If you live with hypermobility or Ehlers–Danlos syndrome (EDS), you may have heard the terms atlantoaxial instability (AAI) or craniocervical instability (CCI) mentioned online, in support groups, or in specialist conversations. These terms can sound intimidating, but like many things in EDS, AAI and CCI are best understood not as isolated diagnoses, but as part of a broader connective-tissue and nervous-system picture. Research over the last several years, including work published in Frontiers journals, is helping clarify how upper cervical instability may develop, how it can present, and how it fits into a whole-body treatment approach.
What Are AAI and CCI?
The craniocervical junction (CCJ) is the area where the skull meets the top two cervical vertebrae (C0–C2). This region houses critical neurological, vascular, and connective tissue structures, including:
- the brainstem
- upper spinal cord
- cranial nerves
- vertebral arteries and venous outflow pathways
- dura and cerebrospinal fluid (CSF) pathways
Atlantoaxial instability (AAI) refers specifically to excessive or poorly controlled motion between C1 and C2.
Craniocervical instability (CCI) is a broader term describing impaired stability or altered mechanics across the skull–C1–C2 complex.
In hypermobility spectrum disorders and EDS, ligamentous and connective tissue differences may reduce the passive stability normally provided in this region, increasing reliance on muscular, neural, and pressure-based stabilization.
Why the Upper Cervical Region Is So Sensitive
The CCJ is not just a mechanical junction. It is a neurological and pressure-regulating crossroads.
Research highlights several relevant features in connective tissue disorders:
- Connective tissue forms part of the meninges, blood–brain barrier, and neural extracellular matrix, influencing how forces and fluids are transmitted through the nervous system.
- Altered connective tissue properties may affect neurovascular mechanics, dural compliance, and CSF flow dynamics.
- Small changes in alignment or motion tolerance at the CCJ can have outsized effects on symptoms because of the density of neural and vascular structures in this area.
These relationships are discussed in recent reviews of neuraxial involvement in EDS and related conditions.
A Systems Perspective: It’s Rarely “Just the Neck”
One of the most important shifts in recent research is moving away from viewing AAI/CCI as a purely structural neck problem.
Instead, current models emphasize interdependent systems, including:
- Connective tissue integrity
- CSF pressure and flow regulation
- Venous outflow from the head and neck
- Autonomic nervous system regulation
- Proprioceptive and sensorimotor control
- Inflammatory and immune factors
The Monro–Kellie doctrine reminds us that brain tissue, blood, and CSF exist in a pressure-balanced system. Changes in one component often lead to compensations in the others. In EDS, connective tissue differences may make this system less tolerant of chronic imbalance.
This helps explain why upper cervical symptoms may overlap with:
- headaches that change with position
- dizziness or balance disturbances
- visual or auditory changes
- dysautonomia or POTS-like symptoms
- cognitive fog or fatigue
- jaw, airway, or swallowing issues
What Symptoms Can Look Like
Symptoms associated with upper cervical involvement exist on a spectrum, and many people with hypermobility experience mild or intermittent features that never progress.
Commonly reported experiences include:
- a “heavy head” or “bobblehead” sensation
- neck fatigue rather than sharp pain
- dizziness or unsteadiness with head movement
- increased symptoms with prolonged upright posture
- visual strain, difficulty focusing, or light sensitivity
- a sense that the nervous system is easily overwhelmed
Importantly, these symptoms are not specific to AAI/CCI and can also arise from vestibular, autonomic, inflammatory, or pressure-regulation factors. This is why a careful, systems-based evaluation matters.
Conservative Care and the Role of Physical Therapy
For many people, management is non-surgical and conservative, particularly when symptoms are mild to moderate.
Physical therapy approaches informed by current evidence may focus on:
- Cervical sensorimotor control, not forceful strengthening
- Proprioceptive retraining to improve head–neck–trunk integration
- Visual–vestibular coordination
- Breathing and pressure regulation strategies
- Postural support that is adaptable, not rigid
- Addressing distal contributors such as thoracic, rib, pelvic, jaw, or diaphragmatic mechanics
Research supports that in hypermobility, improving motor control, proprioception, and nervous system regulation can meaningfully reduce symptom burden even when connective tissue laxity remains.
With thoughtful, individualized care, many patients improve their tolerance, confidence, and quality of life even when connective tissue differences remain.
Selected References
- Frost et al., Frontiers in Neurology (2024): Neuraxial disorders and connective tissue disease
- Wang et al., Journal of Ultrasound (2023): Connective tissue and extracellular matrix behavior in hEDS
- Petrucci et al., Mayo Clinic Proceedings: Innovations, Quality & Outcomes (2024): Phenotypic clusters in hypermobile EDS
- Benson et al., American Journal of Neuroradiology (2023): Reappraisal of the Monro–Kellie doctrine
- Curry et al., Frontiers in Pediatrics (2022): Post-concussion vulnerability in EDS
- Brodbelt et al., British Journal of Neurosurgery (2023): Hypermobility and neurosurgical considerations
- Stott P., Advanced Cranial and Upper Cervical Instability in HSD/EDS (course material)CCI:AAI