What is a sacral cyst? It is unfamiliar to the majority of the population, but in fact it is a common disease. The prevalence of sacral cysts in the population is as high as 4%, and about 15% of them have symptoms of back and leg pain, so there are about 120,000 symptomatic sacral cyst patients in Shanghai alone! However, only hundreds of cases of sacral cysts are diagnosed in major hospitals in Shanghai every year, and only dozens of cases receive formal treatment, reflecting that a large number of patients do not receive timely and reasonable diagnosis and treatment. The sacral canal cyst is located in the sacral canal below the lumbar vertebrae and right after the pelvis, and the cyst is filled with watery and clear cerebrospinal fluid, and the cyst is connected with the normal cerebrospinal fluid in the spine through a one-way valve (i.e. leaky mouth). lumbosacral pain, perineal tingling in the buttocks, perianal pulling-like pain, radiating pain in the lower limbs and intermittent claudication; sexual dysfunction and urinary and fecal incontinence gradually appear as the disease progresses. In this way, sacral cysts have many similarities with intracranial arachnoid cysts: they are both cysts with cerebrospinal fluid components inside, connected to the normal subarachnoid space by a one-way valve, and cerebrospinal fluid flows into the cyst from the subarachnoid space but cannot flow out, so they gradually increase in size, and the main pathology is mechanical compression of the surrounding nerve tissue. So, what is the difference between sacral cysts and intracranial arachnoid cysts? This is a question that many physicians have difficulty answering accurately. The superficial difference is the location and symptoms: the former is located in the lumbosacral region and mainly causes the aforementioned lumbosacral, perineal, perianal, and lower extremity pain, while intracranial arachnoid cysts compress the brain or cerebellum and mainly cause headache and dizziness. The substantive difference lies in the hydrostatic pressure of both: as we know, the entire brain and spinal cord are immersed in cerebrospinal fluid, which is water-like and colorless and transparent, filling the entire cranial cavity and vertebral canal cavity, playing the role of buffering and protecting the cerebrospinal cord; when the body is upright, the hydrostatic pressure at the top of the head is the lowest, while the hydrostatic pressure at the lumbosacral region is the highest, nearly 1000 mm water column higher than the intracranial pressure! Intracranial arachnoid cysts are located at the uppermost end of the entire cerebrospinal fluid system, where the pressure is very low, so surgical treatment of intracranial arachnoid cysts is relatively easy. Recurrence can be avoided by microscopic removal of most of the cyst wall through a locked-hole procedure to relieve the compression and open the arachnoid cyst to the normal brain pool, creating bidirectional flow. This microscopic resection is thorough and safe, with an efficiency of more than 99%. The sacral cyst is located at the lowest end of the entire cerebrospinal fluid system and has the highest pressure. Imagine: if a hole is opened at the bottom of a 1-meter high bucket, will it not gush out? If you put some glue on the outside of the hole, how could it be stopped? In the early days, some neurosurgeons failed to recognize this substantial difference and blindly applied the experience of treating intracranial arachnoid cysts to sacral cysts, and after finding the sacral cysts during surgery, they cut and released the water, and then they ended up with glue. Patients often recur before they are discharged from the hospital, and the recurrence rate is almost 100%. After nearly two decades of exploration and continuous improvement of treatment protocols, the international medical community has now made breakthroughs, and it can be said that the core concept of treatment of sacral cysts today lies in the word “blocking”. Nevertheless, sacral cysts are still one of the most challenging procedures in neurosurgery today.