CLINICAL DESCRIPTION

Introduction

Degenerative Cervical Myelopathy (DCM) is the most common cause of adult spinal cord impairment worldwide, estimated to affect up to 5% of individuals over 40 (~2% of adults) and expected to rise as populations age [1,2]. Although classically DCM refers to any spinal cord disease (myelopathy) in the neck (cervical), it has become synonymous with this form of myelopathy, a progressive spinal cord injury caused by arthritis. In the past this has also been known as cervical spondylotic myelopathy [3].

People with DCM experience a wide range of debilitating symptoms and amongst the poorest quality of life of any chronic disease [4,5]. Cessation and recovery can be achieved with timely surgical decompression [6,7]; however, this is infrequently the case at present, as most people wait years for diagnosis [8]. Today, 95% are left with disabilities [9], 37% left unable to work and 42% left dependent on others for day-to-day care [8]. People with DCM today have amongst the lowest recorded quality of life of any disease [4]. Further this has a significant impact on their loved ones [10] and a high cost to society: we have recently estimated the cost to English society is ~£700 million per year [11].

Given its prevalence and progressive nature, awareness of DCM amongst healthcare professionals is highly important. Healthcare professionals have a critical role to play in the diagnosis and also long-term symptomatic management of DCM. However, awareness of DCM is low [12]. Early diagnosis is challenging for health professionals but life-changing for individuals with DCM.

The Barrow Neurological Institute has produced this booklet which they have kindly given us permission to share this.

How Common Is DCM?

The epidemiology of DCM is poorly understood. Whilst estimates have been generated by evaluating the number of operations performed [2–8 per 100,000], this is recognised to be a significant underestimate, as not all people with DCM receive surgery and DCM is widely underdiagnosed [1–3]. For example, one study reported undiagnosed DCM in 18% of hip fracture patients [4].

A more realistic estimation, albeit crude, comes from the analysis of magnetic resonance imaging (MRI) studies of healthy volunteers. In a recent meta-analysis, including 3,786 healthy volunteers, 24.6% had evidence of cervical spinal cord compression, with a significantly higher prevalence in older populations compared to younger populations. In some of these studies, imaging was followed by clinical examination, and, using this subgroup analysis of 1,202 individuals, the estimated prevalence of DCM in a healthy population was 2.3% [2]. One MRI study found that 59% of randomly recruited individuals between 40 and 80 years had cervical spinal cord compression but were asymptomatic, and 1% had symptomatic cord compression i.e. DCM [5].

Consequently, the true prevalence of DCM is probably closer to 1 in 50 adults, or 1 in 20 individuals over the age of 40 [2].

What Causes DCM?

The longstanding view has been that chronic tissue compression secondary to spinal canal narrowing, from degenerative (e.g. osteophytes, disc prolapse, ligament hypertrophy or calcification) and/or congenital changes, is the direct cause of the spinal cord injury in DCM [1]. This concept is reflected in current surgical practice, where decisions are often based on the extent of cord compression visualised by MRI [2], and less, as current clinical guidelines recommend [3], on the severity of symptoms.

However, this is an oversimplification, as the chronic compression paradigm fails to account for the full spectrum of clinical disease (Figure 1,[4]), namely:

1. Spinal cord compression is common and most frequently incidental and asymptomatic, with approximately 10% of individuals developing symptoms [5].

2. The extent of static spinal cord compression does not correlate well with the severity of symptoms, clinical phenotype, or disease trajectory [6–12].

3. The functional decline in DCM is rarely linear; it can be stable, stepwise, or, particularly in advanced stages, the decline appears to accelerate [13–15].

4. Microstructural MRI has demonstrated that cord damage precedes the loss of spinal cord function and is not restricted to the area of compression [16–23].

Figure 1: Spinal cord compression defined on MRI is not specific for DCM, and only weakly correlated with disease severity [4].

In DCM, other loading forces can place stress on the spinal cord (Figure 2). Examples include stretch, due to deformity of the spine, and shear (frictional injury), due to movement of the spinal cord. For the purpose of clinical practice and research, DCM is therefore better represented as a progressive spinal cord injury brought about by mechanical stress from arthritic changes to the cervical spine [4].

Figure 2: There are 5 principal types of mechanical loading: Compression [A], Tension (or Stretch) [B], Shear [C], Bending [D], and Torsion [E]. At a small element level, compression, tension and shear are the local stresses [Orange] but at a structural level, these can act in combination, to give rise to bending or torsional loads [Grey]. Compression is the application of an inward force. Tension is the application of a force which elongates a material. Shear forces result from sliding contact between two parallel surfaces. Loading methods are figuratively represented in row 1, and how these might apply to DCM in vivo is illustrated in row 2 [4].

The degenerative changes of the spine that can cause this (intervertebral disc prolapse, osteophyte formation, ligament hypertrophy or calcification) are also termed cervical spondylosis. Cervical spondylosis is best thought of as a normal part of ageing; it can be seen on up to 30–90% of imaging studies amongst healthy adults. These changes are thought to arise from degeneration of the intervertebral discs, reducing their ability to handle mechanical load on the spine, and instead loading the surrounding ligaments, bone and muscles. This catalyses a complex sequence of degeneration within the facet and uncovertebral joints and various ligaments of the cervical spine. In some circumstances this can load the spinal cord and lead to DCM [24].

The loading forces in DCM are both static (happening constantly) and dynamic (happening intermittently as a result of spine movement). This includes both pathological movement such as hyperextension and degenerative subluxation, and also normal physiological movement [24].

How these mechanical stresses drive tissue injury is poorly understood [25]. Several cellular and molecular mechanisms have been identified, including decreased vascular perfusion, endothelial cell dysfunction and blood–spinal-cord-barrier dysfunction, which in turn may cause ischaemia and kindle an inflammatory process and apoptosis (death of both neurons and supporting glial cells) [26]. Histological features include demyelination, white matter tract degeneration, grey matter degeneration, gliosis, microcystic cavitation and Wallerian degeneration of ascending and descending tracts. Although these features have been described, the principal or primary mechanisms, and how these processes interact, remain priority research questions. Individual factors such as age and genetics are likely to be important [4, 27].

A new framework has recently been proposed to incorporate these ideas, representing DCM as a function of mechanical stress, vulnerability and time (Figure 3,[4]). Mechanical stress represents the combined effect of loading on the spinal cord (as a result of degenerative pathology of the spinal column), and time (the duration of such loading). This then drives injury to the spinal cord, which, although retarded by repair processes, will subsequently lead to symptoms.

Figure 3: DCM is a function of mechanical stress, time and vulnerability [4].

Vulnerability represents the factors that govern an individual’s ability to resist spinal cord lesion and/or be resilient to developing symptoms as a consequence (Figure 4). These are yet to be clearly characterised, but may include age, genetics, cardiovascular, gastrointestinal and neurological system factors. The analogy is that a jenga tower can absorb some structural changes but will eventually topple.

Figure 4: Vulnerability. Many interacting factors are likely to determine an individual’s vulnerability. Further research is required to definitively characterise these and their causal mechanisms but for illustrative purposes, those outlined in this article are shown around a ‘Jenga’ tower, the preserved stability of which is defined by the interacting support of many elements but can accommodate some structural changes. Working concentrically outwards, potential causal mechanisms, system level factors, and then whole-body factors are considered. This latter distinction is made as genetic and ageing processes are likely to influence systems as well as the spinal cord directly [4].

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What Is the Natural History?

Natural History describes the usual course of a condition in the absence of medical intervention. For DCM this is poorly understood and is a research priority. The majority of individuals with DCM experience a progressive, stepwise deterioration in their symptoms and functional decline [19]. Without treatment, this may progress to severe disability and complete paralysis [20]. However, the rate of this progression is highly variable: some people with DCM can have a long period of neurological stability without progression and more abrupt deterioration can occur following minor trauma.

Moreover, spinal cord compression can also be asymptomatic. It is estimated the population prevalence of asymptomatic spinal cord compression on MRI is 24.6%, with some series identifying 59% of individuals with spinal cord compression [11] demonstrated on MRI imaging, a prevalence which increases with age, but no symptoms of myelopathy. In series providing longitudinal follow up of asymptomatic spinal cord compression, 8% developed DCM after 12 months and 22% developed DCM within 44 months [21]. Moreover, asymptomatic pathology is not necessarily benign; untreated it is a risk factor for acute spinal cord injury.

Asymptomatic spinal cord compression, abnormal electrophysiological investigations (motor and somatosensory evoked potentials) and radiculopathy, which is compression of the spinal nerve roots causing a lower motor neurone picture of weakness, pain, numbness in the distribution of the specific nerve root, are risk factors for developing DCM [1].

CLINICAL DESCRIPTION

Introduction

How Common Is DCM?

What Causes DCM?

What Is the Natural History?

References – Introduction

References – How Common Is DCM?

References – What Causes DCM?

References – What Is the Natural History?

Professionals