Carpal tunnel syndrome
Carpal tunnel syndrome (CTS) or median beuropathy at the wrist occurs when the median nerve becomes pressed or squeezed as it travels from the forearm (antebrachium) through the wrist into the hand. The median nerve controls sensations to the palm side (anterior) of the thumb and fingers (although not the little finger), as well as impulses to some small muscles in the hand that allow the fingers and thumb to move. The pressing or squeezing often results from repetitive stress injury (RSI) in an occupational context, such as using a keyboard and mouse without proper wrist support, or an assembly line job where the same tool is used in the same way for hours. It is also seen during pregnancy due to fluid retention. Prevention of CTS, and RSI in general, is often more cost-effective than disability and not-always-successful treatment.
On the anterior side of the wrist is a sheath of connective tissue that forms a transverse carpal ligament, the flexor retinaculum. It is attached, medially, to the pisiform and the hamulus of the hamate bone; laterally, to the tubercle of the scaphoid, and to the medial part of the volar surface and the ridge of the trapezium. This ligament is superficial to the flexor tendons for the Phalanges (fingers). The carpal tunnel is the space posterior to (deep to) the flexor retinaculum and anterior to (superficial to) the carpal bones, through which the tendons slide during hand and particularly during finger movement. These tendons include the flexor digitorum profundi, and flexor digitorum superficiali groups, as well as the flexor pollicis longus. The median nerve also travels through the carpal tunnel.
A systematic review by the Rational Clinical Examination concluded:
- Carpal tunnel is unlikely if Katz hand diagram is classified as unlikely
If an inflammation exists which causes any of the structures associated with the carpal tunnel to swell appreciably (resulting in a positive Tinel's sign, where gently tapping the nerve produces tingling or pain), or if the size of the canal is reduced (such as when bending the wrist at a 90 degree angle, testing for Phelan's sign), the median nerve can become compressed causing symptoms of pain, paresthesias, and muscle weakness in the forearm and hand. The value of these and other findings on physical examination, however, has been questioned. Nerve conduction velocity and electromyography are valuable tools, and increasing value is seen in magnetic resonance imaging and high-resolution ultrasonography. Plain X-ray radiographs tend not to give useful information.
The differential diagnosis should rule out more general peripheral neuropathy. Especially if there are symptoms in both hands and wrists, genetic testing for hereditary neuropathy with sensitivity to pressure palsies (HNPP) can be informative and give guidance for prevention in the patient and the patient's blood relatives
"Local corticosteroid injection does not significantly improve clinical outcome compared to either anti-inflammatory treatment and splinting after eight weeks or Helium-Neon laser treatment after six months" according to a meta-analysis by the Cochrane Collaboration. 
Treating the underlying cause can offer some guidance. Hypothyroidism for instance is implicated in some cases when the reduced thyroid hormone levels cause synovial swelling. Treatment of the root cause disorder can offer relief from symptoms.
In carpal tunnel release surgery a surgeon physically severs the flexor retinaculum ligament to release the pressure on the median nerve. "Surgical treatment of carpal tunnel syndrome relieves symptoms significantly better than splinting" according to a meta-analysis by the Cochrane Collaboration. 
- Norvell, Jeffrey G & Mark Steele (Nov 13, 2007), "Carpal Tunnel Syndrome", eMedicine
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- Athena Diagnostics, Peripheral Neuropathy
- Inoue K et al. (2001 June), "The 1.4-Mb CMT1A Duplication/HNPP Deletion Genomic Region Reveals Unique Genome Architectural Features and Provides Insights into the Recent Evolution of New Genes", Genome Res. 11 (6): 1018–1033, DOI:10.1101/gr.180401.
- Marshall S, Tardif G, Ashworth N (2007). "Local corticosteroid injection for carpal tunnel syndrome.". Cochrane Database Syst Rev (2): CD001554. DOI:10.1002/14651858.CD001554.pub2. PMID 17443508. Research Blogging. Review in: Evid Based Med. 2008 Feb;13(1):16
- Huisstede BM, Hoogvliet P, Randsdorp MS, Glerum S, van Middelkoop M, Koes BW (2010). "Carpal tunnel syndrome. Part I: effectiveness of nonsurgical treatments--a systematic review.". Arch Phys Med Rehabil 91 (7): 981-1004. DOI:10.1016/j.apmr.2010.03.022. PMID 20599038. Research Blogging.
- Verdugo RJ, Salinas RA, Castillo JL, Cea JG (2008). "Surgical versus non-surgical treatment for carpal tunnel syndrome.". Cochrane Database Syst Rev (4): CD001552. DOI:10.1002/14651858.CD001552.pub2. PMID 18843618. Research Blogging.