RADIOLOGICAL ANALYSIS OF OLFACTORY FOSSA DEPTH : A TERTIARY CARE HOSPITAL BASED STUDY

st th This was a longitudinal study conducted from 1 May to 30 August 2018. Coronal CT scans of nose and paranasal sinuses were taken for analysis of olfactory fossa depth. The landmarks like infraorbital foramen, point of ar cula on of medial ethmoidal roof with lateral lamella of cribriform plate (MERP) and lowest point on the cribriform plate (CP) were taken. The lateral lamella of cribriform plate (LLCP) was calculated by subtrac ng CP height from MERP height (MERP-CP = LLCP).


INTRODUCTION
For the endoscopic sinus surgery the detailed anatomical knowledge of nose and paranasal sinus is important.The knowledge is mainly gained by the computed tomogram of nose and paranasal sinuses.Though there are several other important surgical landmarks, the landmark of olfactory fossa is very important during sinus surgeries to avoid iatrogenic injury to anterior skull base.
The olfactory fossa is occupied by the olfactory nerve and bulb.It lies in the most infero-medial por on of the anterior cranial fossa.The crista galli forms the longitudinal limb above the horizontal limb and the perpendicular plate lies 1 below it.
The fovea ethmoidalis (FE) is the major part in forma on of the roof of the ethmoid bony labyrinth that separates the ethmoidal air cells from the anterior cranial fossa.It ar culates medially with the lateral lamella of the cribriform plate (LLCP).The shape of the fovea is determined by the 2 joining angle between FE and LLCP.
The LLCP is the thinnest bone, so it is the most vulnerable site for iatrogenic injury during func onal endoscopic sinus 3-6 surgery (FESS).
In 1962, Keros' had described the different heights of the horizontal level of the cribriform plate.As per Keros' classifica on, there are 3 types of olfactory fossa depth depending on the length of the LLCP.This thin (mainly ver cal) plate of bone is connec ng the lateral wall of 7 cribriform plate with fovea ethmoidalis.
The three types of olfactory fossa are: Type I is 1-3 mm deep in which the lateral lamella is short, and the ethmoid roof and cribriform plate are almost in the same plane.Type II (most common) is 4 to 7 mm deep, and the lateral lamella is longer.In type III, it is 8-16 mm deep, and the ethmoid roof lies significantly above the cribriform plate.According to Keros, the more the height of LLCP, the greater the risk of 7 iatrogenic injury during endoscopic sinus surgery.Therefore, the type III is at risk during endoscopic sinus surgery for iatrogenic injury.
Since, there are not many studies done in Nepal regarding olfactory fossa depth, this study explores the varia ons of olfactory fossa depth which will help during endoscopic sinus surgery.So, the main aim our study is to analyze the types and frequency of olfactory fossa depth.

METHODOLOGY
This was a prospec ve, longitudinal study conducted in the department of Otorhinolaryngology and Head and Neck st surgery, Dhulikhel Hospital, Kathmandu University from 1 May 2018 to 30 August 2018.The ethical approval was th taken from ins tu onal review commi ee.
All pa ents >/=18 years of age who underwent CT Scans of the nose and paranasal sinuses were included in the study whereas pa ents with previous sinus surgery, age <18 years, maxillofacial trauma, sinonasal malignancy, congenital anomaly and CT images of low resolu on were excluded.
The one hundred one consecu ve pa ents who underwent CT scan of nose and paranasal sinuses and fi ed in the inclusion criteria were taken for the study.
CT scans were performed in 128 slice Siemens somatom perspec ve machine.Pa ent was posi oned in supine posi on and using the parameters-130 kV, 145 mAs, and scan me of 3.5 seconds, a volumetric axial CT scan was taken with 3 mm slices thickness from the frontal sinus to the floor of maxillary sinus.Mul planar reconstruc on was done using 1 mm thin slices with 0.5 mm interval and images were obtained in all planes.For the depth of olfactory fossa calcula on, coronal images were used.All the observa ons were performed by one person to avoid observer bias.
The posi ons of cribriform plate and ethmoidal roof were calculated rela ve to orbital floor as shown by the infraorbital foramen (a plane passing through the two foramina was used (IOP)).Two reference points were chosen at the skull base.They were: point of ar cula on of medial ethmoidal roof with lateral lamella of cribriform plate (MERP) and lowest point on the cribriform plate (CP) as shown in figure 1.
Ver cal height from MERP (MERP height) and ver cal height from CP (CP height) to the horizontal plane through infraorbital foramen were measured on each side (IOP).The LLCP was calculated by subtrac ng CP height from MERP 8 height (MERP-CP = LLCP).The LLCP was classified according to Keros classifica on but with modifica on.In type I we included 0-3.99mm, in type II 4-7mm and in type III >7mm as there was deficit of 1 mm in type I to type II and type II to type III.The LLCP was compared on both sides in males and females.For the sta s cal analysis, sta s cal package for social sciences version 23 (SPSS) was used.The frequency table was used to evaluate the different variables using the descrip ve sta s cal methods (mean, standard devia on and percentage).

RESULTS
There were total 101 pa ents included in the study.The distribu on of age was 33.72+/-15.15years.Regarding the gender, there were 51 males and 50 females.The Keros' varia on type I was the most common in both right and le side as shown in table 1. Regarding the gender distribu on of Keros' varia on, Type I was the commonest in both sides and both genders as shown in Table 2.

Shrestha BL et al
Table 2. Showing the gender related Keros' varia on in right and le side.
Regarding the depth of the olfactory fossa, right side was deeper in males whereas the le side was deeper in females, but overall right side was deeper 52(51.5%)as shown in table 3.

Table 3:
Showing the depth of olfactory fossa in both gender.
The height of the LLCP on right side was 3.331+/-5.010mmwhereas on le side it was 2.845+/-1.084mm.
The figure 2 and 3 show the different varia ons of Keros' classifica on.

DISCUSSION
The endoscopic sinus surgery is done mainly to remove the 9 disease within the sinus and improve the ven la on.This procedure is widely accepted for clearing the disease.The risk of iatrogenic injury to skull base area during surgery is always there and the most vulnerable area is LLCP, as it is 10 the thinnest bone.So, it is important to look for the LLCP and olfactory fossa depth by CT imaging of nose and paranasal sinuses.Coronal CT scan is the best to visualize the olfactory fossa depth.We have performed our study as Our study showed that Keros' type I was the most common whereas type III was the least common with 86.1% pa ents falling in type I and only 0.5% in type III.So, it is somehow safe from the surgeon's point of view to do surgery in Keros' type I popula on.The distribu on of Keros' varia on in different studies in the literature and comparison with our study is shown in table 4. We have found 19 studies of Keros' varia on in different countries and most of them had type II as the commonest varia on (13 out of 19 studies) whereas 6 out of 19 studies had type I as the commonest varia on as in our study.So, there is marked varia on in Keros' types in the literature.
Regarding the gender distribu on, in male and female both type I was more common both in right and le , whereas type III was only seen in right side in male.The reason behind large group falling under type I in our study could be because of taking up to 3.99mm in type I unlike up to 3mm in original Keros' classifica on and another may be because of developmental anatomical varia on within the popula on and also due to difference in the extent of pneuma sa on of the ethmoidal labyrinth and frontal 11 sinus.
Our study showed that the height of LLCP on right side was 3.331+/-5.010mm,whereas on le side, it was 2.845+/-1.084mm.This differs from other studies as most of the studies had LLCP of >4mm.
The difference in height could be because of varia on in ethmoidal roof configura on between different popula on and another reason might be use of different measurement methods.In our study we had taken the infraorbital foramen as a reference point.Different literature has suggested this reference point as of relevant anatomic value during endoscopic middle meatal antrostomy as it helps in telling 14 about the posi on of the ethmoidal roof.
Our study also showed that the depth of the olfactory fossa was more on right in males whereas in females it was more on le side.We could not find any reason for it.However the possible explana on could be hormonal factors in development of craniofacial asymmetry.The literature has also shown varia on in depth of olfactory fossa either on But the overall depth of olfactory fossa was more on the right side (51.5%) in our study.The significance of our results showed that skull base injury with cerebrospinal fluid leaks occur more frequently when endoscopic sinus 23,26 surgery is perform on right side as men oned in literature.So, we have to be very careful while performing surgery on right side.
In our study the type I Keros' varia on was the most common.Therefore it is safer for the surgeon to perform endoscopic sinus surgery in type I popula on like ours as the iatrogenic risk of skull base injury is minimal.

CONCLUSION
1.The study on varia on of Keros' classifica on is mainly to avoid injury in ethmoidal roof during sinus surgery.
2. In our study, type I is the most common Keros' type prevalent and the least prevalent is the type III.
3. A deep olfactory fossa is more common on right as compared to le in our popula on.
4. Careful and precise assessment of CT nose and PNS is a must prior to sinus surgery in order to avoid serious iatrogenic injury.

RECOMMENDATION
The type I Keros' varia on is the most common and a deep olfactory fossa is more common on right as compared to le .However this study is single ins tu onal, so we recommend the mul -ins tu onal study in the different regions of Nepal which will give be er idea regarding the distribu on of Keros' varia on of olfactory fossa.

LIMITATION OF THE STUDY
The main limita on of the study is sample size and short dura on of study.

Figure 1 :
Figure 1: Showing the measurement of different landmarks in CT Scan.

Figure 2 :
Figure 2 : Showing the Keros' varia on I (le side) and II (right side)

Table 4 :
Showing the varia ons of Keros' types in different studies.