Case of the Month #21: Spinal Stenosis by Dr Helen Laycock

Please also see references and this month’s recommended reading section.

Lumbar spinal stenosis occurs when there is compression of the central canal, that can include compression of the nerve roots.  This can cause pain in the lower back and legs and is commonly a consequence of degenerative changes in the lumbar spine.

Although the exact incidence is unknown, it is estimated to affect around 11% of the population, and is a common cause of pain and disability in older people.  Prevalence increases with age, however patients with congenital lumbar spinal stenosis are often younger.

There is narrowing of the lumbar spinal canal and/or intervertebral foramina.  Compression of spinal nerve roots from disk protrusion, ligamentum flavum hypertrophy, facet joint arthrosis and osteophyte formation or spondylolisthesis can displace the nerve roots in either the central canal or neural foramina and lateral recesses, where they exit to the lower extremities.

Narrowing can cause not only compression but also ischaemia of the associated neural and vascular structures.  Although not entirely understood how this leads to back and leg pain, paraesthesia and weakness, it could be small arterioles are compressed leading to nerve ischaemia or normal venous drainage is impeded leading to increased venous pressure, the accumulation of metabolites and consequent nerve root damage.  

Causes
Lumbar spinal stenosis typically develops with older age and is a degenerative condition that is often a combination of disk protrusion, hypertrophy of facet joints and ligamentum flavum and spondylolisthesis.  

Other possible but less common aetiologies include: metabolic syndromes such as Paget disease, where bony overgrowth can compress spinal nerve roots; congenitally small central canal (congenital stenosis); epidural lipomatosis (excess fat in epidural space).  

Clinical picture
There are no history or physical examination findings that are both highly sensitive and specific for lumbar spinal stenosis.  However, a 2016 international consensus agreed the key clinical features to diagnosis spinal stenosis in over 50 year old’s are:

• Leg or buttock pain whilst walking
• Forward flexion required to relieve symptoms
• Using a shopping card or bicycle leading to relief
• Walking causing motor or sensory disturbance
• Normal and symmetrical foot pulses
• Weakness in lower extremities
• Lower back pain

In general, discomfort in the lower limbs and feet can be unilateral or bilateral, and can occur with lower lumbar spine, buttock and thigh discomfort.  Compression of sensory fibres can lead to numbness and paraesthesia of the lower legs and feet, with posterior column fibre compression later causing worsening balance and unsteadiness with a wide based gait.  

Patients mention that standing and walking can worsen symptoms because this increases lordosis and thus stenosis (neurogenic claudication). Conversely, by reversing the lordosis through forward flexion (sitting/bending forward) the canal will open, blood flow will improve and symptoms are relieved.

Because pain reduces walking, limiting function and ability to take part in daily activities, there are often negative psychological effects.

Physical examination can demonstrate a wide based gait and abnormal Romberg sign, which is specific but not very sensitive in diagnosing lumbar spinal stenosis.  Lumbar extension often causes pain in buttocks, thighs and the lumbosacral junction that is relieved by lumbar spine flexion, but examination of the spine is not normally tender.  Neurological examination can be normal with deficits tending to progress slowly, occurring with more severe disease.  Sensory changes tend to be L4 (medial malleolus), L5 (great toe) and S1 (lateral malleolus) deficits to pinprick or vibration, alongside weakness of great toe or ankle flexion or extension, with some patients presenting with absent ankle reflexes.