Adjacent Segment Disease

Overview

Adjacent segment disease (ASD) refers to the development of new, symptomatic degenerative changes at a spinal motion segment immediately above or below a previously fused or instrumented level. It is important to distinguish this from adjacent segment degeneration: a purely radiographic finding that may cause no symptoms at all. The term "disease" is reserved for cases where imaging changes correspond to clinical symptoms such as radiculopathy, neurogenic claudication, or axial spine pain.

ASD occurs across both the cervical and lumbar spine, the two regions where fusion is most commonly performed. Symptomatic adjacent segment disease is reported at a rate of a few percent per year following both anterior cervical discectomy and fusion and lumbar arthrodesis, with cumulative incidence rising over a decade of follow-up.

What causes it

The leading explanation is altered biomechanics. A rigid spinal fusion eliminates motion at the operated segment and transfers increased mechanical load, intradiscal pressure, and facet stress to the neighboring mobile levels. Over time, this accelerated wear promotes disc degeneration, facet joint arthritis, ligament thickening, and bone spur formation, all of which can narrow the spinal canal and nerve passageways.

A coexisting and debated contributor is the natural history of degenerative disc disease, meaning some adjacent-level changes may have progressed regardless of the index surgery.

Recognized risk factors include:

• Longer or multi-level fusion constructs
• Pre-existing degeneration or narrowing at the adjacent level before the original surgery
• Poor sagittal (front-to-back) spinal alignment, including flatback deformity
• Fusion adjacent to a transitional anatomic zone, such as the neck-chest or low-back-pelvis junction
• Older age, obesity, smoking, and osteoporosis
• Surgical factors such as injury to the adjacent facet capsule or nearby ligaments during exposure, and inadequate restoration of lumbar lordosis or cervical sagittal balance

Symptoms and warning signs

Patients typically experience a period of relief following their index fusion before new or recurrent symptoms emerge that are referable to the adjacent level.

In the lumbar spine, this may present as:

• Radiating leg pain in a nerve-root pattern (radiculopathy)
• Leg fatigue, cramping, or weakness with standing and walking (neurogenic claudication)
• Mechanical low back pain

In the cervical spine, adjacent segment disease may cause:

• Radiating arm pain, tingling, or numbness
• Arm or hand weakness or sensory disturbance

On examination, a clinician may find focal motor or sensory deficits, reduced reflexes, a positive straight leg raise in lumbar cases, or a positive Spurling sign in cervical cases, findings that should localize to the level adjacent to the prior fusion. The natural history is variable, and many patients improve or stabilize with non-operative care.

Certain warning signs demand urgent evaluation. Bowel or bladder dysfunction, saddle anesthesia (numbness in the inner thighs, buttocks, groin, or the area around the genitals and rectum), or sudden bilateral leg weakness may indicate cauda equina syndrome, a surgical emergency. Progressive balance problems, gait disturbance, or worsening hand clumsiness can suggest spinal cord compression (myelopathy). New severe pain accompanied by fever, night sweats, unexplained weight loss, or a known history of cancer should prompt urgent workup to exclude spinal infection or metastatic disease before a degenerative cause is assumed.

How it's diagnosed

Diagnosis rests on correlating new symptoms with structural changes at the level adjacent to the prior fusion. Because asymptomatic adjacent degeneration is common on imaging, the central principle is concordance between the imaging abnormality and the patient's clinical syndrome.

• MRI is the primary study. It demonstrates disc desiccation and collapse, central canal stenosis, foraminal narrowing, and nerve root compression, while also helping exclude infection or tumor through marrow signal and contrast enhancement patterns.
• CT scan better characterizes bone spurs, facet arthritis, and the integrity of existing hardware, and can identify pseudarthrosis (failed fusion) at the index level.
• Standing radiographs, including flexion-extension views and full-length scoliosis films, assess instability, sagittal balance, and overall spinal alignment.
• Selective nerve root blocks or diagnostic injections and electrodiagnostic studies (EMG and nerve conduction testing) can help confirm the symptomatic level and exclude alternative diagnoses such as peripheral neuropathy, hip osteoarthritis, or shoulder pathology.

Treatment options

Initial management of adjacent segment disease mirrors treatment of primary degenerative spinal disease and is conservative in the first instance:

• Activity modification and relative rest during symptom flares
• Anti-inflammatory medication
• Structured physical therapy emphasizing core strengthening and postural conditioning
• Short courses of neuropathic agents for radicular pain
• Image-guided epidural steroid injections or selective nerve root blocks, which can provide both diagnostic information and durable symptomatic relief for a subset of patients

Many patients improve or stabilize with these measures, and non-operative care is the appropriate starting point for most.

When surgery is considered

Surgery is considered when symptoms are severe, persistent despite six to twelve weeks or more of appropriate non-operative care, or accompanied by progressive neurologic deficit. Cauda equina syndrome or evolving myelopathy are indications for urgent surgical intervention.

Operative options range from decompression alone (removing compressive bone spurs or disc material without adding further fusion) in carefully selected cases, to extending the existing fusion construct to incorporate the symptomatic adjacent level. When fusion is extended, restoring sagittal alignment is a key surgical goal to reduce the risk of further junctional breakdown. Decisions are individualized, weighing the patient's neurologic deficits, spinal alignment, bone quality, and the recognized possibility that extending the fusion may shift mechanical load to the next mobile segment.