Prevalence and Pattern of Leptomeningeal Pigmentation in the Human Brain and Its Role in the Safe Surgical Excision of Extra-Axial Brain Tumors
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.329529
Source of Support: None, Conflict of Interest: None
Keywords: Arachnoid, extra-axial, leptomeningeal, melanin, microneurosurgery, pigmentation
Leptomeningeal pigmentation, although noted in 1824 by Ollivier d'Angers, was first described in detail by Symmers in 1905. The black pigmentation, discovered to be melanin, covers the spinal cord, brain stem, optic chiasm, and the orbitofrontal cerebral convolutions and seems to have no racial predilection, although reports are conflicting., During the excision of extra-axial tumors, the surgeon dissects the transparent arachnoid and pia mater off the tumor and may occasionally have difficulty in recognizing the arachnoid, leading to an unwanted breach of pia mater and neural tissue. Recognition of this leptomeningeal pigmentation may provide the surgeon with additional useful information thereby enabling a safe excision of the tumor. To our knowledge, no operative neurosurgery textbook mentions leptomeningeal pigmentation; therefore, we studied its prevalence among people of the Indian subcontinent and determined whether recognition of this leptomeningeal pigmentation during microsurgical excision of extra-axial brain lesions was useful.
This prospective cross-sectional, observational, clinical, and cadaveric study was initiated in 2012 after obtaining approval from the institutional review board (IRB No. 7786). It was conducted in the Department of Neurological Sciences in collaboration with the Section of Neuropathology and Department of Anatomy, within a tertiary care referral center in South India.
Cadaveric study design
We harvested brains from human cadavers that were received within 12 hours of death. The study design was in accordance with the QUACS (Quality Appraisal for Cadaveric Studies) scale. We performed a detailed survey of each brain under the operating microscope (Zeiss Universal S3B) to look for pigmentation. For histological analysis, we obtained 1-cm2-sized leptomeningeal samples from the optic apparatus and 2-cm2-sized samples from the ventral pons, irrespective of whether pigmentation was seen under the operating microscope or not.
Clinical study design
Consecutive patients with extra-axial brain lesions in the suprasellar and cerebellopontine angles, in whom the optic apparatus, hypothalamus, brain stem, or the cerebellum would be seen during routine surgical resection of these lesions, were included. Informed consent was obtained prior to surgery; patients with intra-axial lesions were excluded from this study. Based on the senior author's prior surgical experience, we had seen this pigmentation in approximately 75% to 80% of our patients in the brain as well as on the spinal cord. To estimate the prevalence of leptomeningeal pigmentation with a precision of 7% to 9% and a confidence interval (CI) of 95%, the sample size calculated was 126.
Preoperative patient variables
Patients' demographic details such as age, sex, state of origin within India, and whether they had undergone any prior surgery or radiotherapy were noted. Skin color, usually in an area not exposed to the sun, such as under the arm, was graded according to the Von Luschan's chromatic scale, consisting of 36 shades. We had used a simplified modification of this chromatic scale, as shown in [Table 1].
Intraoperatively, we noted the presence and location of pigmentation with particular reference to its relationship to the tumor surface, brain, cranial nerves, and blood vessels. Structures were examined only if they were within the line of approach of the lesion. We also documented whether the recognition of this leptomeningeal pigmentation was useful in the identification of a neural structure as distinct from the tumor or in determining the plane of dissection. Intraoperative photographs of the pigmentation were taken directly from the microscope. In those cases where leptomeningeal pigmentation was identified, we subjectively classified the density of pigmentation as follows: Minimal: Only a few specks of pigmentation seen restricted to one focal region within a neural structure [Figure 1]a; Moderate: Pigmentation seen in discrete clusters, spread over multiple regions within a neural structure [Figure 1]b; Extensive: Widespread contiguous region of the neural structure carpeted with pigmentation [Figure 1]c.
A small region of the pigmented pia/arachnoid was biopsied and sent for histopathological examination if deemed safe by the senior surgeon. All biopsies were fixed in buffered formalin for 6 to 12 hours. At least one section each was cut, for staining with hematoxylin and eosin (H and E) and for melanin bleach. On H and E staining, we looked for brownish stained granules located within leptomeningeal tissue that could represent melanin or hemosiderin. With the melanin bleach, the brown staining of melanin granules would be lost, whereas it would be retained if it was hemosiderin. Immunohistochemistry was then performed on all cases using the Melan-A antibody on the Ventana BenchMark XT autostainer. On the Melan-A immunohistochemistry, a distinct brown stain scattered over the leptomeninges, conforming to the pattern seen on the H and E-stained slides confirmed the presence of melanin pigment within the melanocytes.
Data were entered into a Microsoft Access 2007 database, and the analysis was performed after exporting the data into SPSS Statistics (IBM Version 20). Normative data were presented in terms of mean and standard deviation. The results of Chi-square tests were considered significant if the P value was less than 0.05. The results of logistic regression analysis were denoted in terms of odds ratio (OR), CI, and P value.
The eight cadaveric specimens, including seven males, had a mean age of 48 years, ranging from 19 to 72 years. The commonest site of pigmentation (n = 6, 75%) was the brain stem, on the ventral surface of the pons [Table 2] and [Figure 2]. Leptomeningeal pigmentation was identified in seven of the eight cadavers – a prevalence of 87.5%, and this pigmentation was confirmed to be melanin on H and E and immunohistochemistry. In Cadaver 2, although no pigmentation was seen over the optic chiasm through the operating microscope, melanin pigmentation was identified at histology and confirmed on the Melan-A stains. The melanin pigment within the dendritic leptomeningeal cells had one of two patterns – a branching, reticulate pattern or a serpiginous pattern [Figure 3] and [Figure 4].
A total of 126 patients were included in the clinical component of the study. There was a male predominance in our cohort, with 72 (57.1%) of the patients being males. The mean age of the cohort was 37.2 years ± 16 years, ranging from 4 to 73 years. Twenty-three patients (18.3%) had been previously operated and four had also undergone radiation therapy prior to our surgery: two with suprasellar craniopharyngiomas, one glossopharyngeal schwannoma, and the one patient had a vestibular schwannoma.
The majority of our patients were from West Bengal (27.8%, n = 35), followed by Bangladesh (17.5%, n = 22), Kerala (14.3%, n = 18), and Tamil Nadu (11%, n = 14). Other states of India contributed a few patients each. There were no Africans or Caucasians in the study.
The majority of our patients were intermediate skin colored (n = 100, 78.7%), with only one light skin-colored patient, and the remaining 25 patients were dark skin colored. The mean skin color for our cohort was 23.08 ± 6.05, which fell in the intermediate skin color group.
Appearance of leptomeningeal pigmentation
Under the operating microscope at high zoom, we noticed two different patterns of leptomeningeal pigmentation. Over the optic chiasm [Figure 5]a and olfactory tract [Figure 5]b, the pigmentation was often seen as discrete rounded specks, tightly adherent to the neural structures in the intimal pial layer. On the brain stem [Figure 5]c and lamina terminalis [Figure 5]d, we frequently noticed a loosely arranged, branching, reticulate network-like pattern to the pigmentation in the epipial layer where the arachnoid could be safely elevated off the tumor.
Prevalence of leptomeningeal pigmentation
The overall prevalence of leptomeningeal pigmentation in our cohort was 76.9%, as it was identified in 97 out of 126 patients. In these 97 patients, the density of pigmentation was extensive in 28 patients (28.9%), moderate in 58 patients (59.8%), and minimal in 11 patients (11.3%).
Age: The mean age of patients in whom the pigmentation was identified was 35.5 ± 17 years, as compared with those in whom it was absent (43.2 years ± 11.4 years). This difference in age was statistically significant (P = 0.021). Interestingly, all patients <18 years had leptomeningeal pigmentation.
Gender: Pigmentation was also found to be more prevalent in males as compared with females, seen in 86.3% of males as compared with 63% females, a difference that was statistically significant (P = 0.002).
Skin color: There was a higher prevalence of pigmentation in individuals with an intermediate skin color as compared with those with dark skin colors (78% and 69.3%, respectively), but this was not statistically significant (P = 0.350).
Pathology: The various pathologies in our cohort are listed in [Table 3]. When comparing the two most common pathologies studied, that is, craniopharyngiomas and vestibular schwannomas, there was a higher prevalence of leptomeningeal pigmentation associated with craniopharyngiomas (89.7%) as compared with vestibular schwannomas (74.1%), and this difference tended towards significance (OR = 3.062, 95% CI [0.922, 10.170], P = 0.068).
Location: The prevalence of pigmentation on various neural structures is listed in [Table 4].
On doing a multivariate analysis [Table 5] to assess the association of age, gender, skin color, pathology, and location with the presence of leptomeningeal pigmentation, younger age and male gender continued to remain significant (P values 0.018 and 0.002, respectively).
Leptomeningeal biopsy samples were obtained in 48 of 97 patients in whom the pigmentation could be identified (49.5%). Of the 48 samples, 41 samples were confirmed to have melanin pigment in melanocytes. In seven cases, no melanin could be identified either due to crush artifacts or inadequate tissue. H and E-stained sections showed a fine layer of leptomeningeal tissue containing spindle-shaped cells with dense intracytoplasmic brownish-black granular pigment obscuring other cytomorphological details. The cells were immunopositive for Melan-A.
Surgical implications of the leptomeningeal pigmentation
Recognition of the pigmentation was deemed to be useful; that is, it aided the surgeon in identifying the plane of arachnoid dissection to resect the tumor in 76 cases out of 97 (78.4%) in which the pigmentation was identified. Among these cases (n = 76), the plane of arachnoid dissection was found to be good in 70 cases (92.1%), moderate in five cases (6.6%), and poor in one case (1.3%) of a tuberculum sellae meningioma.
Leptomeningeal pigmentation in humans
Leptomeningeal pigmentation has been described to be of neural crest melanocytic lineage.,,,,,,, Symmers, in 1905, noted a brown to black pigmentation covering the spinal cord, medulla, cerebral peduncles, and ventral surface of the brain, including the optic chiasm and the orbitofrontal cerebral cortex during routine postmortem examinations in Egypt. In a series of 200 postmortem examinations spanning 11 different racial types, the pigmentation was most marked in Egyptians who were light brown skinned. On the contrary, in 1969, a study from Uganda concluded that light-skinned Africans had little or no pigmentation in the leptomeninges, whereas darker skinned people showed more widespread pigmentation. A qualitative and quantitative study in 15 Caucasian cadavers discovered melanin pigmentation limited to the leptomeninges overlying the ventrolateral aspect of the medulla oblongata. The lack of magnification used in the study and limited sampling might explain the restricted distribution of the pigmentation.
Prevalence of leptomeningeal pigmentation in the Indian population
The prevalence of leptomeningeal pigmentation seems to range between 77% and 88% in the Indian population, most commonly on the ventral pons (particularly around the root entry zones of cranial nerves), followed by the optic chiasm, lamina terminalis, and olfactory tract. Our finding that the extent and density of pigmentation did not correlate with skin color needs to be interpreted with caution as the majority of our patients had an intermediate skin complexion. Broniatowsky, in 1911, noticed that pigment-laden branching cells first appeared by the ninth year of life, prior to which the spindle and branching cells lacked pigment. However, in our cohort, we had nine children younger than nine years of age, each with a suprasellar craniopharyngioma, who had moderate to extensive leptomeningeal pigmentation seen over the optic apparatus. We found the pigmentation to be more prevalent in males and in the pediatric population on a logistic regression analysis, both findings that had not been reported earlier. The reasons for these differences remain unknown.
We found that identification of the pigmentation was useful during surgical dissection, particularly at the root entry zones of the cranial nerves in cerebellopontine angle tumors and on the optic apparatus/hypothalamus in suprasellar tumors. One such clinical case is illustrated in [Figure 6].
Identification of the tumor–arachnoid interface is of paramount importance during resection of tumors in the subarachnoid cisterns. Since the pia mater and arachnoid are translucent, identification of the appropriate plane of dissection can pose a challenge to the novice and expert alike, due to tight adherence of the tumor to the pia/arachnoid, tumor infiltration as in meningiomas, intraoperative bleeding, or incorrect plane of dissection. During these times, identification of this leptomeningeal pigmentation enables grasping the pia/arachnoid with a micro-bayonet and peeling it off the tumor toward the normal brain, adhering to the principle that “pigmentation belongs to the patient and not the tumor” [Video 1].
As there is no mention of leptomeningeal pigmentation in operative neurosurgery textbooks, we wondered whether this pigmentation was present at all in the Western population. However, our study does not answer this question as our patient population was restricted to South Asians alone. A broader study that includes Caucasians, Africans, American Indians, and East Asians might yield interesting results with regard to the prevalence of pigmentation in these races.
Leptomeningeal melanin pigmentation is seen in the majority of South Asians, particularly younger male patients. The recognition of this pigmentation provides an additional landmark during arachnoid–tumor dissection in the suprasellar and cerebellopontine angle cisterns, where the pigmentation is most frequently seen.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
This study was funded by the internal institutional grant (FLUID research grant) for postgraduate research activities, at the Christian Medical College, Vellore, Tamil Nadu, India.
Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]