phenol and alcohol neurolysis
/ noo-ROL-ih-sis /
Botulinum toxin is precise and gentle, but it is costly, dose-limited, and wears off in months. For a large, powerful muscle group driving severe stiffness, there is an older, blunter, cheaper tool: inject phenol or alcohol onto the nerve that feeds the muscle, chemically damaging it so it stops carrying the overactive signal. This is neurolysis — literally 'loosening the nerve' — and it long predates botulinum toxin.
Instead of blocking the chemical message at the muscle as botulinum toxin does, phenol and alcohol attack the nerve itself: dropped onto a motor nerve, they destroy part of its insulation and fibres, so fewer signals get through and the muscle relaxes. The clinician finds the right nerve with electrical stimulation, then places the chemical there. Compared with botulinum toxin the effect comes on within minutes, can last much longer — many months, sometimes a year or more — costs far less, and is not capped by a total-dose limit, so it suits big muscles. The price is precision and risk: the nerve also damages partial sensation if it carries sensory fibres, which can leave a burning dysaesthesia (painful abnormal sensation), and the technique is more uncomfortable and more operator-dependent.
In rehabilitation, neurolysis fills a particular niche: severe, focal spasticity in a large muscle, especially when botulinum toxin is too limited by dose or cost, or when long duration is wanted. The two are often used together — botulinum toxin for the small, delicate muscles where a sensory burn would be unacceptable (the hand), phenol for the big hip and knee muscles. The honest caveats are the painful-dysaesthesia risk on mixed nerves and the skill the injection demands; like all these treatments it quiets overactivity but does nothing for a fixed contracture.
A man with a spinal-cord injury has powerful thigh muscles that clamp his knees together so tightly that hygiene is impossible. Because the muscles are large and the botulinum dose would not stretch far enough, his team blocks the nerve to the inner-thigh muscles with phenol — opening the legs for months at a fraction of the cost.
Phenol/alcohol damage the nerve itself — long-lasting and cheap, but riskier on mixed nerves.
Because phenol and alcohol damage whatever the nerve carries, injecting a nerve with sensory fibres can leave a lasting painful dysaesthesia. This is why fine, sensation-rich areas like the hand are usually treated with botulinum toxin instead.