Prevalence of Disease

Before 1982, only 52 cases of true vascular aneurysms of the hand from repetitive trauma were recorded in the scientific literature. Although modalities in diagnosing HHS have improved, it remains difficult to estimate the true incidence and prevalence of HHS owing to its relative rarity and variable degree of associated symptomatology. Nevertheless, a few studies have explored the prevalence of HHS in persons who are thought to be at higher risk for the disease process. Ferris and colleagues prospectively evaluated 1300 subjects who were exposed to vocational or occupational repetitive palmar trauma. Only 21 patients (1.6%) were found to develop symptoms that were consistent with HHS, including pain in the digits and recent onset of cold intolerance in the hands or digits. All but one of these patients also developed unilateral digital ischemia with or without ulceration. Others have reported that in a minority of subjects, ulnar artery injury can be extrapolated to a single episodic palmar trauma event.

Little and colleagues report a slightly higher incidence of HHS among 127 mechanical workshop employees. Of these workers, more than 60% were noted to be habitual hypothenar hammerers, and 8% were found to have symptoms of vascular insufficiency as well as objective evidence of arterial occlusion. Similarly, Kaji and coworkers found that among 330 workers who were exposed to repetitive occupational hand vibration in mining, forestry, and other industries, 24 subjects (7%) had angiographically detectable ulnar artery abnormalities. A survey of the demographics of patients affected by HHS reveals that the majority of subjects are male with a mean age of approximately 40 years. A review of 146 patients with HHS revealed that 89% were habitual cigarette smokers.

Depending on the mechanism and severity of palmar trauma, both the radial and ulnar arteries may also be affected, although the incidence of this is low and is only reported in the form of case reports. Although the majority of patients affected by HHS have unilateral symptoms that typically affect the dominant hand (53%–93%), some subjects also develop HHS in the nondominant hand (12%), and a few have bilateral hand involvement (5%). Several reports demonstrate that among HHS patients, the third, fourth, and fifth digits are the most commonly affected digits, and symptoms in the thumb have never been described.

Anatomy and Pathophysiology

The ulnar artery is the larger medial branch of the brachial artery at the antecubital fossa, and in the majority of people it is the dominant artery that supplies the hand. At the wrist level, the ulnar artery, along with the ulnar nerve, superficializes over the flexor retinaculum before it enters the hand (Figure 1). At this juncture the ulnar artery gives rise to the deep palmar branch, which enters the hand medial to the pisiform and lateral to the hook of the hamate carpal bone. The remainder of the ulnar artery enters the hand by passing over the hamate and the hypothenar muscle, to enter through Guyon’s canal and give rise to the superficial palmar branch. This 2- to 3-cm segment of the ulnar artery is fixed, palpable, superficial, and only protected by skin, subcutaneous tissue, a portion of the palmaris brevis muscle, and the superficial palmar aponeurosis. The relative paucity of overlying tissue makes the superficial portion of the ulnar artery susceptible to blunt forces, which are accentuated by its contiguous bony process (hook of the hamate), which functions very much like an anvil (see Figure 1).

The superficial palmar arch is the dominant vascular supply of the hand, and in 37% of people it is entirely supplied by the superficial palmar branch of the ulnar artery. Extending underneath the palmar aponeurosis, the superficial palmar arch gives rise to three common palmar digital arteries that extend adjacent to the lumbrical muscles, and each divide into the proper palmar digital arteries that supply their respective digits. Notably, only 10% of people have a classic superficial palmar arch that joins the superficial palmar branch of the ulnar artery and to branches of the radial artery. Cadaveric studies demonstrate that 78% of people have a complete superficial arch that supplies all fingers including the ulnar side of the thumb. The remaining individuals with incomplete arches are found to have diverse anatomic branching patterns, and they might have a more dominant deep palmar arch. A complete deep palmar arch occurs in 76% to 96% of the population and describes an arch that migrates from the radial artery, across the metacarpals, to join the deep palmar branch of the ulnar artery.

The presence of collaterals, completeness of the palmar arches, and degree of dominance of the ulnar arterial supply can significantly influence the severity of symptoms in patients with HHS. With a classically dominant superficial palmar arch, disruption of the ulnar artery is more likely to result in more profound symptoms. However, in patients with an incomplete or nondominant superficial palmar arch, symptoms may be less severe, or they can even be masked by the presence of sufficient arterial flow by collaterals.

It is presumed that acute forceful trauma to the ulnar artery induces microtears in the intima and causes denudation of the endothelium. Ordinarily the intima recovers from such insults, but with repetitive trauma, subendothelial matrix proteins are exposed that promote the adhesion of macrophages and platelets to the site of injury. Recruited cells release inflammatory mediators that by way of paracrine mechanisms can induce hyperplasia, fibrosis, and thrombosis. Indeed, pathologic examination of resected ulnar artery segments from patients with HHS demonstrate a high incidence of intimal hyperplasia as evidence of the chronic inflammatory changes the ulnar artery endures in HHS.

Repetitive blunt trauma can also damage the arterial media and disrupt its matrix of elastin and collagen fibers. Histologic studies reveal fragmentation of the internal elastic lamina, hemorrhage into the media, and fibrosis of the vessel wall. Over time these disruptions can lead to gradual arterial wall aneurysmal dilatation, adventitial fibrosis, and periarterial sympathetic fiber disruption. As in other low-resistance vascular arterial beds, aneurysmal dilation of the distal ulnar artery can lead to thrombus accumulation and subsequent microembolization to the terminal arterioles in the digits.

In one study, histologic evaluation of arterial specimens from patients affected by HHS suggested pathologic characteristics of fibromuscular dysplasia. The authors concluded that perhaps certain persons have intrinsically abnormal arteries that predispose them to develop HHS following repetitive trauma to the ulnar artery. However, this argument did not explain why the majority of persons affected are male, whereas fibromuscular dysplasia is more commonly diagnosed in female patients. Moreover, there are no reports of HHS patients who also have evidence of disease in other arterial beds that are typically affected by fibromuscular dysplasia, such as the renal, iliac, or carotid arteries.