Teaching Case

A 36-year-old female patient presented to the department with muscular hypotrophy of the left leg of several months duration. Past medical history of urinary stress incontinence and the operation of a spinal lipoma. The gynaecological status was normal. No other medical complaints were reported.

X-rays of the lumbar spine and MRI without contrast enhancement are presented. The AP X-ray of the pelvis demonstrates non-fusion of the vertebral arches of L5, S1 and S2. Hemivertebra of the S3 with deformed left sacral ala is depicted. Coccygeal vertebras are not observed and their absence is confirmed on the lateral view.
On the obtained MRI images, an extradural formation at the S1-S2-S3 levels is seen, which encloses the dura. It presents with high signal intensity on T1WI and T2WI, with homogeneously suppressed signal on STIR-images. The vertebral canal is dilated caudally from L5, measuring 45/30mm at S2 level. The spinal cord reaches S1 and the central canal is subtly dilated. The terminal filum is thickened, dislocated, elongated, and reaching S1-S2 levels. Additional disc protrusion is observed at level L1-L2 without compression of the nerve roots or the dural sac.

Sacral agenesis is defined as a congenital absence of the whole sacrum or part of it. Embryologically, it is part of the group of closed spinal dysraphisms, due to disorders of notochordal formation. Approximately 15-25% of mothers of these children have insulin-dependent diabetes mellitus. It can be with autosomal dominant form of inheritance, which defines the need for genetic counselling [1]. The worldwide incidence of neural tube defects ranges between 1.0 and 10.0 per 1, 000 births, with almost half of these cases falling under the category of spinal dysraphisms [2].
Sacral agenesis has been classified into four types: types 1 and 2 refer to partial sacral agenesis, where part of the sacrum remains; types 3 and 4 refer to total absence of a sacrum, i.e. complete sacral agenesis; type 4 also involves the absence of one or more lumbar vertebrae. The defective left sacroiliac joint can be linked to a fracture, but history of trauma is required. On MRI imaging there is no oedema of bone marrow and soft tissues [3]. Early detection is important, as there is a significant association with neuropathic bladder, recurrent urinary tract infections, and incontinence.

In our case complex syndromes like VACTERL are excluded, because there is no evidence for cardiac pathology and anal stenosis. Evidence exists to suggest this can be a progressive neurological disorder due to growth and traction on abnormally positioned sacral roots, or in association with a tethered cord. Like tethered cord syndrome, diastematomyelia is a form of spina bifida occulta, but two spinal cords are clearly observed, separated by a fibrous or bony membrane in a single or double dural sac. A defect between S2 and S4 is most likely to lead to bladder dysfunction. The diagnosis is usually made prenatally and in early childhood, but mild and asymptomatic forms can be found in adulthood, as in our case. Mega sac can also lead to urinary incontinence and leg problems, but the sacrum is well-formed and only thinned. Vertebral scalloping is usually present. Clinical, neuroradiological and developmental features should be correlated when classifying spinal dysraphism [4, 5].

Sacral agenesis can be an isolated finding or it can be associated with other congenital anomalies. It can lead to clinical symptoms not only in neonates and early childhood, but in adulthood too. Early imaging is very important to start appropriate treatment and define comorbidities. As there is autosomal dominant inheritance, genetic counselling should be considered.