Carpal Tunnel Syndrome
One of the most common pain conditions seen in clinical practice, carpal tunnel syndrome may affect up to 10% of workers. Treatment varies based on the severity of the disorder and ranges from stretching/splinting to surgery.
Carpal tunnel syndrome (CTS) is the clinical presentation of symptomatic dysfunction of the median nerve at the transverse carpal ligament and is the most common mononeuropathy seen in clinical practice. The incidence in the general population has been estimated at approximately one per 1,000 persons, although recent surveys have shown that the prevalence may be up to five times that, with CTS twice as common among women than men.1,2 Other studies have described the prevalence among working populations (as diagnosed by either electrophysiologic or symptomatic testing) to be as high as 10%.3 The annual medical cost of CTS treatment in the United States has been estimated to be $2 billion with 400,000 to 500,000 carpal tunnel release (CTR) surgeries being performed on an annual basis.4,5 Non-medical costs, including lost productivity and functional impairment, are also significant.
Clinical, Anatomic, and Pathological Features
The carpal tunnel is an inelastic space on the volar aspect of the wrist, which is bounded anteriorly by the transverse carpal ligament—anchored to the bones of the wrist—and dorsally by the carpal bones themselves. In addition to the median nerve, the carpal tunnel contains the finger flexor tendons as they pass from the forearm to the wrist (Figure 1).
The sensory representation of the median nerve can be seen in Figure 2. Hand muscles supplied by the median nerve include first and second lumbricals, opponens pollicis, abductor pollicis brevis, and half of the flexor pollicis. These are remembered by the acronym LOAF.
Carpel tunnel syndrome symptoms often presents with nocturnal or activity-related paresthesias (tingling, burning, pricking, numbness) caused by transient ischemia of the median nerve resulting in conduction block. In these circumstances, increased volume within the fixed carpal tunnel or external compression puts pressure on the nerve. Typical perineural pressure is generally in the range of 2.5 mmHg in healthy individuals. With exposure to elevated pressure, which may reach 30 to 50 mmHg or more, segmental demyelination and ultimately axonal injury occur in patients with CTS. Typically, sensory symptoms and nerve conduction abnormalities precede motor abnormalities. It is thought that this is related to the higher compression sensitivity of the more thickly myelinated sensory fibers.
Related Links for Patients
The symptoms of CTS relate primarily to median nerve dysfunction caused by compression in the carpal tunnel. Mechanical wrist and thumb pain caused by weakness surrounding the first metacarpal phalangeal joint may also occur. The typical progression of the neuropathic symptoms of CTS may be highly variable. Some individuals present with intermittent paresthesia and maintain relatively normal motor and electrophysiological function. Others, particularly older patients or those with constitutional illness, may present with profound atrophy and sensory loss with little or no discomfort.
The natural history of CTS appears to vary significantly in individuals. This probably stems from the wide variety of physical and environmental factors in play in different individuals. Reviews of the natural history of CTS have identified different longitudinal courses from long-term stability and spontaneous improvement to progressive deterioration and complete median nerve dysfunction. Hypothyroidism, diabetes, and rheumatoid arthritis are risk factors for CTS as they can increase volume and thus pressure within the carpal canal and are also frequently associated with impaired nerve resiliency. Cases of CTS caused by increased carpal canal pressure related to the hormonal changes and volume expansion of pregnancy generally resolve after delivery.
Sensitivity to nociception also varies significantly among individuals and is likely influenced by genetic and other factors. It is interesting to note that CTS is most often a bilateral disorder. In a series of 131 patients, 59% had bilateral symptoms at presentation. Of those presenting with unilateral symptoms, 66% had electrophysiological abnormalities in the contralateral hand and 73% developed symptoms in the initially asymptomatic hand after a mean interval of 3.2 years.6
Known risk factors for CTS include anatomy, occupational exposure, genetic predisposition, and systemic causes. Anatomic factors relate primarily to the dimensions of the carpal tunnel in relation to its contents and to the generation of pressure exerted on the median nerve within its confines. Occasionally other anatomic factors may contribute to compression of the nerve including ganglion cysts, lipomas, or other masses. Compression as an immediate or late complication of fracture or scarring may also initiate CTS.
Intrinsic patient characteristics clearly influence the development of CTS and likely determine what degree of environmental exposure, if any, is required. Hakim and colleagues studied the relative risks of CTS in female monozygotic twins, dizygotic twins, and singletons.7 They found heritability to be the strongest determinant of CTS, conferring approximately half of all risk.7
In addition, it is well established that certain individuals, for example those with peripheral nerve myelin protein abnormalities or those with familial amyloid deposition, have an increased risk of the development of nerve dysfunction in response to pressure. In one recent study, individuals with a body mass index (BMI) >29 were 2.5 times as likely as individuals with a BMI <20 to develop CTS.8 Diabetes, neuropathy, rheumatoid arthritis, concurrent cervical radiculopathy, and a number of other disease states also confer a predisposition to the development of CTS. Non-occupational exposures such as crutch use and wheelchair propulsion are also risks.
In a recent meta-analysis, a strict case definition of CTS, which included nerve conduction abnormalities and typical symptoms, was used to evaluate the relationship between various risk factors found in the workplace and the occurrence of CTS. Vibration (odds ratio [OR] 5.40), hand force (OR 4.23), and repetition (OR 2.26) were described as risk factors for development of CTS.9 Combinations of high force, awkward hand positions, repetition, and high levels of vibration may afford the highest occupational risk.10 While typing on a computer keyboard has been demonstrated through ultrasonography to acutely increase median nerve cross-sectional area, the relative contribution of this occupational activity to the development of carpal tunnel syndrome remains controversial.11