In recent years there has been renewed interest in the relation between Chiari I malformation and spinal cord tethering.
A 3-year-old girl presented with right leg pain. At spinal MR scan there was diastematomyelia with a bony peg at L1 and syringomyelia at T10-11. She had a surgical repair of the diastematomyelia without filum division. Symptoms settled postoperatively. A few months later she started displaying weakness in the right leg and loss of bulkiness of the right calf. A new MR scan showed a new Chiari I malformation, which was not present in the preoperative MR scan. The syringomyelia cavity was smaller. There was low-ending conus medullaris at L3 and a thickened filum. The filum was divided 15 months after the first operation. At MR scan obtained 18 months after filum division, the previously noted acquired Chiari I malformation had disappeared.
There is no obvious explanation for the formation of acquired Chiari I malformation and its disappearance after filum division. Recently, there has been description of patients with Chiari I malformation and normal posterior fossa volume, who were treated with filum division, and their Chiari malformation regressed without craniovertebral decompression surgery. Nevertheless, it is difficult to explain how traction on the lowest aspect of the spinal cord could lead to herniation of the cerebellar tonsils through the foramen magnum.