Disease description.
Sacral duct cysts are dural cysts that originate from the periaqueductal membrane of the spinal cord and are therefore referred to by the general term “intravertebral spinal cysts”; there are no exact statistics on the incidence of sacral duct cysts in the population. In fact, understanding the causes of sacral cysts and their treatment can greatly reduce such concerns.
Disease classification.
Sacral duct cysts are a type of spinal cyst and are broadly classified into two types.
Type IB
Epidural spinal cysts that do not contain spinal nerve root fibers, due to congenital dural diverticulum or congenital dural defects caused by arachnoid herniation, mostly located at the level of S1-3 of the sacral canal, common in adults, no significant difference between men and women.
Type II
Epidural spinal cysts containing spinal nerve root fibers are abnormal expansions of the distal spinal nerve root sleeves forming cysts, usually located at the level of S2-3 spinal ganglia or their distal ends, and are commonly seen in adults.
Pathogenesis.
Sacral canal cysts are dural cysts that originate from the spinal cord tegmentum, so the term “intradural spinal cysts” is used to refer to these diseases in general. Most intradural spinal cysts are thought to be congenital, but some are acquired, and the causes of each type vary. Regardless of the cause, cysts always form because they initially communicate with the subarachnoid space, where cerebrospinal fluid enters with arterial pulsation and eventually enlarges due to poor outflow or due to fluid hydrostatic pressure.
Disease symptoms.
Most patients with epidural spinal cysts that do not contain spinal nerve root fibers are asymptomatic; 25% of patients with epidural spinal cysts that contain spinal nerve root fibers are symptomatic. The sacral canal contains sensory and motor nerves innervating the saddle area, dorsal thigh, and perineal area, as well as parasympathetic nerve fibers innervating urination and defecation. Therefore, the clinical manifestation of sacral cyst is mainly chronic pain and discomfort in the lower back, sacrococcygeal region and perineum; it may also be accompanied by dorsal thigh pain, sciatica and even neurogenic claudication.
As mentioned above, sacral cysts are spinal cysts, not tumors, and there is no possibility of malignant transformation. The pressure of cerebrospinal fluid inside the cyst increases, compressing the surrounding sacral nerve and bone, which may cause bone destruction in severe cases. If the cyst persistently compresses the peripheral nerve fibers, severe patients may develop sensory and motor dysfunction, and even abnormal urinary and fecal functions. Rarely, cyst rupture causes chemical inflammation.
Ancillary examinations.
X-ray examination
Erosion of sacral bone can be detected, mainly showing enlargement of the sacral canal and fan-like lace-like changes of bone erosion at the posterior edge of the vertebral body. Sometimes congenital malformations of the lumbosacral region such as occult spina bifida, spondylolisthesis, kyphosis, etc. can also be found at the same time.
CT examination
It can clearly show the bone destruction and occupying lesions, especially for sacrum.
MRI examination
MRI is the most reliable method to diagnose intraspinal spinal cysts, which are bag-shaped, ovoid and irregular in shape, and the signal of the cyst fluid is similar to that of cerebrospinal fluid, with low signal in TlWI and high signal in T2WI. type IB is located in the sacral canal and is separated from the dural sac by fat. Type II is located lateral to the dural sac with the presence of nerve roots within the sac.
Differential diagnosis.
Lumbar disc herniation
Intraspinal spinal cysts have a slow course, mild and atypical clinical symptoms, and clinical manifestations and signs are similar to those of lumbar disc herniation. Features of sacral cyst: the cyst is a benign lesion with slow growth, long course, and intermediate remission of symptoms; symptoms are characterized by lumbosacral pain, perineal hyperalgesia, and chronic course; the cyst is a distending lesion, and spinal X-ray plain film shows enlargement of the spinal canal cavity in the lesion area, thinning of the pedicle, and widening of the pedicle spacing; MRI can clearly differentiate.
Disease treatment.
Sacral canal cysts are common and most of them are asymptomatic; for asymptomatic ones, treatment is generally not needed and can be observed first. For those with symptoms, active surgery should be performed on the premise of excluding disc herniation, spinal stenosis or tumor in the sacral canal.
Indications for surgery
Generally speaking, surgery is feasible in the following cases: back and leg pain or intermittent claudication for which conservative treatment is ineffective and which affects normal life or work; back and leg pain with reduced muscle strength and sensation in the lower limbs; pain or reduced sensation in the perineum, urinary and faecal or sexual dysfunction.
Surgical treatment
Surgery should be performed under a microscope to trim and shape the cyst wall. The cyst wall should be sharply excised with microscissors and should not be bluntly peeled to avoid damaging the nerve roots and not to force complete removal of the cyst wall. Surgery should locate the cerebrospinal fluid leak and close it with a vascular anastomosis line (type IA) or remodel the nerve root sleeve (type II).
Precautions.
Postoperatively, patients should be placed in a head-low-hip-high prone position as much as possible, the wound should be compressed with sandbags, and downward movement should be done after 1 week,wearing a peri-waist after surgery, and functional exercise after 3 weeks.
Disease prognosis.
With the widespread use of MRI in clinical practice, clinicians’ understanding and diagnosis of intraspinal spinal cysts have improved greatly. As long as the indications for surgery are well chosen, the results of surgical treatment are satisfactory.