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Monday, September 8, 2014

Deep-Brain Stimulation — Entering the Era of Human Neural-Network Modulation


Michael S. Okun, M.D.
September 8, 2014






Deep-Brain Stimulation.
Scribonius Largus, the court physician for the Roman emperor Claudius, used an electrical torpedo fish in 50 A.D. to treat headaches and gout. More than 1000 years elapsed before the idea of therapeutic brain stimulation was rekindled. In 1786, Luigi Galvani demonstrated that he could conduct electricity through the nerves in a frog's leg. Later, Alessandro Volta conducted electrical current through wires and built crude but effective battery sources. Yet none of these experimenters could have predicted the usefulness of their technology in treating human disease by applying an electrical current within the human brain.
This year's Lasker–Debakey Clinical Medical Research Award, announced September 8, recognizes the contributions of two pioneers in deep-brain stimulation (DBS): Alim-Louis Benabid, a neurosurgeon, and Mahlon DeLong, a neurologist. Their research and its translation into clinical practice have improved the lives of more than 100,000 people with Parkinson's disease or other neurologic or neuropsychiatric disorders.
Typically, people with Parkinson's disease receive the diagnosis in the sixth or seventh decade of life. Age is the most important risk factor for the disease, and it has been estimated that 1 to 2% of people older than 60 years of age are affected. The disability associated with Parkinson's disease arises from a broad spectrum of motor symptoms (masked face, soft voice, tremor, small handwriting, rigidity, bradykinesia, dystonia, balance issues, and shuffling steps) and nonmotor symptoms (depression, anxiety, apathy, disordered sleep, and cognitive difficulties), as well as problems of the autonomic nervous system (sexual dysfunction, constipation, gastrointestinal problems, and orthostatic hypotension). Of every three patients diagnosed with Parkinson's disease, one will become unemployed within 1 year, and most will be unemployed after 5 years. On average, patients with Parkinson's disease will spend $1,000 to $6,000 per year on medications, and their annual risk of hospitalization exceeds 30%.
Before the late 1960s, pioneers sectioned the human brain's motor pathways, and later investigators intentionally ablated many basal ganglia regions with alcohol or the application of heat; this approach met with limited success, however, partly because of inaccurate, imprecise, and inconsistent targeting. Moreover, intentionally created bilateral brain lesions frequently led to irreversible deficits in speech, swallowing, and cognition. This surgical approach faded in popularity with the discovery of levodopa (dopamine replacement).
Before levodopa's introduction, life for patients with Parkinson's disease was dreadful. Many were institutionalized. After levodopa, it became routine for patients with Parkinson's disease to “awaken” from frozen states, and nearly all were able to live at home. Tremors faded, stiffness waned, and many patients regained their ability to walk. Yet important and unexpected challenges emerged. The most worrisome were dopamine-related, medication-induced complications. Patients began to report fluctuations (doses wearing off), freezing (especially when walking), and dance-like movements (chorea), later termed levodopa-induced dyskinesia. Many reported tremors that did not respond to pharmacotherapy. In addition, there was a growing realization that levodopa was not a cure and that the disease progressed despite miraculous “awakenings.”
In the early 1970s, shortly after levodopa's introduction, Mahlon DeLong began studying a complex and neglected area of the brain. By the time DeLong joined Edward Evarts' laboratory at the National Institutes of Health, all the “good stuff” (such as the motor cortex and cerebellum) had been assigned to other researchers. He was stuck with the basal ganglia. The paucity of knowledge of even the normal anatomy and physiology of this part of the brain did not deter DeLong, who published a seminal description of electrical activity patterns in primate basal ganglia neurons and a complete description of these neurons' responses to movement.
DeLong, along with Garrett Alexander and Peter Strick, broke open research on basal ganglia and Parkinson's disease in 1986 when they introduced the segregated circuit hypothesis — the idea that the basal ganglia and associated areas of cortex and thalamus could be divided into separate territories, with little functional or anatomical cross-talk.1 This observation seeded a new understanding of human neural networks, paving the way for electrical modulation. It also clarified that many of the symptoms of neurologic and neuropsychiatric diseases could be associated with dysfunction in specific cortical–basal ganglia brain circuits. DeLong, Hagai Bergman, and Thomas Wichmann tested this hypothesis by destroying the subthalamic nucleus in a primate model of Parkinson's disease, and they demonstrated improvement in disease symptoms.2 Soon thereafter, electricity was introduced as a modulation-based approach to the brain circuits in Parkinson's disease (see Figure 1
FIGURE 1

The DeLong “Box” Models of Basal Ganglia Circuitry and Their Use in Guiding Deep-Brain Stimulation (DBS).
). A French neurosurgeon, Alim-Louis Benabid, would take the courageous step of leaving a wire that could provide continuous electrical current inside a human brain.
In 1987, Benabid operated on an elderly man who had tremor. He had previously created a brain lesion to treat this tremor, but he was concerned about the potential adverse effects associated with doing the same in the other hemisphere. And so, in a second procedure, he addressed the contralateral tremor. He passed a large test probe several centimeters below the brain's surface. He knew from previous surgeries that low-frequency stimulation worsened tremor and that faster pulses suppressed it. Benabid left a neurostimulator in the man's brain. He implanted a wire with four metal contacts at its tip. This wire, the DBS lead, was then connected to an external battery source. Benabid and colleagues programmed the device using a small box with buttons and archaic-looking switches. As simple as the system was, it turned out to be very powerful, allowing Benabid and Pierre Pollack to individualize the settings; the results are described in several seminal articles.3,4
Although the biology and mechanisms underpinning DBS therapy remain unclear, we now know that normal human brain function is largely mediated through rhythmic oscillations that continuously repeat. These oscillations can change and modulate, ultimately affecting cognitive, behavioral, and motor function. If an oscillation goes bad, it can cause a disabling tremor or other symptom of Parkinson's disease. Rogue brain circuits stuck in states of abnormal oscillation in many diseases have become candidates for DBS therapy. Changes in neurophysiology, neurochemistry, neurovascular structures, and neurogenesis may also underpin the benefits of DBS therapy.5
Before therapeutic DBS was developed, neurologists, neurosurgeons, psychiatrists, and rehabilitation therapists labored largely in isolation from one another when treating patients with Parkinson's disease. DBS therapy's success spurred the formation of multidisciplinary teams, whose members evaluate candidates for DBS and together personalize the therapy. This personalization includes selecting, on the basis of symptoms, the brain regions to target and planning preoperative and postoperative care. Although DBS teams typically have many members, I believe the most important element for success has been the partnership between neurologist and neurosurgeon. It is therefore fitting that the Lasker Award for DBS therapy has been given to a neurologist and a neurosurgeon.
Smaller, sleeker, more energy-efficient units are on the horizon. Better lead designs will permit more precise current delivery. Real-time monitoring of the neural-circuit physiology is driving the field toward smarter technologies. Remote monitoring and adjustment of devices may become possible. In its current form, however, the technology has several limitations. Current can spread into unintended brain regions, causing side effects, and DBS usually doesn't effectively treat all symptoms. Most commonly, the battery source for neurostimulators has been placed in the subclavicular region (see Figure 2
FIGURE 2



Devices for DBS.
), but this configuration has been associated with high risks of lead fracture and infection.

Nevertheless, DBS has had an enormous effect on the treatment of Parkinson's disease. It has also been used to treat essential tremor, dystonia, and epilepsy and in experimental treatments of obsessive–compulsive disorder, depression, Alzheimer's disease, and Tourette's syndrome (see interactive graphic, available with the full text of this article at NEJM.org). DBS therapy is usually considered only after all other treatments have been exhausted, but becoming “bionic” has provided many patients with a new lease on life. Thanks in large part to the contributions of two extraordinary scientists, we have entered the era of human neural-network modulation.

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