Under normal conditions, the bony elements of the cervical spine and the craniocervical junction are supported by the integrity of the ligamentous elements and the craniocervical muscular elements. Therefore, all situations involving an intrinsic weakness of ligaments or muscles, allow an increase in mobility of the cervical spine and specifically of the craniocervical junction. Especially in hypermobility syndromes such as Ehler Danlos syndrome, there is an increase in mobility due to the hyperlaxity of the ligamentous elements given the abnormalities in the expression or structure of the fibrillar collagen (craniocervical instability Ehler Danlos). In other words, in these cases there is an incompetence of the ligamentous elements (and also bony) to support both the cervical spine and the craniocervical junction. This increased mobility of the craniocervical junction, which causes clinical symptomatology, is called craniocervical instability or atlanto-occipital instability.
Therefore, craniocervical instability basically is an incompetence of the ligamentous and bony elements to support or hold the weight of the skull and the movements of the skull with respect to the cervical spine.
This increased mobility of the craniocervical junction may cause neurological symptoms in these patients, either due to neuronal compression at the level of the brainstem and/or cervical spine or symptomatology derived from the compression/distortion of the Vertebral Artery.
In these hypermobile cases, instability can occur at any level of the spine, but it is common to present as craniocervical instability (CCI) –affectation of the joints between the skull and the first cervical vertebra or C1 (Atlas)- and atlantoaxial instability (AII)- affectation of joints located between the first cervical vertebra (C1-Atlas) and the second cervical vertebra (C2-Axis). These instabilities can be expressed along with other instabilities in other spine levels such as the subaxial cervical vertebrae (C3 C4 C5 C6). In these cases a head and/or neck traumatic antecedent may have occurred and may result in the symptomatology being triggered, although in many cases this traumatic antecedent does not exist. This leads to severe head and neck pain, in addition to other neurological symptoms we define as “cervicomedullary syndrome” or “cervicocranial syndrome”. This syndrome consists of a set of symptoms potentially translating an affectation of these patients’ cervicomedular area:
Craniocervical instability diagnosis is given by performing an Upright Magnetic Resonance Imaging (MRI) that allows evaluating the craniocervical junction in an upright position as well as the evaluation of the patients’ anatomy in flexion & extension and neck rotation. In many cases, performing a supine (standard) MRI does not allow an accurate diagnosis and may be diagnosed as a normal MRI or with nonspecific signs. The surgeon should evaluate the MRI and check the existence of direct signs (angle measurements and distances between bone elements of the craniocervical junction) and also the existence of indirect signs of craniocervical mobility. The most commonly used measures in the radiological evaluation are CXA, Grabb, BDI, BAI, ADI (see below). It is extremely important to perform the clinical-radiological correlation to estimate the congruence between the clinical symptomatology and the MRI. In many cases, it is also important to perform a 3D bone CT scan of the craniocervical junction.
In this group of patients, the most frequent situations consist of:
1-Craniocervical instability, levels C0-C1 (Occipital-atlas).
2-Atlantoaxial instability, levels C1-C2 (atlas-axis).
3-Cranio-atlanto-axial instability, levels C0-C1-C2.
This division has important considerations for the surgical treatment since the location of the instability must be determined with precision and try to fusion only what is necessary. It should be remembered that approximately 50% of neck’s flexion/extension takes place at C0-C1-C2 levels and that up to 50% of neck’s rotation occurs at C1-C2 level. Therefore fusion in this area may imply a marked reduction of the mobility of the head and neck and should always be considered before proposing a surgical procedure like such.
Abbreviations: BDI: basion dens interval, CXA: clivo axial angle, BAI: basion-axial interval, ADI: Atlantoaxial interval
-Dr. Vicenç Gilete, MD, Neurosurgeon & Spine Surgeon.
-Henderson FC, Wilson WA, Mott S, Mark A, Schmidt K, Berry KJ, et al. Deformative stress associated with an abnormal clivo-axial angle: A finite element analysis. Surg Neurol Int 2010;1:30
Last Update 2020-05-12 10:25:27