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Epithelium Is Absent from the Subcutaneous Tunnel in Long-Term Peritoneal Dialysis Patients
Exit-site infections (ESIs) and tunnel infections (TIs) are important causes of peritonitis. Because these infections are not clearly differentiated clinically, they are collectively called catheter-related infections (1). The lack of differentiation poses difficulties for clinicians in daily clinical practice and might contribute to a high rate of peritoneal dialysis (PD) technique failure from catheter infections. It also hampers research on catheter infections because of ambiguity in the definition of ESIs and TIs.
We thought that one main reason for the ambiguity might be a lack of understanding concerning the morphology relevant to exit sites and subcutaneous tunnels. To date, few studies have examined the morphology of subcutaneous tunnels, especially with respect to long-term PD. In animals (2-4) and humans (5) alike, the entire sinus tract—that is, the part of the tunnel between the exit at the skin and the external cuff—was found to be lined with epidermis, and epidermal downgrowth was inhibited by collagen fibers invading the cuff. In contrast, Twardowski et al. demonstrated that, in short-term PD (mean duration < 1 year), the epithelium extends only several millimeters from the exit rim (6). That lack of consensus about the extent of epithelialization, especially in long-term PD patients, led us to examine tunnel morphology, with special attention to the extent of epithelialization from the exit site.
METHODS
From August 2010 to August 2011, we used a histologic approach to prospectively investigate the epithelialization of the subcutaneous tunnel in 10 consecutive PD patients from our unit. Table 1 shows details of clinical backgrounds of the patients. Written informed consent was obtained from all patients.
The 10 patients were undergoing a surgical intervention either for a reason other than catheter infection (group A, n = 5, catheter removal) or for catheter infection (group B, n = 5, partial catheter implantation). The catheters used were a double-cuff type (MediTech Medical Polymers, Tokyo, Japan). The diagnosis of catheter infection was made in the presence of purulent drainage, erythema of the skin, and edema or tenderness over the subcutaneous pathway. All patients started PD after 3 - 7 days after catheter implantation.
During surgery, the subcutaneous tunnel from the exit site to the superficial cuff was excised in all patients. These tissue samples were immediately fixed in 20% buffered formalin. After overnight fixation at room temperature, the samples underwent routine processing for light microscopy and were embedded in paraffin. The extent of epithelialization of the subcutaneous tunnel was evaluated independently by 2 examiners—1 pathologist (Y. Takara) and 1 nephrologist (M. Tsukamoto)—who were blinded to the clinical backgrounds of the patients. The extent of the epithelialization as measured by the 2 examiners was averaged to reach a final value for each patient.
RESULTS
In the 10 patients, mean PD duration was 5.0 ± 2.8 years. As shown in Figure 1, specimens obtained from the Tenckhoff catheter exit site demonstrated epithelialization of the entire circumference of the tunnel exit. Compared with the exit site, the subcutaneous tunnel away from the exit site was not covered with epithelium. The epithelialization boundary was clear, and the length of epithelialization along the tunnel from the exit site was 4.4 ± 4.1 mm in group A and 4.2 ± 3.4 mm in group B (Table 1).
— Subcutaneous tunnel morphology in a long-term peritoneal dialysis patient (A2 in Table 1) (A) Epithelialization of the subcutaneous tunnel is limited. An asterisk denotes the limit between epithelium (solid line) and granulation tissue (dashed line). (B) High-power magnification of the epithelium-granulation tissue junction in the subcutaneous tunnel. As the subcutaneous tunnel extends from the exit site, the density of basal cells of epithelium declines, suggesting that basal cells of epithelium are migrating from skin.
We next examined the histology in detail. Just outside the exit site, hyperkeratosis, elongation of rete ridges, and basal pigmentation were detected [Figure 2(a)]. Mild dermal lymphatic infiltration was also seen. The subcutaneous tunnel near the skin was covered with epithelial cells [Figure 2(b)]. The epidermis without rete ridges showed orthokeratinization. Dermal fibrosis was prominent. Near the edge of the epithelialization, splits between the dermis and epidermis were frequently seen, suggesting that attachment between the epidermis and dermis was very weak at that point [Figure 2(c)]. Keratinocytes at the edge of the epithelialization had nuclear karyolysis and pyknosis with homogeneous pink cytoplasm [Figure 2(d)]. The granular layer was lost. In the portion of the subcutaneous tunnel not covered with epithelium, macrophages, lymphocytes, and plasma cells infiltrated around the catheter, which was surrounded by fibrosis [Figure 2(e)]. A closer look revealed multinuclear cells and many blood vessels [Figure 2(f)].
— Subcutaneous tunnel morphology in long-term peritoneal dialysis patients without tunnel infection. (A) Just outside the exit site, hyperkeratosis, elongation of rete ridges, and basal pigmentation, with mild dermal lymphatic infiltration, are evident. (B) The subcutaneous tunnel near the skin is covered with epithelial cells. (C) Near the edge of the epithelialization, splits between the dermis and epidermis are frequent. (D) Keratinocytes at the edge of the epithelialization have nuclear karyolysis and pyknosis with homogeneous pink cytoplasm. (E) Macrophages, lymphocytes, and plasma cells infiltrate around the catheter where the subcutaneous tunnel not covered with epithelium. (F) At higher magnification, multinuclear cells and many blood vessels can be seen.
Little histologic difference was observed between the tunnels in infected and noninfected patients. That observation might be a result of treatment of the infections with antibiotics before surgical intervention. However, some infected tunnels demonstrated distinctive findings. In 3 of the 5 infected tunnels, mild-to-severe acanthosis at the edge of epithelialization was found [Figure 3(a)]. Granulation tissue around the tunnels tended to be thicker in the infected than in the noninfected patients [Figure 3(b)]. Multinuclear giant cells showing phagocytosis were detected, and remarkable blood vessel proliferation was observed in 2 infected patients [Figure 3(c,d)].
— Subcutaneous tunnel morphology in long-term peritoneal dialysis patients with tunnel infection. (A) Acanthosis is evident at the edge of the epithelialization. (B) Granulation tissue around the tunnel tended to be thicker in infected patients, and in 2 infected patients, (C) multinuclear giant cells showing phagocytosis were detected, and (D) remarkable blood vessel proliferation was observed.
DISCUSSION
Our study demonstrates for the first time that the subcutaneous tunnel in long-term PD patients is not epithelialized. Epithelialization around the Tenckhoff catheter throughout the subcutaneous tunnel has been believed to be important for protection from infection. However, our study demonstrates that, in long-term PD patients, only a small part of the tunnel near the skin is covered with epithelial cells. The extent of epithelialization from the exit-site was almost the same for all patients regardless of the reason for catheter removal.
Keratinocytes at the edge of epithelialization had nuclear karyolysis and pyknosis with homogeneous pink cytoplasm, suggesting necrosis. The density of basal cells decreased with distance from the exit site, which might suggest that tunnel epithelialization develops from basal cells of skin. The length of epithelialization from the exit site measured in the present study may be the limit of keratinocyte stem cell migration and dermal tissue maintaining the epidermis. Our observation accords with previous reports that epithelial cells at a wound edge may become senescent or mitotically inactive and unable to carry out the DNA replication necessary for the process of proliferation (7).
Tunnels or cysts covered with epithelial cells have been seen in specimens of chronic pyoderma or atheroma—diseases that are both frequently accompanied by bacterial infections. In that context, epithelialization around the Tenckhoff catheter throughout the subcutaneous tunnel, even if possible, may not be ideal for the prevention of bacterial or fungal infection. Although epidermal tissue has a barrier function, and keratinocytes produce many antimicrobial agents, the skin hosts a number of bacteria and fungi. In fact, ESIs are thought to be associated with epidermal downgrowth after a dead space is formed at the exit site, with accumulation of the products of skin metabolism. To prevent downgrowth, creation of an outward-directed exit site at the time of surgery and avoidance of trauma in daily care would be desirable ways to avoid catheter infections.
CONCLUSIONS
Epithelium is absent from the subcutaneous tunnel in long-term PD patients. Because the exit site and the subcutaneous tunnel are not clearly differentiated histologically, diagnostic criteria to differentiate ESIs and TIs should be further explored.
DISCLOSURES
The authors have no conflicts of interest to declare.