique involves using micro suturing to block the leak with a small muscle fragment without harming the traversing nerve root. (2) Cerebrospinal fluid enlarges the nerve root sleeve (i.e., cyst wall). Post-successful occlusion, cerebrospinal fluid within is aspirated to confirm no further leakage. At this juncture, the cyst wall is folded, wrapped around the nerve root, returning it to a snug-fitting sleeve.
Figure 6: Addressing an Enlarged Bony Sacral Canal. Since bone regeneration is impossible, autologous muscle and fat are utilized to fill the bony sacral canal, as adipose tissue naturally resides there under physiological conditions. Consequently, post-sacral cyst surgery, magnetic resonance imaging reveals fat and muscle. After several months, slight fat liquefaction results in a subtle liquid signal on imaging.
Figure 7: Surgical Conclusion with Titanium Mesh Application. To prevent sacral canal content protrusion, titanium mesh should be employed to repair the sacral canal’s posterior wall.
A 25-year-old male patient was admitted to the hospital due to a growing mass in the lower right abdomen for two months, accompanied by stool weakness and constipation. Additionally, he experienced soreness and pain in the lumbosacral region, rendering him unable to stand or sit for prolonged periods and hindering his ability to maintain work performance.
Magnetic resonance imaging revealed a huge sacral cyst that had protruded through the sacral foramen into the pelvic cavity, forming a spherical cystic mass exceeding ten centimeters in size. Furthermore, the cyst had penetrated the posterior wall of the sacral canal, creating a conical cystic mass within the intermuscular space.
During surgery, it was discovered that the sacral cyst had a big leak, which was promptly sealed while ensuring the protection of transversing nerves.
Follow-up magnetic resonance imaging conducted within one year post-surgery demonstrated successful occlusion of the cyst with no signs of recurrence. To date, the patient has been under follow-up care for nearly two years.
All symptoms subsided rapidly, and bowel movements recovered to normal. The patient resumed his work responsibilities three months after surgery.
Dr. Zheng Xuesheng emphasized that the prolonged exposure of sacral cysts to substantial static water pressure can lead to damage to the sacral vertebral body, which typically has a thickness of 3-4 centimeters. This damage allows the cyst to protrude into the pelvic cavity. Once in the pelvic cavity, the cyst loses the bone restriction, enabling rapid enlargement. In this particular case, the cyst affected bowel function within two months, causing stool weakness and constipation. If left untreated, it was anticipated that urinary and sexual function would suffer irreversible damage in the near future. Fortunately, the patient underwent timely surgery, resulting in swift postoperative recovery. Given his youthful and robust constitution, he was able to resume his duties within three months.
To determine the responsible nerve root in sacral cysts based on pain location, doctors rely on understanding dermatomal distributions. The accompanying two images depict the areas of skin sensation corresponding to various nerve roots in the lower limbs, buttocks, and perineum. Here, T denotes the thoracic nerve, L the lumbar nerve, and S the sacral nerve, with S2 representing the second sacral nerve root, for instance.
The first sacral nerve root (S1) is primarily responsible for skin sensation behind the calf, heel, and sole, while the second sacral nerve root (S2) covers the sensation behind the thigh, upper thigh, and posterior calf. The skin sensation of the buttocks, anus, and perineum is mainly innervated by the third, fourth, and fifth sacral nerve roots (S3, S4, S5).
Sacral cysts can compress nerve roots, leading to varying symptoms depending on the pressure. Typically, prolonged standing or sitting increases pressure and exacerbates pain, which improves upon resting in a supine position.
Large sacral cysts can compress nerve roots on both sides due to the limited width of the sacral canal (approximately 1.5cm). For instance, a cyst originating from the left side can affect the right-side nerve roots if sufficiently large. Similarly, cysts originating from upper nerve roots may compress both the corresponding and lower nerve roots, causing pain in various areas such as the heel, posterior calf, anus, and perineum. However, cysts from lower nerve roots typically do not affect upper nerve roots. These principles contribute to the wandering pain sites and fluctuating pain levels observed in sacral cysts.
It’s crucial to note that painful skin segment distribution varies significantly among individuals and should not be interpreted mechanically. Instead, it requires comprehensive evaluation alongside other neurological signs.
A commonly held belief suggests that sacral cysts exhibit high pressure due to their leakage mechanism resembling a one-way valve, akin to the valve in a basketball, allowing fluid to flow in but not out, causing the cyst to progressively enlarge. According to this perspective, injecting a contrast agent into a sacral cyst would prevent it from escaping the leak or entering the subarachnoid space. Therefore, performing an imaging study of a sacral cyst could validate this theory.
However, the imaging study depicted above illustrates the injection of a contrast agent into a sacral cyst. Upon reaching the level of the leak, the contrast agent promptly escapes and enters the subarachnoid space. Evidently, the aforementioned perspective is incorrect.
Since the leakage in sacral cysts is not a one-way valve but rather a bidirectional flow, allowing for the free movement of fluid in and out, the question arises as to why sacral cysts continue to grow. The answer lies in the high hydrostatic pressure of cerebrospinal fluid in the sacrum when the body is upright, continuously expanding the cyst.
Given that sacral cyst leaks are bidirectional, surgical diversion techniques that open up the sacral cyst and dural sac may exacerbate the condition by increasing the size of the leak and the flow rate.
A middle-aged female patient has been experiencing sacrococcygeal pain accompanied by numbness in the buttocks for four years. Over the past three months, her symptoms have worsened, particularly when standing or sitting for prolonged periods, but find relief when lying flat. Her urination and defecation remain normal.
Her symptoms are reminiscent of a sacral cyst.
Despite undergoing magnetic resonance imaging (MRI) at multiple hospitals, the diagnosis remained uncertain, with each facility concluding that the imaging findings were similar to a sacral cyst.
However, Dr. Zheng’s team conducted a thorough review of the images and noticed discrepancies between the MRI signals and those typically associated with a sacral cyst or a sacral cyst with bleeding, which led them to suspect a sacral tumor. They prepared for surgery with two potential diagnoses in mind.
During the surgery, the team discovered that the lesion was indeed a sacral canal tumor, intimately associated with the cauda equina nerve. The tumor was successfully resected under microscope, and the patient’s postoperative symptoms have virtually disappeared without any neurological damage.
The postoperative pathological report confirmed the diagnosis of ependymoma, a tumor that typically originates in the ventricles of the brain and can spread into the spinal canal through cerebrospinal fluid. A subsequent whole brain and spinal MRI examination ruled out the presence of any other lesions, indicating that this was a rare case of a primary ependymoma of the sacral canal. The key to successful treatment involved complete resection of the tumor and postoperative adjuvant radiotherapy.
