Introduction

Spinal cord stimulation (SCS) is an established modality for treatment of chronic pain. Over the years it has become the most common surgical intervention for medically intractable pain and is now used worldwide with full acceptance by the medical community, patients, and third-party payers. The underlying principle appears to involve electrical activation of the dorsal columns of the spinal cord that is delivered through epidurally placed cylindrical or paddle-type electrodes. Production of paresthesias in the region of pain strongly correlates with the pain relief, and the steering of paresthesias is an integral part of SCS trial procedures.

However, in addition to the beneficial effect on chronic pain, SCS has been used successfully in many other conditions. Indications for clinical and laboratory SCS applications other than pain may be divided into several large categories (Box 14-1):

Motor control, including relief of spasticity, dystonia, and, most recently, parkinsonian symptoms

Vasoactive SCS effects that are used in treatment of peripheral vascular disease and coronary ischemia

Genitourinary applications ranging from incontinence control in paraplegics to augmentation of female sexual function in anorgasmia

More esoteric indications such as impaired consciousness caused by various cerebral pathologies, management of autonomic hyperreflexia, prevention and treatment of cerebral arterial vasospasm, and improvement in tissue perfusion in brain tumors aimed at increased radiosensitivity and chemosensitivity

Box 14-1 Current and Future Indications for Spinal Cord Stimulation (Other than Pain)

Motor System

Spasticity after stroke

Spasticity after spinal cord injury

Spasticity caused by demyelination

Spasticity in cerebral palsy

Motor recovery after spinal cord injury

Parkinson disease

Vascular System

Peripheral vascular disease

Coronary ischemia/angina pectoris

Cerebral ischemia caused by vasospasm

Cerebral ischemia caused by occlusive disease

Genitourinary System

Urinary incontinence/neurogenic bladder

Female orgasmic dysfunction

Other

Autonomic hyperreflexia

Impaired consciousness/vegetative state

Brain tumors

Radiation-induced brain injury

Motor Control

Beneficial effects of SCS on spasticity were discovered early; multiple reports in the 1970s documented usefulness of SCS in improvement of spasticity. Objective evaluation of stretch and H reflexes was used to support clinical results,¹ and the most responsive cause of spasticity was dysfunction of the spinal cord as a result of injury or demyelination.² Developed as an alternative to destructive interventions,^3,4 SCS was used in many clinical centers throughout Europe, Asia, and America with impressive long-term results.^5–8 In addition to patients with spinal cord injuries, SCS was tried in patients with multiple sclerosis, poststroke hemiparesis, dystonia, and cerebral palsy. Animal experiments were used to confirm clinical observations and to find an explanation for the SCS effect and putative mechanism of SCS action in these circumstances.⁹

It has been postulated that spasticity may be relieved with electrical inhibition of impulses transmitted through the reticulospinal tract. The anterior location of the reticulospinal tract in the spinal cord does not allow direct stimulation of this structure from the posterior epidural space without impulses traveling through the dorsal columns. This may explain (a) the observed need in higher-than-usual settings for spasticity control, (b) the fact that paresthesia coverage may not correlate with spasticity relief, and (c) that the spasticity control seems to be more pronounced in patients with more advanced stages of demyelination when sensory impairment allows one to use higher electrical stimulation parameters.

Although the initial impression suggested that spasticity of cerebral origin does not respond to SCS,² subsequent studies showed sustained benefits of SCS in patients with poststroke weakness,^10,11 dystonia,¹² and posthypoxic encephalopathy.¹³ The general enthusiasm was lowered by reports indicating a lack of clinical long-term effectiveness^14,15 or cost-effectiveness of SCS in spasticity,¹⁶ but the main reason for almost complete abandonment of this once popular SCS indication was introduction of intrathecal baclofen administration.¹⁷ However, in countries where intrathecal baclofen is not available because of regulatory barriers, SCS remains a useful tool for treatment of otherwise refractory spasticity through nondestructive intervention.^18,19

In addition to suppression of spasticity in symptomatic patients, SCS may be effective in recovery of motor function in paraplegic patients. A study of 10 patients with complete motor spinal cord injury indicated that epidural SCS at the lumbosacral spinal cord level recruited leg muscles in a segmental-selective way, generating integrated motor behavior of sustained extension and rhythmic flexion and extension movements.²⁰ In the case of an incomplete spinal cord injury, a wheelchair-dependent patient was able to walk with a walker essentially in effortless manner after prolonged SCS. The superiority of gait assisted by SCS was particularly impressive in ambulation at longer distances.²¹

The latest surge of interest to SCS in treatment of motor disorders came from an experimental study showing improvement in locomotion in an experimental model of Parkinson disease (PD).²² The improvement in mobility and restoration of normal patterns of neuronal activity were observed with dorsal column stimulation in both the acute PD model of pharmacologically dopamine-depleted mice and the chronic PD model of hydroxydopamine lesioned rats.²²

Vasoactive Applications of Spinal Cord Stimulation

With the primary intent of pain relief, early SCS implanters noticed that in addition to paresthesias and/or sense of vibration, patients described a sensation of warmth in their extremities; along with this subjective sensation there may have been objective vasodilation and blood flow augmentation. As early as 1976, multiple groups described changes in peripheral blood flow in response to SCS, laying a foundation for subsequent widespread clinical applications.^23,24

This consistent and reproducible effect on autonomic functions became the basis of SCS application for blood flow augmentation and ischemic pain relief in treatment of vascular disorders such as peripheral arterial occlusive disease,²⁵ coronary ischemia/intractable angina,²⁶ and vasospastic disease in extremities.²⁷

A significant wealth of information for these indications exists in the current literature; in this text there are separate chapters dealing with peripheral vascular disease and intractable angina as indications for SCS.

Genitourinary Effects of Spinal Cord Stimulation

Conus medullaris SCS for micturition control in a paraplegic patient was first performed in 1970; this approach was later used in a group of 10 other paraplegic patients with long-lasting symptomatic improvement.²⁸ Improved bladder control was one of the major, results of SCS in a group of 24 patients with upper motor neuron disease, including multiple sclerosis, traumatic spinal cord injury, and neurodegenerative conditions,²⁹ and another group of 11 patients with multiple sclerosis.³⁰

When SCS was implanted specifically to treat neurogenic bladder, most patients developed complete or almost complete normalization of urination with relief of bladder spasticity, marked increase of bladder capacity, and reduction or abolition of postvoid residual urine volume.³¹ The same group of authors noticed no changes in bladder striatal activity or detrusor reflexes in patients who underwent SCS for pain treatment and had intact bladder function.³¹