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extrinsic (outside the tunnel) compression, although anatomical factors can produce compression from within the tunnel itself. Soft-tissue cysts or tumors can develop inside the canal and are one of the more common factors of intrinsic compression. There are also reports of anatomical anomalies such as unusual muscles or tendons that can occur within the tunnel. 5 Hypertrophy or enlargement of the flexor carpi ulnaris muscle can also cause a decrease in tunnel volume, as can irregularities in the shape of the hook of the hamate bone. It is difficult to identify any of these anatomical anomalies through physical examination—most are identified through high-tech diagnostic studies or surgical exploration. From a clinical perspective, recognizing the possible existence of anatomical anomalies helps narrow the assessment when no apparent signs indicate a likely cause for nerve compression. The most common causes of Guyon's canal syndrome involve external pressure on the base of the hand that compress the nerve. If the wrist is in hyperextension when a compressive force is applied to the base of the hand, the nerve is at risk of injury because the nerve is stretched and exposed in this position. The FOOSH (fall on outstretched hand) injury is an example, as the person's body weight lands on the base of the hand with the wrist in full hyperextension. This position makes the ulnar nerve vulnerable to compression injury. The ulnar nerve can also be injured by bone displacements or fractures in the wrist that occurred during the FOOSH. Guyon's canal syndrome is common among long-distance cyclists because their body position places upper body weight onto a hyperextended hand holding the bike's handlebars. Within the cycling community, this condition is frequently referred to as handlebar palsy. It is especially aggravated with long periods of downhill riding, when more pressure is put on the hands. A comparison of FOOSH injury and handlebar palsy illustrates the differences in degree of nerve injury in this condition. The degree of nerve compression is related to (1) the amount of compressive force and (2) the length of time that force is applied. In a FOOSH injury, there is a high degree of compressive force, but the force is applied only for a short time. In handlebar palsy, the compressive force is much less, but the force is applied over a longer period. The practitioner should identify key factors in the client history that indicate both the amount of compressive force applied and the length of time it was applied in order to determine the severity of nerve compression. Other factors that lead to Guyon's canal syndrome include walking on crutches, operating power tools, and various sporting activities such as handball, basketball, tennis, squash, golf, martial arts, and break dancing. Another cause of ulnar nerve compression of the wrist is sometimes described in the literature as hypothenar hammer syndrome. This is essentially a fancy name for banging something with your hand. We would all be wise to remember that the hand is not a hammer and using it like one can easily lead to nerve compression. ASSESSMENT AND EVALUATION Recognizing clinical signs and symptoms along with a detailed physical examination remains the most accurate means of assessing ulnar nerve compression in the canal. In addition to the information reported during the client history (pain in the hand, weakened grip strength, history of wrist compression, etc.), there are other key clinical indicators. During range of motion testing at the wrist, pain or neurological symptoms are usually exaggerated by wrist extension movements, as this will bowstring the nerve within the tunnel. However, keep in mind that Type II compression primarily affects motor fibers, so the sensory symptoms would not show up during a Type II compression problem, which is the most common. As noted earlier, hypothenar or thenar muscle atrophy may be visible (Image 5). There may also be some lack of coordination in specific hand movements or grasping activities, which can be tested during a physical examination. Sometimes clawing of the hand may be apparent (Image 6). If atrophy is visible in the hand, it usually indicates a chronic condition, as atrophy does not occur rapidly. When performing the physical examination, keep in mind that other locations of ulnar nerve entrapment can occur throughout the upper extremity and may give similar symptoms. In fact, ulnar nerve compression in the cubital tunnel at the elbow is actually the second most common upper extremity nerve entrapment condition. Compression of the early branches of the ulnar nerve are also far more common in thoracic outlet syndrome than Guyon's canal syndrome, so screen for this possibility as well. It's important to identify any more proximal location of nerve entrapment and treat the entire length of the ulnar nerve so that all potential locations of nerve compromise can be addressed. This will decrease the likelihood that multiple nerve compression sites (called multiple crush phenomenon) will cause additional nerve impairment. 5 Atrophy of hand muscles (flatness of palm) from ulnar nerve compression. Image courtesy Wikimedia Commons. 96 m a s s a g e & b o d y w o r k m a r c h / a p r i l 2 0 1 8