It is possible the presence of the tethered cord and the occult tethered cord in patients with the Chiari I malformation but it is not simple, nor is it always certain, where to put the limit between the symptoms of the two pathologies, in the diagnosis, in the indication to the treatment and in the technique with which to perform it.

The neurosurgical team of The Chiari Institute in New York said that a certain percentage of the presence of the occult tethered cord associated with the Chiari I malformation cannot be given, but it is probably 5% in the population. 

Colleagues from The Chiari Institute in New York conducted a study published in a medical scientific journal: "Association of Chiari malformation type I and tethered cord syndrome: preliminary results of sectioning filum terminal. Surg. Neurol 2009; 72 (1): 20-25 ". 

The data concluded in this study were: the tethered cord syndrome was present: 

- in 14% (408 cases) of patients with Chiari I malformation (herniation of cerebellar tosillas of 5 or more millimeters); 

- in 63% (182 cases) of patients with low lying cerebellar tonsils (LLCT: tonsillary herniation between 1 and 4 millimeters). 

Symptoms, after resection of the intradural filum terminale, improved or disappeared in 69 children (93%) and 203 adults (83%), unchanged in 5 children (7%) and 39 adults (16%), worsened in 2 adults (1%) in a follow-up period between 6 and 27 months (average 16.1 months). 

Magnetic resonance imaging 1 to 18 months after surgery (average 5.7 months) found a cranial migration of the medullary cone (average 5.1 mm), and an ascent of the cerebellar tonsils (average 3.8 mm). 

The conclusions of this scientific work were: the association between Chiari I malformation and tethered cord is a clinical entity in its own right and represents the progression of the association of low lying cerebellar tonsils and tethered cord. 
However, at The Chiari Institute in New York and we, too, DO NOT recognize the filum terminale section as the surgical treatment of Chiari type I malformation.
The Chiari I malformation can be represented as a box not with a single product but with several products inside. In the suspicion that there is the comorbidity of the pathology of the tethered cord and occult tethered cord in patients with Chiari I malformation, the main problem is deciding when to indicate the section's surgical intervention of the filum terminale and with which technique.
If an occult tethered cord is suspected, lumbar MRI in the prone position is useful as instrumental support for clinical diagnosis; see the dedicated page within this website.

In cases where there is a clear indication for the posterior fossa decompression for Chiari malformation, should be avoided the surgery of sectioning of the filum terminale.

Symptoms of occult tethered cord are numerous, none specific, and may in part overlap with those of Chiari malformation.
The filum terminale section is reasonable to perform in patients with symptoms and clinical signs of tethered cord, with symptoms therefore present at the lumbar level, in the lower limbs and/or with impaired control of the urinary and faecal sphincters. In my experience with the minimally invasive surgical treatment extradural section of the filum terminale under local anaesthesia and the improvement of the tethered cord's classic symptoms, there have often been benefits for the associated symptoms, not present in the classic aforementioned list.

Specifically, there are two surgical techniques for the sectional surgery of the filum terminale:

- Extradural section of the filum terminale under local anaesthesia without bone tissue removal, by the microsurgical technique with minimum skin incision (MIMS) of about 4 cm, published by us in the scientific journal "Operative Neurosurgery" in 2018. We proceed with the sacrococcygeal ligament opening, under which there is the extradural filum terminale which is sectioned. We have used this minimally invasive surgical technique since 2010, and the series includes more than 200 patients from 17 of the 20 Italian regions and 10 foreign patients from Europe and America.

- Section of the intradural filum terminale under general anaesthesia: classic neurosurgical intervention with laminectomy, removal of the posterior bone portion of the vertebrae, the opening of the meninges, recognition between the nerve roots of the intradural filum terminale, which is then sectioned.
The difference between the possible surgical complications is significant as with the minimally invasive surgery, and they are linked only to the skin incision. In classic surgery under general anaesthesia, they depend on the anatomical bone, muscle and nerve structures present in that surgical approach.

PS I cannot comment on the surgical results of other Institutes because they are covered by copyright. Therefore, those who want to pursue patent protection, which means obtaining a patent on something that is kept secret, renounces publication and therefore, as a researcher does not exist, in the sense that it does not he can claim to be recognized by the scientific community for having made known his work, the result obtained and the method adopted.