INTRODUCTION

 

Chronic sensorimotor distal symmetric polyneuropathy is the most common type of neuropathy among diabetic patients and is defined as “the presence of symptoms and/or signs of peripheral nerve dysfunction in people with diabetes, after the exclusion of other causes”.^1,2 Diabetic neuropathy represents a major complication of type 1 diabetes mellitus (T1DM) in young people, and is detected in up to 57% of this group, by using complete electrophysiological evaluation.^3,4

Clinical neuropathy is defined as an abnormal neurologic examination consistent with peripheral sensorimotor polyneuropathy, plus either abnormal nerve conduction in at least two peripheral nerves or unequivocally abnormal autonomic nerve testing.^3,5

In addition to retinopathy, ocular surface disorders, including corneal abnormalities (keratopathy) and tear film dysfunction (dry eye), have been identified as complications of T1DM.^6,7 Keratopathy associated with diabetes mellitus comprises superficial punctate keratopathy, recurrent corneal erosion, persistent epithelial defect, and corneal endothelial damage.⁸

These ocular abnormalities are attributed to damage of the nerve supply and might be correlated to diabetic polyneuropathy (DPN).^9-11 The association between various chronic diabetic complications was addressed by numerous studies in type 2 diabetes mellitus (T2DM),^7,12 but there has been a lack of research related to the changes of ocular surface and their correlation with DPN in young people with T1DM.^10,11,13

The aim of this study is to evaluate the clinical characteristics of clinically and electrophysiologically diagnosed DPN in children and adolescents with T1DM, and its relation to ocular surface complications.

 

SUBJECTS AND METHODS

 

This cross sectional study was conducted at Ain Shams University in the period from April 2008 to December 2009. Forty patients with type 1 diabetes mellitus, including 21 male and 19 females, were recruited from Pediatric Diabetes Clinic at Children's Hospital. Their ages ranged from 5 to 17 years old with mean age of 13.51±2.78 years. Twenty age and sex matched healthy children and adolescents were included as a control group. Written informed consent was taken from all parents, after approval of Local Ethical Committee.  Comprehensive history and physical examination was conducted for all patients.

Mean random blood glucose (RBS) and mean glycated hemoglobin (HbA1c) over the last year follow-up were recorded. HbA1c was calculated by using high purified liquid chromatography (HPLC).¹⁴ HbA1c target range of <7.5% was recommended for all age groups.¹⁵  To diagnose nephropathy; urinary albumin excretion was performed using immune turbidimetric methods. Persistent microalbuminuria was defined when two of three samples showed an excretion rate of 30-300 ug/mg creatinine.¹⁶ 

The patients were divided into 2 groups according to the clinical and/or electrophysiological diagnosis of DPN, into; Group I; 19 patients with DPN (mean age =14.1; 11 males (58%) and 8 females (42%)) and Group II; 21 patients without DPN (mean age= 12.9; 13 males (62%) and 8 females (38%)). The two groups were compared regarding age, duration, glycated hemoglobin, microalbuminuria, and different ocular tests.

To assess neuropathy, neurological assessment and nerve conduction studies (NCS) were done at department of neurology. Motor conduction velocities (MCVs), distal motor latencies (DMLs) and compound muscle action potential (CMAPs) amplitudes of the median, tibial, and peroneal nerves of both sides were measured using EVOMATIC 8000 Apparatus (Dantec, Denmark). Additionally, sensory conduction velocities (SCVs) and amplitudes of the sensory nerve action potentials (SNAPs) of the median and sural nerves were studied. These nerves, especially of lower limbs, are the most useful and practical nerves for the electrophysiological study in diabetic patients.^{17, 18}

The diagnosis of peripheral neuropathy was based on the presence of symptoms and signs of diabetic polyneuropathy such as dysesthesias, paresthesias, glove and stocking hypesthesia, abnormal tuning fork vibration perception, abnormal motor functions and abnormal deep tendon reflexes, and/or on abnormal nerve conduction velocity.

Bipolar surface stimulating electrodes were used to stimulate both sensory and motor nerves of the skin, while two types of recording electrodes were used (ring sensory electrodes with an active recording electrode placed at the proximal phalanx and a reference electrode placed at least 2 cms distal to the active electrode, and surface electrodes with an active recording electrode placed over the motor point of the muscle and the reference one placed distally over the tendon (at least 2 cms distal to the active electrode)). If two or more of the nerves had at least one abnormal attribute according to normative values,^18-20 it is electrophysiologically considered a diabetic neuropathy.

Detailed ophthalmological examination was done for eighty eyes of the 40 patients including fundus examination to diagnose diabetic retinopathy (DR). Tear film changes were tested by variable techniques. Schirmer test was used to quantitatively evaluate basal tear production. A reading of less than 10 mm was considered to show dry eye. Tear break up time (BUT) test reflected tear stability and composition, using a fluorescein filter paper.²¹ A BUT of less than 10 seconds was considered abnormal. Keratoepitheliopathy was evaluated by staining the cornea with fluorescein staining test, using slit-lamp biomicroscopy.²² The staining area was graded on a numerical scale of 0 to 3, with 0 representing no punctate staining, 1 representing fine punctate staining, 2 representing coarse punctate staining, and 3 representing diffuse staining. Rose bengal staining was used to detect corneal and conjunctival desiccation associated with insufficient tear flow, using diffuse white light. Staining was graded of 0 to 3 for each of the lateral and medial corneal and conjunctival regions of the exposed intrapalpebral ocular surface. If the total staining score was ≥ 3, it was considered abnormal. Conjunctival impression cytology (IC) was conducted to detect conjunctival metaplasia. Three days after tear function examinations, an impression cytologic specimen for light microscopy was obtained. Scoring of tear film tests was based on criteria of Sood and his colleagues.²³ Exclusion were made for patients with contact lenses, eye infection, eye trauma, ocular surgery, any other surgeries within the previous 6 months, severe blepharitis with meibomian gland dysfunction, blinking abnormality, or severe pterygium.

Statistical analysis was done using the statistical package for the social sciences (SPSS version 18, Chicago, IL, USA). Qualitative data was analyzed using Chi-square test, and exact tests such Fisher exact and Monte Carlo was applied to compare any two categories. Non-parametric quantitative data was analyzed using Mann Whitney U test to compare between two categories. p-value <0.05 was considered significant. Comparison of NCS findings was done using independent sample t-test.

RESULTS

 

Mean duration of T1DM was 5.58±3.43 years and 19 patients (47%) were diagnosed to have DPN, after abnormal nerve conduction studies. The abnormalities in tear film were detected by Schirmer, BUT, impression cytology and fluorescein staining in 17.5, 45, 63.7% and 66.2% of patients’ eyes, respectively, compared to none of the controls (p <0.001), while rose bengal staining abnormalities were detected only in 11.3% of patients’ eyes and none of the controls, but this did not reach a statistical significance (p >0.05) (Table 1).

Abnormalities in Schirmer, rose bengal, fluorescein and impression cytology tests were not significantly related to age, sex, RBS, HbA1c, acute or chronic diabetic complications (p >0.05), while; abnormalities in BUT test were significantly related to age (p= 0.047) , high mean RBS (p=0.01), and high HbA1c (88.9% of abnormal BUT test were among those with HbA1c > 7.5%) (p <0.001). Moreover; 55.6% and 75% of patients with severe hypoglycemia and severe DKA respectively had abnormal BUT test (p=0.015 and 0.007, respectively). 63.2% of patients with neuropathy had abnormal BUT test results (p =0.008).

The two studied groups of patients, with and without neuropathy, were statistically matched with respect to age (p= 0.341), sex (p= 0.752), mean RBS (p=0.295) and HbA1c (p=0.331), while group 1 (T1DM with neuropathy) had significantly tibial MCV values (36.15± 7.29 versus 45.85 ± 2.43; p =0.01), and longer duration (mean 6.6 versus 4.6 years; p= 0.03). 16% (3 patients) had nephropathy, while none in group 2) (T1DM without neuropathy) (Table 2).

Patients with DPN had lower mean values of Schirmer and BUT tests than the group without neuropathy, and statistically there was significant difference (p<0.05). Also, conjunctival impression cytology (Figure 1) test was more significantly abnormal (p= 0.007 and 0.035, respectively) in patients with DPN, compared with patient without DPN. Moreover, more patients with DPN had abnormal rose bengal test, but without significant difference (Table 2).

Within group of patients with neuropathy, Schirmer and BUT tests are significantly related (p <0.005) to duration of diabetes and to each other, while not related to age, or RBS. Also, Schirmer test is significantly correlated with Hb A1C level.

 

 

Table 1. Tear film tests among studied patients and controls.

 

	Patients	Control	P-value
Schirmer test (score) (no of eyes,%)
Normal	66 (82.5)	40(100)	0.005*
Abnormal	14 (17.5)	0 (0)
Median & range (mm/5 min)	14 (8-20)	16 (14-17)	<0.001*
BUT (score)(no,%):
>10 seconds	44 (55)	40 (100)
6 – 10 seconds	29 (36.3)	0 (0)	<0.001a
3- 6 seconds	7 (8.7)	0 (0)
Median & range (mm/5 min)	11 (5-20)	18 (16-22)	<0.001*
Impression cytology (grade) (no,%)
Normal Mild  Moderate Median &range	29 (36.3) 45 (56.2) 6 (7.5) 0 (1-3)	40 (100) 0 (0) 0 (0) 0	<0.001 a   <0.001*
Rose bengal (score) (no,%)
Normal Abnormal	71 (88.7) 9 (11.3)	40 (100) 0 (0)	0.154
Fluorescein staining findings (no,%)
Absent Fine punctuate Coarse punctuate	37 (33.8) 26 (45) 17 (21.2)	40 (100) 0 (0) 0 (0)	p <0.001 a

^a Monte Carlo test was used

* Significant at p<0.01

Table 2. Comparison of characteristics between diabetic patients with and without neuropathy.

 

	Patients with DPN	Patients without DPN	P-value
No of patients/eyes	19/38	21/42
Gender (F/M)	8/11	8/13	0.752
Age (yr, mean±SD)	14.1±2.25	12.9±3.13	0.341
Duration of diabetes (yr, mean±SD)	6.6±3.09	4.6±3.51	0.03*
RBS	177.8±30.56	164.8±29.54	0.295
HBA1C (%, mean±SD)	9.9±2.88	14.1±2.31	0.331
Microalbuminuria (no)	3 (16%)	0 (0%)	0.062
Tibial MCV (mean±SD)	36.15± 7.29	45.85±2.43	0.01**
SCH (mean±SD)	12.5±2.84	12.3±2.44	0.001**
BUT(mean±SD)	10.2±3.67	13.2±3.7	0.0003**
Rose Bengal staining (no)	5 (26%)	3 (15%)	0.885
Fluorescein staining (grade)	1  (1-2)	0.5 (0-1)	0.035*
Impression cytology (grade)	1.5 (1-2)	0.5 (0-1)	0.007**

HB A1C glycosylated hemoglobin, MCV motor conduction velocity, SD standard deviation.

* Significant at p<0.05 ** Significant at p<0.01

 

 

 

Figure 1. Conjunctival impression cytology grades among patients with DPN (pn) and without DPN (nopn).

 

 

DISCUSSION

 

Recently, establishing a surrogate marker for diabetic neuropathy (DN) is extensively studied, to define progression or response to treatment via repeated assessments. So, the ideal surrogate marker should be easy to use, reliable, sensitive, and noninvasive.²⁴ Various markers have been advocated, including invasive and noninvasive techniques. Invasive procedures (e.g., sural nerve biopsy²⁵ and skin biopsy^24,26) proved to be accurate, while clinically noninvasive techniques (e.g., electrophysiology, quantitative sensory testing (QST), and assessment of neurological disability²⁷) showed less accuracy.

Nerve conduction studies are the most sensitive noninvasive technique for identification of PDN, compared with other methods as vibration tactile perception.¹⁸ As an alternative noninvasive markers, ocular surface disorders, such as corneal dysfunction assessment or tear film dysfunction using different tests, have been extensively studied on diabetic patients, especially patients with T2DM, searching for their correlation with DN.^10,12,24,28

Using the ocular surface parameters as a marker for DPN is more appropriate and applicable in younger patients with T1DM. Invasive (biopsy) and painful (electrophysiology) techniques are difficult to advocate among children and adolescents with T1DM. Also, age–related decline in these ocular functions is absent compared with elder people.⁶ This was confirmed in the present study by absence of any ocular abnormalities in all tests among age and sex matched healthy controls.

In this study, ocular surface disorders have been significantly detected among patients with T1DM, especially those with neuropathy. Other studies indicated that 47% to 64% of diabetic patients have primary corneal lesions during their life time.^6,12 A prevalence of dry eye symptoms of 20.6% in diabetics and 13.8% in non-diabetics was reported in previous studies.^7,29

In the present study, 17.5 and 45% of examined diabetic eyes, respectively, had abnormal Schirmer and BUT tests, compared to none in the controls. This agrees with reports of other studies that diabetic patients had a decreased BUT test and lower values of basal Schirmer test.^12,30 Moreover; this study proved that diabetic patients showed significantly more frequent and more pronounced signs of conjunctival metaplasia as assessed by impression cytology test. This is consistent with other studies that demonstrated pronounced signs of conjunctival squamous metaplasia in patients with insulin-dependent diabetes³⁰ and loss of mucin-secreting goblet cells, indicating chronic irritation.¹²

Moreover, using clinical assessment and routine nerve conduction studies of lower extremities in this study, 47% of patients with T1DM had DPN. Karsidage and his colleagues suggested that the most useful and practical nerves for the electrophysiological study in diabetic patients were the motor and sensory nerves of lower extremities.¹⁷ Also, previous studies found that the peroneal nerve was the highest³¹ and most sensitive abnormality in diabetic patients.²⁰

Percentage of neuropathy varied across studies due to different methods of assessment (clinical scores, special tests as vibration perception thresholds (VPT) and tactile perception thresholds (TPT), electrophysiological assessment, or nerve biopsy), duration of diabetes, age of group, and involvement of small fiber or autonomic neuropathy^18,28,32.

Similar to previous studies,^10,17,18 patients with neuropathy are of older age, longer duration of diabetes, and significantly correlated with other chronic complications. Moreover, patients with neuropathy are more vulnerable to ocular surface disorders compared with patients without neuropathy. This was detected by few reports in T1DM¹¹ and many studies in T2DM patients.^10,24,28

In diabetic patients, correlation between DPN severity and corneal keratopathy is attributed to loss of corneal nerve fibers secondary to abnormal glucose metabolism and activated polyol pathway.^8,10 Also, the presence of dry eye in the studied diabetic patients, which agrees with other studies, is attributed to the decrease in reflex tearing due to a decreased sensitivity of the conjunctiva resulting from neuropathy. Likewise, the BUT test was decreased, especially in diabetes with peripheral neuropathy, suggesting a neuropathy involving the innervations of the lacrimal glands.^11,12,28

The present study detects the percentage of DPN in young patients with T1DM, and confirms its correlation with duration of DM, other chronic diabetic correlation, and ocular surface disorders. It also suggests the usefulness of corneal and tear film parameters as markers for DPN, but more extensive prospective studies are needed to confirm this association, with long term follow-up, repeated assessments, using standard measures for DPN and ocular surface disorders, and correlation to DPN severity.

Confirming the relationship between DPN and ocular surface disorders is of high clinical value. It suggests that corneal keratopathy and tear film dysfunction could be a non-invasive, simple marker or screen for DPN. Also, it recommends regular ophthalmological assessment and ocular care in patients with DPN.

 

[Disclosure: Authors report no conflict of interest]

 

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